volume 30 number 3 june 2020 - aappsbulletin

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ASSOCIATION OF ASIA PACIFIC PHYSICAL SOCIETIES

v o l u m e 3 0 N u m b e r 3 J U N E 2 0 2 0

Feature Articles Plasma and Ions Help Slice

and See Surfaces Better First Results of the Event

Horizon Telescope

Physics Focus Novel Optical Microscopies

to Unravel Brain Function The Tango of Rotating Black

Holes and Spinning Particles

APCTP Section How Do Novel Viruses

Threaten Humankind? We Are Not Prepared

Review and Research Gaussian Expansion Method and

its Application to Nuclear Physic with Strangeness

Processes at Plasma-Matter Interfaces: An Overview and Future Trends

Tohru Motobayashi (Editor-in-Chief) / 2017.01-2020.06RIKEN Nishina Center for Accelerator-Based Science 2-1, Hirosawa, Wako, Saitama 351-0198, JapanE-mail: [email protected]

Akira Yamada (Deputy Editor-in-Chief) / 2017.01-2020.06Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8550, JapanE-mail: [email protected]

Qing Wang / 2017.01-2020.06Tsinghua University, Haidian DistrictBeijing 100084E-mail: [email protected]

Brian James / 2017.01-2020.06Sydney University, New South Wales 2006 AustraliaE-mail: [email protected]

Leong Chuan Kwek / 2017.01-2020.06Center for Quantum TechnologiesNational University of Singapore, Block S15 3, Science Drive 2, 117543, SingaporeE-mail: [email protected]

Shozo Suto / 2017.01-2020.06Graduate School of Science, Tohoku University 6-3, Aramaki Aza-Aoba, Aoba-ku, Sendai 980-8578, JapanE-mail: [email protected]

Jeong-Hyeon Song / 2017.01-2020.06School of Physics, Konkuk University 120 Neungdong-ro, Gwangjin-gu, Seoul, 05029, KoreaE-mail: [email protected]

Chong-Sun Chu / 2017.01-2020.06National Center for Theoretical Sciences 101, Section 2 Kuang Fu Road, Hsinchu, 300E-mail: [email protected]

Jan-e Alam / 2017.10-2020.09Variable Energy Cyclotron Centre, 1/AF Bidhannagar, Kolkata - 700 064, INDIAE-mail: [email protected]

Kaoru Minoshima / 2019.04-2020.03The University of Electro-Communications (UEC) 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585 JAPANE-mail: [email protected]

Gui-Lu Long / 2020.01-2023.06Tsinghua University, School of Sciences Building Beijing 100084, China E-mail: [email protected]

Yu-Bo Sheng / 2020.01-2023.06Nanjing University of Posts and Telecommunications, 180 Siwangting Road, Yangzhou City, Jiangsu Province, P.R. China 225002E-mail: [email protected]

Woo-Sung Jung / 2020.01-2023.06Asia Pacific Center for Theoretical Physics, Hogil Kim Memorial Building #501 POSTECH, 77 Cheongam-ro, Nam-gu, Pohang-si, Gyeongsangbuk-do, 37673, KoreaE-mail: [email protected]

Masao Ogata / 2020.01-2023.06University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, JAPANE-mail: [email protected]

Bongjin Simon Mun / 2020.01-2023.06Gwangju Inst. of Science and Technology,Department of Physics and Photon Science 123 Cheomdan-gwagiro, Buk-gu, Gwangju, 61005, KoreaE-mail: [email protected]

Honghao Zhang / 2020.01-2023.06Sun Yat-Sen University,Guangzhou 510275, China E-mail: [email protected]

Jiunn-Wei Chen / 2020.01-2023.06National Taiwan University,No.1 Sec.4 Roosevelt Road Taipei 10617,TaipeiE-mail: [email protected]

Akira Uritani / 2020.03-2021.02Graduate School of Engineering and School of EngineeringNagoya University Furo-cho, Chikusa-ku, Nagoya, 464-8603, JapanE-mail: [email protected]

Association of Asia Pacific Physical Societies

EDiTOriAL STAff

Seunglae Cho (APCTP)E-mail: [email protected]

Eunjeong Lee (APCTP)E-mail: [email protected]

Susan Song-One KangPrincipal Language and Technical Editor

Applied physicsBae Ho Park, KoreaMasaaki Tanaka, JapanHyeon-sik Cheong, Korea

Astrophysics & GravitationEnwei Liang, China/BeijingPrajval Shastri, IndiaMisao Sasaki, JapanSang Pyo Kim, Korea

Condensed matterTae Won Noh, KoreaSadamichi Maekawa, JapanShunqing Shen, China/Hong KongFuchun Zhang, China/BeijingTing-Kuo Lee, China/TaipeiJun Sung Kim, Korea

Kwon Park, KoreaHyoung Joon Choi, KoreaMyunghwa Jung, KoreaSetsuko Tajima, JapanXucun Ma, China/BeijingMichiYasu Mori, JapanSeiji Yunoki, JapanAndrew Wee, Singapore

Particle & HEPHong-Hao Zhang, China/BeijingPyungwon Ko, KoreaSang-Jin Sin, KoreaJun Cao, China/BeijingMitsuaki Nozaki, Japan

Plasma physicsCormac S Corr, AustraliaShuyan Xu, SingaporeHyeon K Park, KoreaGun Su Yun, KoreaMichael Keidar, The US

Accelerator ScienceWon Namkung, KoreaGao Jie, China/BeijingTadashi Koseki, JapanIn Soo Ko, Korea

Quantum informationYu-Bo Sheng, China/BeijingLeong Chuan Kwek, SingaporeChao Yang Lu, China/Beijing

Optics, Photonics, LaserDeb Kane, Australia

AMOChuan-Gang Ning, China/Beijing

Nuclear physicsYongseok Oh, KoreaShoji Nagamiya, JapanYong Liang Ma, China/BeijingJane Alam, India

Statistical physicsJian Sheng Wang, SingaporePaul Pearce, Australia

Biological physics & Soft MatterMeishan Wang, China/Beijing

EDiTOriAL BOArD MEMBErS

SCiENTifiC COMMiTTEE (fOr rEviEW AND rESEArCH)

cONteNtsJune 2020 vol. 30 no. 3

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feature articlesPlasma and ions Help Slice and See Surfaces BetterAnnalena Wolff , Kostya (Ken) ostrikov, nunzio Motta

First Results of the event Horizon telescopeKeiichi Asada, Masanori nakamura

activities aNd research NeWs Higher Precision Mirrors Set to Benefi t ‘next Gen’ Gravitational Wave Detectors

Reducing noise of Quantum light Below the Sound of Silence

Physicists Shine light into Primordial universe

iNstitutes iN asia pacificthe Department of Physics at northeastern university, ChinaWei-Jiang Gong, Qi Wang, Yong Hu

phYsics fOcusnovel optical Microscopies to unravel Brain FunctionChiao Huang, Kuo-Jen Hsu, Han-Yuan lin, Shi-Wei Chu

the tango of Rotating Black Holes and Spinning ParticlesYu-tin Huang

apctp sectiONHow Do novel viruses threaten Humankind?Kang-Seuk Choi

We Are not Prepared Sungsil Moon

revieW aNd researchGaussian expansion Method and its Application to nuclear Physic with Strangenessemiko Hiyama

Processes at Plasma-Matter interfaces: An overview and Future trendsigor levchenko, Kateryna Bazaka, oleg Baranov, oleksii Cherkun, Michael Keidar, Shuyan Xu

Due to the uncertainty caused by COVID-19, all in-person academic events are canceled or will be postponed. The calendar of events will return in a future issue.

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Bulletinfeature articles

ABSTRACT

Focused Ion Beam (FIB) devices are versatile analytical tools in physics and materials research. Today, different FIBs, including gallium- and xenon plasma Focused Ion Beam Scanning Electron Microscopes (FIB/SEMs) as well as the Helium Ion Microscope (HIM) help answer research questions that no other technology can. This article looks into the physics of the FIB devices to help understand the difference between the effects of ion spe-cies (Ga, Xe, He) and FIB systems.

INTRODUCTION

Richard Feynman’s speech “There is plenty of room at the bottom’ [1] is considered to be one of the most well-known and influential speeches in science. In 1959, Feynman proposed that one day there will be technology available that can be used as our eyes and hands in the microscopic domain. In his visionary speech, he pre-dicted the use of focused ion beams to see and manipu-late matter at the tiniest scales. It would take another 40 years until the first Focused Ion Beam/Scanning Electron Microscope (FIB/SEM) would be available commercially. Gallium FIB/SEMs have established themselves as one of the key instruments in many facilities and labs due to their unique ability to cut a wide variety of materials with nanometre precision while analysing the sample and re-vealing sub surface features with nanometre resolution. Today, they are the go-to tool for transmission electron microscope (TEM) lamella preparation and are increas-ingly being used for site specific cross-sectioning, 3D re-construction as well as nanofabrication [2], e.g. pattern-ing graphene on SiC [3].

Throughout the past 20 years, Ga FIB/SEMs have domi-nated the market. A focused beam of gallium ions, how-ever, can have significant drawbacks. This becomes ap-parent when looking at the ion-solid interactions. As the ion beam hits the sample, the incident ions interact with the sample atoms in various ways.

ION-SOLID INTERACTIONS

The ions, no matter what ion species, lose their energy in collisions with the sample atoms (Fig. 1). With every in-teraction, energy is transferred (and/or lost). The entire energy loss dE /dx consists of both contributions, the nu-clear (elastic) energy losses [dE /dx]nucl and the electronic (inelastic) energy losses [dE /dx]lec :

dE /dx = [dE /dx]nucl + [dE /dx]elec

What type of interaction occurs depends on the ion en-

Plasma and ions Help Slice and See Surfaces BetterAnnAlenA Wolff, KostyA (Ken) ostriKov and nunzio MottA

sCHool of CHeMistry AnD PHysiCs institute for future environMents AnD Centre for MAteriAls sCienCe

QueenslAnD university of teCHnoloGy BrisBAne QlD 4000, AustrAliA

Doi: 10.22661/AAPPsBl.2020.30.3.02

fig. 1: electronic and nuclear interaction of an ion with a sample atom.

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June 2020 vol. 30 no. 3 feature articles

ergy, the energy transfer, the ion species as well as on the sample itself (Fig. 2): the collisions can lead to sputtering of the sample surface atoms, secondary ion emission as well as backscattered ions, sample atom displacements such as vacancies, a collision cascade, replacement colli-sions and phonons. Secondary electron emission as well as polymerization are additionally caused by the ion-sol-id interactions. The interactions occur until the incident ion has lost all its energy and comes to rest in the sample at a certain depth, leading to ion implantation [4].

GALLIUM VS XENON

Significant problems arise when processing samples with a Ga FIB/SEM including phase transformations as well as changes in physical properties of the sample as a result of ion implantation [5, 6]. It has long been recognized that Ga changes the physical properties of semiconduc-tors when processing them in the Ga FIB/SEM. It is less known that the gallium ions can accumulate along grain boundaries in aluminium, leading to a completely differ-ent deformation and fracture behaviour of the material [7].

To avoid the drawbacks which are associated with Ga FIB/SEMs, different ion species are explored in labora-tories and utilized commercially. Today, systems with a

conventional gallium liquid metal ion source as well as multi-species (liquid metal alloy ion source) can be pur-chased. The plasma FIB/SEM technology, available since 2012, has had a major impact with Xe being one of the most popular ion species. Xe is inert, avoiding doping of semiconductors or alloying (such as Ga in Cu → Cu3Ga, see Fig. 3) when processing samples with a Xe plasma FIB/SEM. Furthermore, the combination of a higher sputtering yield for a Xe plasma FIB than Ga (factor 1.5) and the possibility of using μA range currents (rather than nA range currents for the Ga FIB) makes patterning 30 times faster in comparison to a Ga FIB/SEM [8]. As a result, large area cross-sectioning or 3D volume recon-structions, with dimensions around 500 μm (in each di-rection) become feasible while maintaining nm precision of the cross-section placement. Being able to prepare site specific large area cross-sections/volumes and analyse those with the SEM in situ opens up new opportunities, especially in materials engineering, life sciences as well as geology. These applications often require larger area removal which was not feasible with Ga FIB/SEMs previ-ously. A recent report [9] revealed that Xe plasma-FIB is a superior instrument with respect to Ga FIB in slicing materials for TEM analysis. Reduced amorphization lay-er thickness and the absence of residual Ga contaminants improve the TEM lamella quality significantly.

HELIUM VS XENON

Let us briefly recap the physics of operation of the He-lium Ion Microscope (HIM). A lighter ion species like He predominantly interacts with the sample atom electrons

fig. 2: Schematic of ion-solid interactions: the ion solid interactions can lead to (1) Sputtering, secondary ion emission; (2) back scatter ions, (3) dislocations and vacancies, (4) interstitials, (5) phonons, (6) ion implantation, (7) polymerization as well as (8) secondary electron emission.

fig. 3: top: Scanning transmission electron Microscopy images of a teM foil: Cu (left), Ga FiB irradiated Cu (right). the corresponding electron diffraction measurements (bottom) show that the irradiated area phase transformed to Cu3Ga.

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(inelastic collisions), producing many ‘secondary elec-trons’ (roughly 10 times more signal than is created when using an electron beam in an SEM) which can be detect-ed using a conventional Everhart Thornley detector [10]. The beam stays collimated within the secondary electron escape depth. This in combination with its atomically sharp and cryogenically cooled source gives the HIM its superior imaging capabilities in nanomaterials research (Fig. 4) [10]. Nuclear interactions still occur and become statistically significant and lead to sputtering when using higher ion doses. This gives rise to the sub-10 nm fab-rication capabilities of the HIM when these interactions occur near the sample surface. It is not well recognized within the science community though, that nuclear inter-actions become the dominant interaction type below the sample surface once the He ion’s energy has fallen be-low 1 keV as a result of the ion-solid interactions. As the atoms cannot be removed under such conditions from within the material, dislocations are created and this can be used for defect engineering, creating novel material properties [11]. Figure 5 shows a defect engineered alu-minium oxide layer on silicon which exhibits superplas-tic behaviour, opening an exciting opportunity to write “nano-tattoos” on solid surfaces.

Heavy ions like Xe predominantly interact via nuclear interactions in the entire energy range that is available

in FIB/SEMs, making these ions ideal for sputtering samples. The new Xe plasma FIB/SEM (Fig. 6) very re-cently installed at QUT [12] is therefore ideally suited for preparing and analysing large area cross-sections, large volume reconstruction and TEM lamella preparation with reduced amorphous layer thickness. It also extends QUT’s HIM noble gas structuring range, making it pos-sible to precisely fabricate features ranging from sub-10 nm (HIM) to > 500 μm (Plasma FIB).

fig. 4: Colorized HiM micrograph of MoS2 on graphene. fig. 5: Defect engineered aluminium oxide on Si, reprinted from Aramesh, M., Mayamei, Y., Wolff, A. et al. Superplastic nanoscale pore shaping by ion irradiation. nat Commun 9, 835 (2018). https://doi.org/10.1038/s41467-018-03316-7

fig. 6: tescan S8000X Xe FiB/SeM installed at Qut. this system is equipped with oxford Symmetry eBSD and ultim Max eDS. image courtesy tescan.

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CONCLUSION

Using different ion species and focused ion beams (FIBs) presents new opportunities to study as well as create materials of tomorrow. From using the Xe plasma FIB/SEM to prepare high quality site-specific cross-sections or TEM lamellae of various materials to the HIM’s abil-ity to study the surface features and to defect engineer novel materials, the underlying physics of the ion solid interactions helps choose the appropriate ion species for the job.

Acknowledgements: The authors acknowledge the fa-cilities of the Australian Microscopy and Microanalysis Research Facility at the Central Analytical Research Facil-ity operated by the Institute for Future Environments at the Queensland University of Technology. The authors greatfully acknowledge the support from the ARC LIEF LE180100090 as well as the partner institutions the Uni-versity of Queensland, Griffith University, Monash Uni-versity and the University of Technology Sydney for the new Xe plasma FIB/SEM, located at QUT.

References

[1] calteches.library.caltech.edu/1976/[2] D. Drobne, M. Milani, v. leser, f. tatti, Microsc. res. tech. (2007), 70, 895-903. [3] M. Amjadipour, J. Macleod, J. lipton-Duffin, f. iacopi, n. Motta,

nanotechnology (2017), 28, 345602.[4] l. A. Giannuzzi, f.A. stevie, introduction to focused ion Beams, springer

(2005).[5] J.r. Michael, Microsc. Microanal (2006) 12 (supp2), [6] J. einsle, J. Bouillard, W. Dickson, A.v. zayats, nanoscale research letters

(2011) 6, 572.[7] y. Xiao, v. Maier-Kiener, J. Michler, r. spolenak,J.M. Wheeler, Materials and

Design (2019), 181 107914.[8] t. Hrncir et al novel plasma fiB/seM for high speed failure analysis and real

time imaging of large volume removal istfA conference proceedings paper 2012.

[9] t. l. Burnett, r. Kelley, B. Winiarski, l. Contreras, M. Daly, A. Gholinia, M. G. Burke, P. J. Withers, ultramicroscopy (2016), 161, 119-129.

[10] G. Hlawacek, A. Goelzhaeuser, Helium ion Microscopy, springer, 2016[11] M. Aramesh, y. Mayamei, A. Wolff, K. ostrikov, nature Communications

(2018), 9, 835.[12] infocus Magazine, royal Microscopical society, issue 54 June 2019.

Nunzio Motta is a Professor in the School of Chemistry and Physics at the Queensland university of technology, Brisbane, Australia. He graduated in Physics at università di Roma la Sapienza in 1981 and obtained his PhD in 1986 (Scuola normale Superiore di Pisa) He was the first scientist in italy to achieve atomic resolution on Si(111) 7×7 by Scanning tunneling Microscopy, and he is internationally recognized in the field of material science, with over 30 years’ experience in growth and characterization of semiconductors and nanostructures. He is currently leading surface science and nanotechnology research at Qut, developing new 2D heterostructures, graphene-based supercapacitors, solar-powered nano-sensors and thin film solar cells. He has published more than 200 papers, with more than 5400 citations and an h-index of 42. He has been chair of the international school and conference nanoS-e3 since 2007.

Kostya (Ken) Ostrikov is a Professor with Queensland university of technology, Australia, and a Founding leader of the CSiRo–Qut Joint Sustainable Processes and Devices laboratory. His research on nanoscale control of energy and matter contributes to the solution of the grand challenge of directing energy and matter at nanoscales, to develop renewable energy and energy efficient technologies for a sustainable future.

annalena Wolff is a Research infrastructure Specialist for Focused ion Beams at the Queensland university of technology, Australia. She manages the universities new Xe plasma FiB/SeM as well as the Helium ion Microscope and supports the instrument user groups. Her research interest is the physics behind the systems which allow the development of novel FiB approaches and techniques.

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ABSTRACT

The Event Horizon Telescope (EHT) collaboration has revealed the first-ever images of a black hole shadow at the heart of a giant elliptical galaxy Messier 87 (M87). The EHT links ground-based radio telescopes around the globe to form an Earth-sized virtual telescope with an unprecedented highest angular resolution using very long baseline interferometry (VLBI) at millimeter wave-lengths. Images visually reveal the strongest evidence of an existence of a black hole in the universe. The bright compact radio source with a diameter of 42±3 micro-arcsecond (μas) suggests a supermassive black hole (SMBH) of (6.5±0.7)× 109 M⨀ (solar mass). An asym-metric ring-like morphology strongly suggests that we see gravitationally lensed emission from plasma rotating around the very vicinity of the SMBH event horizon. The image also supports the longstanding hypothesis that a SMBH powers an active galactic nucleus (AGN). The EHT collaboration demonstrates that VLBI at milli-meter/sub-millimeter bands offers a powerful method to explorer gravity in its most extreme limit and at a previ-ously inaccessible mass scale.

INTRODUNTION

On April 10, 2019, the Event Horizon Telescope (EHT) collaboration released the “photograph” of a black hole into our visual world. This day becomes a memorable day in human history, because a black hole, first predict-ed by Albert Einstein over a century ago with his general theory of relativity, has been visually captured by the EHT collaboration. The EHT Collaboration, founded by Prof. Shep Doeleman at CfA/Harverd university, is an international collaboration established in order to im-age the shadow of a supermassive black hole (SMBH) for the first time. More than 200 researchers from 13 stake-holder institutes including the Academia Sinica Institute

of Astronomy and Astrophysics (ASIAA) in Taiwan, the National Astronomical Observatory of Japan (NAOJ) and the East Asian Observatory (EAO) from the Asian region and more than 20 affiliated research institutes/universities participate this collaboration. While the EHT collaboration formally started in 2017, project planning and test observations were initiated in the early 2000s. In this report, we describe how the first image of a black hole was taken together, its impact and future prospects.

Black Hole and its shadowAlthough we are unable to resolve the event horizon in the image, we expect to resolve a ring-like structure with a photon capture radius of Rc = 27rg, where rg ≡ GM / c2 is the gravitational radius of a black hole, in a nonrotating Schwarzschild [1] black hole. The photon capture radius is larger than the event horizon in the Schwarzschild metric, the so-called Schwarzschild radius RS ≡ 2 rg. Photons at radius < Rc are captured by the black hole [2], while photons at radius > Rc could escape to infinity. There is an unstable circular orbit at radius ≈ Rc that produces an enhanced bright emission due to gravitational lensing. For a Kerr [3] black hole with spin angular momentum, Rc varies with the dimensionless spin parameter a = J / Jmax, where Jmax ≡ GM 2/c is the maximum value of the black hole angular momentum, and the cross section of the lensed photon ring departs from a true circle [4]. This change is very small for a low inclination angle (< 20°) with respect to the black hole spin vector up to ~2 % [5].

Event Horizon Telescope and its observationsBased on the theory of general relativity, the angular size of the shadow is simply determined by the mass (and spin) of the black hole together with its distance from the earth. The largest one is the BH of Sgr A*, which is at the

First Results of the event Horizon telescopeKeiiCHi AsADA1, MAsAnori nAKAMurA1,2

1 institute of AstronoMy & AstroPHysiCs, ACADeMiA siniCA,11f AstronoMy-MAtHeMAtiCs BuilDinG, As/ntu no. 1, tAiPei 10617, tAiWAn

2 nAtionAl institute of teCHnoloGy, HACHinoHe ColleGe, 16-1 uWAnotAi, tAMonoKi, HACHinoHe City, AoMori PrefeCture, 039-1192, JAPAn

Doi: 10.22661/AAPPsBl.2020.30.3.06

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center of our Milky Way Galaxy, and the second largest one is that of M87 in the Virgo Cluster. The apparent an-gular sizes are expected to be 50 and 40 Äsl211Äslmult0 (μas), respectively. Therefore, we need to have a sufficient angular resolution in order to see the shadows of BHs. In keeping with this aim, Lo et al. [6] measured the size of the Sgr A* using Very Long Baseline Interferometry (VLBI) at 7 mm, and Shen et al. [7] pursued further finer structure by reducing the observing wavelength to be 3 mm. There is a growing recognition that further shorter millimeter/submillimeter (mm/submm) VLBI could be a unique technique to achieve a sufficient angular resolu-tion to image the shadow of the black hole in the coming decade [8, 9]. The Greenland Telescope project is one of the innovations led by ASIAA, Taiwan, targeting the im-aging of the shadow of the BH in M87 [10]. The project started at the end of 2000s, and has now joined the EHT collaboration [11].

VLBI is a technique that links radio telescopes across the globe to form a virtual telescope with an aperture that is equivalent to the size of the earth. Observing the same astronomical source simultaneously with all the mm/submm telescopes, signals from the object are re-corded using specialized data recorders at each telescope site. Then, the data are sent to the correlator site, and weak astronomical signals are extracted by taking the correlation between the data taken at each site.

From the early 2000s, experimental VLBI observations at 1.3 mm (230 GHz) using only a few telescopes in the mainland US and at Hawaii have been conducted and

the results revealed that presence of the very compact structures those are equivalent with the sizes of the shad-ows of black holes in Sgr A* and M87 [8, 9]. Following these successes, the EHT collaboration conducted new observations aiming to make images of BH shadows in M87 and Sgr A* [12, 13]. M87 was observed on April 5, 6, 10 and 11, 2017 with 7 stations (ALMA, APEX, IRAM 30m, JCMT, LMT, SMA, SMT) located at 5 different sites on the Earth (note that the SPT cannot observe M87).

Data Calibration and ResultsAll the data taken at each site were sent to the MIT Hay-stack Observatory and the Max Plank Institute fur Radio Astronomy (MPIfR). Data were synchronized and cor-related in order to extract astronomical signals recorded in the hard disks using a super computer. Information was reduced extensively in order to extract only essential information during this process. For instance, the total amount of data size was reduced to 1 TB from 5 PB. Once data were correlated, we calibrate the data for fur-ther analysis.

Three pipelines have been developed and used for the a prior calibration and extraction of the astronomical signals (fringe finding); the development and usage of one of the pipelines have been led by the East Asian data calibration team [14].

The first Black Hole Shadow Image

After a long process of careful data validation, calibration and many crosschecks by the EHTC, the first attempt to image the shadow of M87 was conducted in 2018 June/July. In order to avoid any possible biases, four inde-pendent imaging teams have been created, and they at-tempted to image the data without sharing any informa-

fig. 1: eHt array used for 2017 observations together with Greenland telescope, which started to participate in eHt observations from 2018.

fig. 2: the first image of a BH shadow taken by the eHt collaboration. (credit: eHtC, 12)

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tion among teams at the initial imaging stage. Two of the teams have been led by institutes in East Asia (the ASIAA in Taiwan and NAOJ in Japan). Three different imaging methods have been used. There were 4 days of the data for M87, and all the four teams imaged the black hole shadow image very similarly for all the four days of the data. Based on this similarity, it is concluded that the images we obtained were robust and had a very high reli-ability [15].

After that, the imaging teams collaboratively refined the image and the final 1.3 mm VLBI image of M 87 revealed asymmetric ring morphology of the central compact radio source with a diameter of 42±3 μas [16]. The ring is almost circular and encompasses a central brightness depression of > 10:1. The robustness of this feature over multiple days strongly suggests that we see gravitationally lensed emission originating from near the black hole event horizon.

Interpretations of the Asymmetric RingThe theory working group of the EHT collaboration built physical models in order to reproduce the observed images by utilizing massive numerical simulations [17]. This helps to understand the behavior of the plasma under the influence of the intense gravity in the vicin-ity of the black hole. General relativity suggests that the orbital paths of photons, which originated from the ac-creting gas, are eventually captured by the gravity of the black hole. Therefore, there is a dark region where the intensity is much lower than the surrounding area within a few gravitational radii (rg) including the black hole it-self (inside the event horizon). Beyond this dark region (we call it a “shadow”), the so-called photon ring with an angular radius of 27rg (in reality, it is a photosphere) where photons will survive to rotate several times (gain-ing in intensity) and finally arrive at the earth. The EHT took photographs of the brightest photon ring around the SMBH with a depressed central darkness (the black hole shadow).

M87 is one of the typical sources that is categorized as a low-luminosity AGN and is also well-known to possess a jet. The jet from the nucleus of M87 travels more than 5,000 light years beyond the central region of the host galaxy. It is considered that a very hot (electron tempera-ture of ~ 1011 K) accretion flow surrounds the SMBH in the center of M87 with dominant synchrotron radiation at millimeter/sub-millimeter wavelengths. The simula-tion library consists of general relativistic magnetohydro-dynamic (GRMHD) simulations together with general relativistic radiative transfer (GRRT) calculations. They are essential tools for modeling the accretion flow and formation of jets around the black hole.

The theory working group performed extensive simu-lations using a wide variety of parameters including the black hole angular momentum, the event horizon-threading magnetic flux, and the electron temperature of the accretion flow. In total, more than 60,000 snap-shots were used for cross checking with observed images during the EHT2017 campaign.

Overall the structure of simulated images reproduce the asymmetric photon ring with a diffusive component of the accretion flow and a funnel wall (outflow) which sur-rounds the funnel jet from a spinning black hole (Fig.4). Furthermore, it was beyond our expectation, but there are many models having different parameters that are consistent with the observations. This implies that the ob-

fig. 3: estimated size of the ring (eHtC, 17). Both image and visibility domain fitting for all the epochs using different methods give consistent values.

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served emission of the asymmetric ring does not depend much on the dynamics and/or structure of the accreting material, but it strongly indicates that the behavior of the photons can be described in the curved spacetime of GR. We note that a non-rotating black hole is ruled out because it will not support the jet power apparent in ob-servations (≥ 1042 erg s–1). In this case (the black hole in M87 is spinning), the spin vector axis is uniquely deter-mined as pointed away from Earth and the asymmetric emission (the lower part is brighter than upper part) is produced primarily by Doppler breaming: the bright re-gion corresponds to the side approaching us.

fig. 4: An example a of self-consistent GRMHD + GRRt simulated image (left). the model image convolved with a 20 μas FWHM Gaussian beam (right) (credit: eHtC, 16).

The theory working group adopted the working hy-pothesis that the central object in M87 is a black hole described by the Kerr metric. Under this assumption, observed images are pretty much consistent with our GR-based numerical models. However, it is interesting to consider whether or not observed images are also consis-tent with alternative models for the central object such as black hole “mimickers”, i.e., compact objects, both within GR or in alternative theories. For example, regu-lar horizonless objects without a surface are boson stars [18], while mimickers with a surface are gravastars [19]. A comparison of EHT2017 data with both the boson star model (as a representative horizonless and surfaceless black hole mimicker) and the gravastar model (as a rep-resentative of a horizonless black hole mimicker) would be important [20]. Further examinations will bring a strong constraint on the spacetime property around the central object in M87.

The asymmetric ring in EHT2017 also provides a robust estimation of the black hole mass by extracting charac-

teristic properties, such as size and degree of asymmetry from the geometric model, GRMHD simulation model, visibility data, and reconstructed images. We find that > 50% of the total flux at arcsecond scales comes from near the event horizon with a dramatically suppressed interior emission by a factor > 10, providing solid evi-dence of a black hole shadow. Across all methods above, which are consistent with EHT2017 data, a diameter of 42±3 μas was obtained. Folding in a distance measure-ment of 16.8+0.8

– 0.7 Mpc gives a black hole mass of (6.5±0.7) × 109 M⨀. This supports the value obtained by the stel-lar dynamical measurements [21].

fig. 5: the structural profile of the M87 jet [34]. the structural transition was found by Asada & nakamura [24] for the first time and observations were conducted during recent decades.

Future ProspectsThroughout the EHT2017 campaign running for ~ a week, the asymmetric ring structure was stably imaged, while unexpectedly we could not capture the innermost emission of the jet. How/where is the jet initiated? This long-standing question has remained unsolved for over a century since the astrophysical jet was found in M87 in 1918 [22]. It may be due to a lack of the sensitivity of current arrays (the expected photon flux from the jet and the accretion flow could be orders of magnitude lower than that of the photon ring). In 2018, the ASIAA-led Greenland telescope (GLT) joined the EHT observa-tions. Furthermore, two more telescopes are planned to join after 2021. We therefore expect to image the launch-ing regions of astrophysical jets for the first time.

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During recent decades, East Asian (EA) astronomers have conducted various research programs in relation to M87 and play a major role in the community; Hada et al. [23] measured the VLBI core shift using multi-frequency VLBI observations and nailed down the possible loca-tion of the black hole in the upstream of the core at 43 GHz. Asada & Nakamura [24] explored the parabolic jet structure and discovered the jet collimation break at around the Bondi radius (see also, 25, 26, 27, 28). Kino et al. [29] argued that the innermost magnetic field at 230 GHz was associated with the jet base. Asada et al. [30] found possible spine jet emission in VSOP (VLBI Space Observatory Programme) observations (see also, 31 for a theoretical interpretation). Hada et al. [32] and Park et al. [33] studied the jet kinematics by utilizing the East Asia VLBI Network (EAVN). Nakamura et al. [34] inves-tigated the parabolic jet from the spinning black hole using GRMHD simulations and applied it to the M87 jet. Takahashi et al. [35] analyzed a synthetic synchrotron map with a force-free jet model and constrained the BH angular frequency. Kim et al. [36] analyzed the nuclear spectrum with the Korean VLBI network and suggested the possibility of its flatness (indicating a strong magnetic field near the black hole). Park et al. [37] measured the Faraday rotation obtaining an indication of winds from hot accretion flows confining the parabolic jet.

fig. 6: the velocity profi le of the M87 jet [33]. A complicated profi le was revealed by utilizing the eAvn facilities.

Fig. 5 represents one of extensive efforts led by EA as-tronomers (Nakamura et al. 2018). They compiled the jet emission at frequencies, ~ 1 GHz to ~ 230 GHz, to

reveal the jet collimation structure in M87 and com-pared it with the theoretical model. Based on this effort, the EHT collaboration will seek an origin for the M87 jet with connecting to lower frequency emissions. The EAVN large program is currently conducting a very high cadence monitoring of the M87 jet (a few day to a week) and this is the one and only program that can investigate how/where the jet is accelerated to a relativistic speed. The latest result is summarized by Park et al. [33] as shown in Fig. 6. The velocity profile suggests a gradual acceleration of the jet and that it transits into a decelera-tion beyond the Bondi radius. This result is consistent with Fig. 5 and we speculate that the jet acceleration and collimation take place in a co-spatial manner, supporting the MHD jet paradigm. Thus, EA astronomers are very active in determining the acceleration and collimation in M87, which is one of the most fundamental quests in the field of astrophysical jets. Besides, EAVN observations are scheduled quasi-simultaneously with the EHT cam-paign. This provides the jet position angle and other im-portant information in the downstream, giving a further constraint on the horizon-scale modeling of observed EHT images.

After 2017, the EHT collaboration continued to observe M87 and further investigations will be conducted includ-ing time variation of the photon ring (the stability of the asymmetric ring structure). Also, polarimetric observa-tions will reveal the magnetic field morphology on the horizon scale. Thanks to the GLT (improving a north-south baseline), the resolution along the transverse direction of the jet has been improved. The EHT col-laboration is planning to conduct 0.8 mm (345 GHz) ob-servations as well as space VLBI (next generation EHT). As a result of imaging the black hole shadow at 345 GHz, Kawashima et al. [38] proposed a constrain on the BH spin in M87.

We remark that observations of our galactic center Sgr A*, another EHT target for imaging a BH shadow, are now under analysis by the EHT collaboration. The mass ratio between Sgr A* and M87 is about three orders of magnitude. If we could image BH shadows in both sourc-es, it will provide a solid milestone in radio astronomy showing that VLBI is a powerful tool for studying the ex-tremely curved spacetime around a black hole in a visible way. It also indicates that the general theory of relativity can be examined in regions of the strongest gravitational fields around black holes using electromagnetic waves. There is no doubt that a big leap will be brought to BH

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astronomy by the success of the EHT project. Studying a BH shadow may also play a complementary role with testing the general theory of relativity in stellar mass black holes observed in gravitational wave astronomy.

EA-led EHT arrays such as JCMT (EAO), SMA, and GLT (ASIAA/CfA) play an important role in providing criti-cal baselines for calibrating data and maximizing spatial resolution. JCMT and SMA are key to accurately captur-ing the structure and time variation that affect an inter-pretation of the BH shadow. EA regional efforts (EAVN high frequency observations at 230 GHz: SPART in Nobeyama, Japan and SRAO in Seoul, Korea) have also begun. Further rapid progress in BH astronomy is surely promising in the coming decade.

Acknowledgements: This research was conducted by the EHT collaboration, which is an international collabora-tion for imaging the black hole shadow.KA is supported by the Ministry of Science and Technol-ogy of Taiwan grant MOST108-2112-M-001-051.

References

[1] K. schwarzchild, sitzungsberichte der Königlich Preußischen Akademie der Wissenschaften (Berlin: Deutsche Akademie der Wissenschaften zu Berlin) (1916).

[2] D. Hilbert, nachrichten von der Königlichen Gesellschaft der Wissenschaften zu Göttingen—Mathematisch-physikalische Klasse (Berlin: Weidmannsche Buchhandlung) (1917).

[3] r. P. Kerr, Phys. rev. let. 11, 237.

[4] J. M. Bardeen, in Black Holes, ed. C. DeWitt & B. s. DeWitt (new york: Gordon and Breach), 215.

[5] C.-K. Chan, D. Psaltis, f. Özel, ApJ, 77, 13 (2013).[6] K. y. lo et al. nature 362, 38 (1993).[7] z. Q. shen et al. nature, 438 62(2008).[8] s. s. Doeleman et al. nature, 455, 78 (2008).[9] s.s. Doeleman et al. science, 338, 355 (2012).[10] M. inoue, J.C. Algaba, K. Asada et al. 2014, rasc. 49, 564.[11] M.-t. Chen, P. raffin, P.t.P Ho, et al. sPie 2018, 10700e, oHC.[12] eHt Collaboration, ApJl, 875, l1 (2019a).[13] eHt Collaboration, ApJl, 875, l2 (2019b).[14] eHt Collaboration, ApJl, 875, l3 (2019c).[15] eHt Collaboration, ApJl, 875, l4 (2019d).[16] eHt Collaboration, ApJl, 875, l5 (2019e).[17] eHt Collaboration, ApJl, 875, l6 (2019f).[18] D. J. Kaup, Phys. rev. 172. 1331 (1968).[19] P. o. Mazur and e. Mottola, PnAs, 101, 9545 (2004).[20] H. olivares, z. younsi, C. M. fromm et al., Phys. rev. D submitted

(arXiv:1809.08682).[21] K. Gebhardt et al. ApJ, 729, 119 (2011).[22] H. D. Curtis, Plico, 13, 9 (1918).[23] K. Hada, A. Doi, M. Kino et al., nature, 477, 185 (2011).[24] K. Asada and M. nakamura, ApJl, 745, l28 (2012).[25] M. nakamura and K. Asada, ApJ, 775, 118 (2013).[26] K. Hada, M. Kino, A. Doi et al., ApJ, 775, 70 (2013).[27] K. Hada et al. ApJ, 817, 131 (2016).[28] J.-y. Kim, t. P. Krichbaum, r.-s. lu et al. A&A, 616, A188 (2018a).[29] M. Kino, f. takahara, K. Hada et al., ApJ, 803, 30 (2015).[30] K. Asada, M. nakamura, H.-y. Pu, ApJ, 883, 56 (2016).[31] t. ogihara, K. takahashi, K. toma, ApJ, 877, 19 (2019).[32] K. Hada, J. Park, M. Kino, PAsJ, 69, 71 (2017).[33] J. Park, K. Hada, M. Kino et al., ApJ, 887, 147 (2019).[34] M. nakamura, K. Asada, K. Hada et al., ApJ, 868, 146 (2018).[35] K. takahashi, K. toma, M. Kino et al., ApJ, 868, 82 (2018).[36] J.-y. Kim, s.-s. lee, J. A. Hodgson et al., A&A, 610, l5 (2018b).[37] J. Park, K. Hada, M. Kino et al., ApJ, 871, 257 (2019).[38] t. Kawashima, M. Kino, K. Akiyama, ApJ, 878, 27 (2019).

Masanori Nakamura is an associate professor at the national institute of technology, Hachinohe College in Japan (since April 2020) and a visiting scholar at Academia Sinica, institute of Astronomy & Astrophysics (ASiAA) in taiwan. After receiving a D.Sci. from the tokyo university of Science, he worked at nASA Jet Propulsion laboratory/California institute of technology, los Alamos national laboratory and the Johns Hopkins university/Space telescope Science institute before joining ASiAA in 2010. His research fields are theoretical and computational astrophysical fluid dynamics and high energy astrophysics. He is a member of the event Horizon telescope Project.

Keiichi asada is an associate research fellow at the Academia Sinica, institute of Astronomy & Astrophysics (ASiAA) in taiwan. After receiving a D. Sci. from the Graduate university for Advanced Studies, he worked as a postdoc at the national Astronomical observatory in Japan, institute of Space and Astronautical Science (iSAS), the Japan Aerospace exploration Agency (JAXA) and Academia Sinica, institute of Astronomy and Astrophysics. After that, he was appointed as an assistant research fellow, then associate research fellow at the same institute. He is a project scientist of the Greenland telescope Project and a member of the event Horizon telescope (eHt) Project. He also serves as a Science Council member for the eHt.

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Bulletinactivities aNd research NeWs

Scientists have developed a new type of deformable mir-ror that could increase the sensitivity of ground-based gravitational wave detectors.

“In addition to improving today’s gravitational wave de-tectors, these new mirrors will also be useful for increas-ing sensitivity in next generation detectors and allow de-tection of new sources of gravitational waves,” says Huy Tuong Cao.

Cao is a research team leader at the University of Ad-elaide node of the Australian Centre of Excellence for Gravitational Waves Discovery (OzGrav) and a member of the University’s School of Physical Sciences.

The illustration shows the cross-section of a thermal bimorph mirror and its constituents. Controlling the temperature of the mirror changes the curvature of the reflected wavefront. Overlaid on the cross-section is the simulated radial stress, showing a concentration of stress at the boundary of the two layers, where the adhesive holds the structure together. Detectors such as the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) are set to benefit from the new technology.

Advanced LIGO measures faint ripples in space time called gravitational waves, which are caused by distant events such as collisions between black holes or neutron stars.

Deformable mirrors, which are used to shape and control laser light, have a surface made of tiny mirrors that can each be moved, or actuated, to change the overall shape of the mirror.

Published in Applied Optics [1], Cao and colleagues have, for the first time, made a deformable mirror based on the bimetallic effect in which a temperature change is used to achieve mechanical displacement. “Our new mirror provides a large actuation range with great preci-sion,” says Cao. “The simplicity of the design means it can turn commercially available optics into a deformable mirror without any complicated or expensive equipment. This makes it useful for any system where precise control of beam shape is crucial.”

The new technology was conceived by Cao and Aidan Brooks of LIGO as part of a visitor program between the

Higher Precision Mirrors Set to Benefit ‘next Gen’ Gravitational Wave Detectors

Cross-section of a thermal bimorph mirror and its constituents. image credit: Huy tuong Cao, university of Adelaide.

[reproduced from the university of Adelaide and optical society of America newsrooms]

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June 2020 vol. 30 no. 3 activities aNd research NeWs

University of Adelaide and LIGO Laboratory, funded by the Australian Research Council and National Science Foundation.

Building a better mirrorGround-based gravitational wave detectors use laser light traveling back and forth down an interferometer’s two arms to monitor the distance between mirrors at each arm’s end. Gravitational waves cause a slight but detect-able variation in the distance between the mirrors. De-tecting this tiny change requires extremely precise laser beam steering and shaping, which is accomplished with a deformable mirror.

“We are reaching a point where the precision needed to improve the sensitivity of gravitational wave detectors is beyond what can be accomplished with the fabrication techniques used to make deformable mirrors,” says Cao. Most deformable mirrors use thin mirrors to induce large amount of actuation, but these thin mirrors can produce undesirable scattering because they are hard to polish. The researchers designed a new type of deform-able mirror using the bimetallic effect by attaching a piece of metal to a glass mirror. When the two are heated together the metal expands more than the glass, causing the mirror to bend.

The new design not only creates a large amount of pre-cise actuation but is also compact and requires minimum modifications to existing systems. Both the fused silica mirrors and aluminium plates used to create the deform-able mirror are commercially available. To attach the two layers, the researchers carefully selected a bonding adhe-sive that would maximise actuation.

“Importantly, the new design has fewer optical surfaces for the laser beam to travel through,” Cao says. “This reduces light loss caused by scattering or absorption of coatings.”

Precision characterisationCreating a highly precise mirror requires precision char-acterisation techniques. The researchers developed and built a highly sensitive Hartmann wave front sensor to measure how the mirror’s deformations changed the shape of laser light.

“This sensor was crucial to our experiment and is also used in gravitational detectors to measure minute chang-es in the core optics of the interferometer,” Cao says.

“We used it to characterise the performance of our mir-rors and found that the mirrors were highly stable and have a very linear response to changes in temperature.” The tests also showed that the adhesive is the main limit-ing factor for the mirrors’ actuation range. The research-ers are currently working to overcome the limitation caused by the adhesive and will perform more tests to verify compatibility before incorporating the mirrors into Advanced LIGO.

References

[1] “High dynamic range thermally actuated bimorph mirror for gravitational wave detectors” Huy tuong Cao, Aidan Brooks, sebastian W. s. ng, David ottaway, Antonio Perreca, Jonathan W. richardson, Aria Chaderjian, and Peter J. veitch, Applied optics, 59(9), 2784-2790 (2020); https://doi.org/10.1364/Ao.376764

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An international team of researchers has developed a technology that manipulates quantum states of light at noise levels that are quieter than the sound of silence. The technology is an important development towards quantum computers, which could solve problems that are impossible for to-day’s computers. Quantum computers promise to drastically increase processing speeds compared with current state-of-the-art technology.

Scientists at The Australian National University (ANU), in collaboration with colleagues from Nan-yang Technological University, National University of Singapore and Shanxi University, tested the technology in a series of scientific experiments.

Quantum mechanics predicts the presence of “vacuum noise” – a surprising phenomenon where silence does not equate to the complete absence of noise. ANU Professor Ping Koy Lam is one of the lead senior researchers who developed the technology.

Reducing noise of Quantum light Below the Sound of Silence

Dr Sophie Zhao. image credit: Centre for Quantum Computation and Communication technology, Anu.

[Reproduced from www.anu.edu.au/news]

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“Our device enables us to encode and manipulate information within a quantum state to a resolu-tion that is typically drowned out by vacuum noise,” he said. “Even in an empty, dark room, there is still noise in the form of energy that permeates all space. While imperceptible at everyday scales, the fluctuation of this energy can cause problems, by distorting the signal or information encoded in situations where extreme precision is required,” he said.

One way to reduce the effect of this vacuum noise is to use a light source known as “squeezed light”. Squeezed light is quieter than emptiness. “Our team has been working on squeezed light for more than two decades. We have used this technique to increase sensitivity of optical measure-ments, such as in kilometre-long optical interferometers for detecting ripples in space and time known as gravitational waves,” Professor Lam said. “Our focus in recent years has been to use this technique for information processing in quantum computing and encryption.”

Dr Jayne Thompson from the National University of Singapore worked on the theory behind the technology. “This level of fine control has many technological benefits. From certain viewpoints, such states also appear to have temperatures below that of absolute zero – and, as such, they can also act like a power resource for information processing at the quantum scale,” Dr Thompson said.

Lead author Dr Sophie Zhao from ANU said the experiment does not guarantee success every time. “However, when it works, it can efficiently squeeze many quantum states to a level that was once thought to be unachievable,” she said.

Dr Zhao presented the team’s initial results in a recent global “Rising Star of Light” competition organised by Springer Nature and was awarded the first prize.

Dr Syed Assad is the team leader at ANU. “Conventional approaches for an in-line squeezing gate always require a lot of non-classical resources. The heavy reliance on resources ultimately limits their performance. However, with our probabilistic protocol, this fundamental constraint is circum-vented,” Dr Assad said. “Our experiment may be a useful logic gate for quantum computation.”

The results of the research are published in Nature Photonics [1].

References

[1] “A high-fidelity heralded quantum squeezing gate”, zhao, Jie and liu, Kui and Jeng, Hao and Gu, Mile and thompson, Jayne and lam, Ping Koy and Assad, syed M, nature Photonics (2020), https://doi.org/10.1038/s41566-020-0592-2.

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Following the Big Bang, the Universe passes through a series of epochs, each one longer than the last. For physicists to be sure that they understand the Universe, the predicted events in each of these eras must combine to match what astronomers see when they look into the sky.

University of Auckland researchers have now taken a major step forward in understanding one of these epochs in the evolution of the Universe, the mysterious “primordial dark age”, when the Universe is entirely devoid of both light and all presently known subatomic particles.

Scientists think the primordial dark age lasted a trillionth of a second or even less, but that the Universe grew up 100 trillion times larger during this time. As the primordial dark age begins, the Universe is filled with a mirror-smooth, cold, ultra-dense quantum condensate, an exotic state of matter. This condensate can survive for much of this time, but must eventually fragment into par-ticles and radiation due to the force of gravity.

In a paper published in Physical Review Letters [1], University of Auckland researchers PhD student

Physicists Shine light into Primordial universe

Primordial dark age research. image credit: university of Auckland.

[Reproduced from www.auckland.ac.nz/en/news]

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Nathan Musoke, Research Fellow Shaun Hotchkiss and Professor Richard Easther have shown that interactions between this condensate and its own gravitational field are captured by the so-called Schrodinger-Poisson equation. This equation describes the gravitational interactions of quantum matter.

Using this insight, the researchers performed the first numerical simulations of the gravitational collapse of the condensate, showing that the peak density would quickly grow to be hundreds of times larger than the average density once gravitationally-driven collapse begins.

This marks a key step forward in our un-derstanding of the very early Universe. The work will allow cosmologists to bet-ter predict the properties of the “ripples” in the early Universe that eventually grow into galaxies and improve our ability to test theories of the Big Bang. In particular, it offers insight into the hypothetical inflationary phase which would precede the primordial dark age and generate the quantum condensate, a key part of most theories of the evolving Universe for close to 40 years.

The research could also offer insight into the production of dark matter and the origin of the mis-match between matter and anti-matter in the early universe which ensures that our present-day cosmos is built from regular matter alone.

“This is an exciting result, and provides a pathway to understanding the predictions according to our theories about the first moments after the Big Bang, and to testing new ideas in ultra-high en-ergy particle physics,” Professor Easther says.

References

[1] “lighting the Dark: evolution of the Postinflationary universe”, nathan Musoke, Shaun Hotchkiss, and Richard easther, Phys. Rev. lett., 14, 061301, https://doi.org/10.1103/PhysRevlett.124.061301

Professor Richard easther. image credit: university of Auckland.

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BulletiniNstitutes iN asia pacific

Northeastern University Northeastern University, China (NEU) was founded over 90 years ago, on April 26, 1923. The university’s motto is “to strive constantly for improvement and to behave in conformity with the knowledge”. NEU is affiliated with a number of high-level scientific research achievements, such as the creation of China’s first analog computer, the creation of the first domestically produced computed tomography (CT) scanner, the fabrication of the first piece of super steel, the development of new technology for smelting vanadium titanium magnetite, the develop-ment of the controlled rolling and controlled cooling technique, and the development of mixed intelligent optimization control technology. By establishing the first university science park in China, NEU has created a series of high-tech enterprises, such as Neusoft Corpo-ration and Neunn Technology Inc., and has developed

unique strengths in the areas of technological innova-tion, technological transfer, and industry-university co-operation.

NEU is situated in Shenyang, the central city of north-eastern China, and NEU also has a campus in Qin-huangdao City, Hebei Province. The university occupies a total area of 2,550,000 square meters, of which the architectural area amounts to 1,680,000 square meters. It has 4,538 faculty members, among whom 2,711 are full-time instructors. The university has four innovation groups of the National Natural Science Foundation and three innovation teams of the Ministry of Education. It has more than 100 research institutes, including three National Key Laboratories, four national engineering (technology) research centers and three national engi-neering laboratories. In addition, it has two national

the Department of Physics at northeastern university, China

Wei-JiAnG GonG, Qi WAnG, AnD yonG Hu DePArtMent of PHysiCs, nortHeAstern university

fig. 1: northeastern university in China.

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collaborative innovation centers and three collaborative innovation centers in Liaoning province.

DEPARTMENT OF PHYSICS

As early as 1958, NEU had instruction in metal physics, semiconductor materials, semiconductor devices and other physics topics. In 1977, the Department of Physics was established, and the department began admissions of undergraduate students in physics. In July 1986, the de-partment began to obtain the right to confer a master’s degree in condensed matter physics. In June 2003, NEU established a master’s degree program in theoretical physics, and in January 2006, an authorized master’s degree program of physics was approved. In 2011, the department was granted the right to confer a doctoral degree in physics. In 2019, it was approved to build a physics postdoctoral mobile station.

The Department of Physics has 47 faculty members, 42 of whom are full-time instructors. Among the teaching staff, there is one awardee of the National Ten Thousand Talents Plan; one winner of the National Excellent Youth Fund; one winner of the special government allowance of the State Council; one “Excellent Talent” awardee of the Ministry of Education in the New Century; one “Hundred Talents Level” awardee and three “Thousand Talents Level” awardees of Liaoning Province; one “Famous Teacher” of Northeastern University; and one “Young Thousand Plan (Xinjiang project)” awardee.

The department attaches great importance to student education, especially undergraduate education, and considers teaching to be its central task. Guided by the motto of the university, the Department of Physics strives to provide a well-rounded education for our students; in particular, we endeavor to have our students master not only the fundamental theories, knowledge and skills of physics (and advanced research and compound applica-tion abilities for those students with the requisite capa-bilities), but we also hope to instill a civic sense and an awareness of social responsibilities to our students. In ad-dition, the Department of Physics aims to foster the ex-perimentalists, theorists and applied engineers who will to contribute to the critical science projects of the present and the future. In parallel with the developments of sci-ence and technology and combined with the directions of our discipline, the physics department at NEU works to provide an education that merges applied physics with outstanding academics and business acumen.

NEU has doctoral level disciplines in physics, and stable research teams in fields including theoretical physics, astroparticle physics and cosmology, condensed mat-ter physics, optics and radio physics. Condensed matter physics is one of Liaoning province’s key disciplines. Ac-cording to the direction of the discipline and the devel-opments of physics, key teachers have been involved in different academic terms, including astroparticle phys-ics and cosmology, nanophysics and nanodevices, con-densed matter physics, optics and radio physics.

RESEARCH GROUPS

Astroparticle Physics and Cosmology GroupThe members of this group focus on research in cosmol-ogy, and in particular, in exploring the fundamental na-ture of dark energy with cosmological observations, which has always been one of the major subjects of research in cosmology. In recent years, they have been investigat-ing issues such as whether or not dark energy directly interacts with dark matter, how such a subtle interaction influences the evolution of the Universe and how to de-tect this interaction, how the properties of dark energy influences the cosmological measurements of the total neutrino mass, and how to precisely measure the cosmo-logical parameters relevant to dark energy. Many impor-tant research achievements in the studies for these issues have been made, including, e.g., the establishment of a theoretical framework for calculating the cosmological perturbations for the interacting dark energy scenario, the systematic exploration of issues associated with the cosmological weighing of neutrinos and the cosmological search for sterile neutrinos, and a systematic study on the precise measurements of cosmological parameters.

fig. 2: Department of Physics at northeastern university.

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BulletiniNstitutes iN asia pacific

Professor Xin Zhang, the leader of this group, has pub-lished more than 120 research papers in peer-reviewed journals, which have been cited more than 5400 times so far by international experts in the same community throughout the world; his H index is now 39. He is also the recipient of numerous awards and honors, including the Natural Science Award of Liaoning Province, and the Special Government Allowance from the State Council. His group currently still focuses on the essential ques-tions of cosmology, including, e.g., systematic studies on the cosmological measurement of the coupling param-eter between dark energy and dark matter, issues con-cerning gravitational-wave multi-messenger astronomy and dark energy, and cosmology studies based on SKA (the Square Kilometre Array project).

Condensed Matter Physics GroupOne research topic of this group is condensed matter theory. The members pay attention to quantum trans-port phenomena dominated by quantum interference, such as the AB (Aharonov-Bohm) effect, the Fano effect, decoupled state, antiresonance, and the application pos-sibilities of these phenomena in electronic manipulation or thermoelectric conversion. In recent years, the quan-tum transport behavior driven by the Majorana bound states has become one important research area. They clarified the competition mechanism of Andreev reflec-tion and electron tunneling by considering the coupling of the Majorana bound state with the quantum dot struc-ture or the change of the topological superconductor. In addition, they elucidated the phenomenon of superfluid phase transitions by embedding quantum dots into the Josephson junction.

The group’s other main area of research focuses on the fundamental concept of quantum physics. This group studies the underlying physics of non-Hermitian quan-tum systems. They clarified the influence of Parity-Time-symmetric complex potentials on the transport proper-ties and topological phase transitions of low-dimensional systems. The quantum interference behavior regulated by PT-symmetric complex potential energy has been discussed, and the influence of the existence manners of complex potentials on topological states in different sys-tems has been analyzed. It has been found that changing the ways of introducing complex potentials or consider-ing the coupling mechanisms can lead to the emergence of new topological states and can enrich the phase transi-tion phenomenon of the original topological mediocrity region.

Magnetic Materials and Nanophysics GroupThe magnetic materials group studies the preparation and physical properties of various magnetic materials. This includes, mainly, high anisotropic magnetic films and the preparation and processing technologies and physical properties of soft and hard magnetic permanent magnet alloys and amorphous nanocrystals. This area of research provides basic data for exploring and under-standing the principle of magnetic memory, understand-ing the magnetic nature of materials, developing ultra-high recording density disks, and obtaining magnetic materials with excellent soft or hard magnetic properties.

The nanophysics group pays attention to the self-assem-bly behavior of nanoparticles in the fields of physics, chemistry and materials science. Based on the theoretical numerical method, the group carried out a qualitative analysis of the self-assembly behavior of nanosystems. The results fully correspond to the experimental process and clearly explain the underlying mechanism. Further-more, are performing studies on field-driven oscillation and rotation of multifrequency clusters in nanodisks and the magnetothermal effect of nanosystems.

Optics GroupThe group focuses on laser technologies and applica-tions. The group carries out systematic research on the cutting-edge issues in the cross-fields of laser technology and materials research. The main research areas include laser surface modification of medical metal materials; la-ser texturing technology to improve the comprehensive performance of metal surfaces; and, in laser welding, joint dynamic tensile and structural properties of auto-mobile steel plates. The common thread that binds these research areas together is the desire to use laser technol-ogy to resolve problems in modern engineering applica-tions. We have established cooperative relationships with the Institute of Laser Application Technology, the School of Materials, the Metal Institute of NEU and many do-mestic and foreign universities; therefore, we have access to excellent research equipment and research conditions.

Radio PhysicsThe radio physics group focuses on the applications of superconducting single-photons, optical measurements, and stereoscopic displays. They have successfully devel-oped an excimer laser dioptric correction instrument, a rapid colorimetric optical temperature measurement system and other application equipment. For training tasks such as teaching and applied research, they par-

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ticipated in a 0.23T permanent magnetic resonance imaging system developed by the Neusoft company. In high-temperature superconducting magnetic resonance imaging research, they have several domestic and in-ternational advanced technologies and have the ability to independently develop high-temperature supercon-

ducting magnetic resonance equipment. In addition, they study plasma physics, including the characteristics of electromagnetic fields in low-pressure discharge fre-quency sources, and the application of radio frequency (RF) discharge in advanced manufacturing fields such as material processing, synthesis and preparation processes.

Yong hu received his undergraduate degree from northeastern university, China (neu) in 2005. He obtained his doctoral degree in materials physics and chemistry in April, 2011. in 2011, he joined the faculty of neu, where he is now an associate professor of physics. in 2016-2017, he worked in the physics department of the university of California, Davis as a visiting professor. His scientific interests mainly focus on nanomagnetism and spintronics. He has worked on the development of a modified Monte Carlo method by which the spin energy can be exactly calculated to consider the influence of energy barriers on the spin reversal. this modified method has been successfully used in simulations to understand the spin glass dynamics and the exchange bias effect for the first time.

Qi Wang is an associate professor and also the vice dean of the College of Science at northeastern university, China (neu). Since 2007, he has taught physics experiment courses for more than 10 years, including "college physics experiments" and "modern physics experiments”. While teaching, he encourages students to explore new physics phenomena. in 2018, he won the national virtual Simulation experiment teaching Project of China. His research interests are mainly in the field of nanomaterials, and he performs studies regarding the realization and application of nanosensors.

Wei-Jiang Gong is a professor and is also the dean of the Department of Physics at northeastern university, China (neu). in 2008, he works in the Department of Physics after obtaining his doctoral degree from Jilin university. From 2015 to 2016, he worked as a visiting scholar at the university of texas at Dallas. He carries out research on the electronic structure and quantum transport properties of low dimensional semiconductor systems and the phase transition of strongly correlated systems. He is also interested in the physical properties of topological quantum systems.

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BulletinphYsics fOcus

ABSTRACT

Brain function stems from emergent properties of inter-connected neuron networks, which requires in vivo volu-metric imaging with high spatial/temporal resolution. Here, we introduce our recent technical developments for Drosophila brain imaging.

INTRODUCTION

The brain is one of the most important organs in our body, but it is functionally the least understood one. It is composed of millions of neurons, whose interconnection, i.e. connectome, determines its function. Although the interaction of neurons in vitro has been well studied in the past century, no existing tool can capture whole-brain emergent properties at single neuron or even synapse resolution. To understand functional connectome, an imaging system that can cover a whole brain in vivo with spatial resolution of micrometers (neuron) to nanometers (synapse) as well as temporal resolution in sub-seconds (calcium) to milliseconds (action potential) is highly desir-able. In this focus article, we introduce our recent efforts to improve optical microscopy in terms of speed, depth, and spatial resolution, toward the goal of understanding the brain of Drosophila, which offers a small brain with so-phisticated functions and genetic control capabilities.

High speed volumetric imaging with millisecond temporal resolutionNeurons are distributed in three dimensions in the brain, and their firing dynamic is in the millisecond scale. To observe their functional connections in vivo, we developed a high-speed volumetric imaging system by combining a conventional two-photon microscope with a tunable acoustic gradient-index (TAG) lens [1]. The TAG lens is based on acoustic resonance, which in turn creates

periodic gradient index modulation up to MHz speed, thus enabling high-speed axial scanning when combined with a microscope objective [2, 3]. Fig. 1 (a) shows that we can convert a plane scan in the xy plane into volumetric imaging with sub-second imaging speed. Fig. 1(b) is a line scan in the xy plane converted into a “ribbon” scan in xyz volume, with temporal resolution approaching the millisecond scale.

fig. 1: Fast three-dimensional imaging in Drosophila brains. (a) left: conventional two-photon imaging, taking 0.5 s for one 2D optical section; right: our new system observes 3D brain volume with the same acquisition time. (b) Ribbon imaging with 3.8 millisecond temporal resolution, see [1] for more details.

Volumetric all-optical physiology To further unravel the in vivo functional connections, it is necessary to incorporate high-precision stimula-tion capabilities into a volumetric imaging system, i.e., a volumetric all-optical physiology that uses photons to manipulate and report neuron activities [4]. Fig. 2 pres-ents our results in Drosophila’s visual pathway, where we were able to stimulate upstream neurons and record the

novel optical Microscopies to unravel Brain FunctionCHiAo HuAnG, Kuo-Jen Hsu, HAn-yuAn lin, sHi-Wei CHu

DePArtMent of PHysiCs, nAtionAl tAiWAn university, tAiWAn

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June 2020 vol. 30 no. 3 phYsics fOcus

downstream neuronal responses in 3D, thus resolving the neural coding scheme of vision.

Whole-brain super-resolution imagingIn the above results, optical microscopy provides sub-μm spatial resolution, limited by diffraction. The neural fi-bers and synapses inside a Drosophila brain can be much smaller, so super-resolution techniques that breaks the diffraction barrier are required. However, conventional super-resolution modalities are mostly not applicable at the tissue level due to their susceptibility for aberration and scattering. We recently developed COOL (Confocal lOcalization deep-imaging with Optical cLearing) [5], which combines advanced techniques using blinking flu-orescence proteins, confocal microscopy, optical clearing, and localization microscopy to achieve 20-nm spatial res-olution across a whole brain of Drosophila. Fig. 3(a) shows resolving densely entangled dendritic fibers in an intact Drosophila brain with unprecedented depth/resolution performance in 3D (inset: in the same structure mapped

by confocal microscopy, no fibrils can be observed).

Whole-brain imaging in a living Drosophila brainIt is well known that two-photon microscopy provides ~1-mm penetration depth in a mouse brain, but when imaging the Drosophila brain, it is mysterious that the imaging depth cannot exceed 0.1 mm! We recently un-raveled the underlying mechanism as an strong optical aberration from the trachea, which delivers oxygen in insects. As shown in Fig. 3(b), we used long-wavelength three-photon microscopy to reduce the aberration and therefore achieved whole-brain observation with single neuron resolution in a living Drosophila brain [6].

CONCLUSION

For developing a bio-imaging system, the most impor-tant factors are contrast, resolution, speed, and depth. By combining interesting physics concepts (such as acoustic resonance for volumetric imaging, high-precision focus-

fig. 2: in-vivo volumetric all-optical physiology in Drosophila’s visual pathway. (a) Structures of upstream (Aotu) and downstream (Bu) neurons, where the neural signal coding is hidden in >70 tightly assembled 2-μm Bu microglomeruli. (b) volumetric response of downstream Bu microglomeruli, some of whose temporal responses are shown in the right-hand side [4].

fig. 3: Deep-tissue imaging in Drosophila. (a) intertwined neural fibrils of two neurons imaged by Cool and a confocal microscope (inset), respectively. only Cool can distinguish fibrils with nanometer resolution [5]. (b) Comparison of single-, two-, three-photon in-vivo imaging at the bottom of a Drosophila brain (170-190 µm), with excitation wavelengths at 488-, 920-, and 1300-nm, respectively. only three-photon imaging provides whole-brain observation [6].

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ing for precise stimulation, localization calculations for super-resolution microscopy, and three-photon excitation for deep-tissue imaging) with innovative biological con-cepts, (including calcium/voltage sensitive fluorescent pro-tein labeling, optogenetic stimulation, and optical clear-ing), we push the limits of optical microscopy in all four areas. These novel techniques will benefit not only brain science research, but also studies in other bio-tissues.

Acknowledgements: The above work was supported by the Outstanding Young Scholarship Project of Ministry of Science and Technology (MOST), Taiwan, under grant MOST-105-2628-M-002-010-MY4, and MOST-108-2321-

B-002-058-MY2. The work was also supported by the Higher Education Sprout Project funded by the Ministry of Science and Technology, and the Ministry of Educa-tion of Taiwan.

References

[1] K.-J. Hsu, et al., opt. lett. 44, 3190 (2019). editor's pick[2] l. Kong, et al., nat. Methods. 12, 759 (2015).[3] K.-J. Hsu, et al., opt. express. 25, 16783 (2017).[4] C. Huang, et al., iscience. 22, 133 (2019).[5] H.-y. lin, et al., iscience. 14, 164 (2019).[6] K.-J. Hsu., et al., Biomed. opt. exp. 10, 1627 (2019).

shi-Wei chu is a professor in the Department of Physics, national taiwan university (ntu). He serves as the vice director for both the Center for teaching and learning Development, and the Digital learning Center, at ntu. He is also the associate director on innovative teaching at ntu’s D.school. His research focuses on improving optical microscopy for interdisciplinary research applications. He has received the outstanding Young Scholar Research Project Award from the Ministry of Science and technology, taiwan; the Young Scholars’ Creativity Award from the Foundation for the Advancement of outstanding Scholarship; the excellent Mentor Award of ntu; and the outstanding teaching Award of ntu. As a faculty member, he is particularly proud of his supervised students, who have received more than 50 international and domestic research awards.

Kuo-Jen hsu is a researcher at ASMl in the netherlands, and he is working on the development of lithography machines for the semiconductor industry. He received his PhD from the Department of Physics, national taiwan university (ntu) in 2018. His dissertation addressed the development of optical imaging techniques for brain functional studies.

han-Yuan lin received his master’s degree from Department of Physics, national taiwan university (ntu) in 2019. His master’s thesis concentrated on deep-tissue super-resolution microscopy in an intact Drosophila brain.

chiao huang is a PhD student at the university of Arizona. She received her master’s degree from the institute of Applied Physics, national taiwan university (ntu) in 2018. Her master’s thesis focused on building an all-optical physiology platform for observing neuron connection in the living Drosophila brain.

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The no-hair theorem states that black holes are char-acterized by only their mass, charge and angular mo-mentum. As these are the same quantum numbers that are associated with elementary particles, a natural ques-tion arises: to which extent does a black hole mimic an elementary particle? A naive answer would be that at sufficiently large distances, the dynamics of black holes (e.g., the orbital motion of in-spiral black holes) should be well approximated by that of an elementary par-ticle. For example, in the limit where the impact factor b ≪ p, m, where (m, p) are the mass and spatial momenta of the black holes, we expect that the dynamics of binary black holes to be well described by the gravitational in-teractions of elementary particles. On the face of it, this proposal does not seem particularly enlightening, for at large distances, doesn’t everything look like a point par-ticle?

Let us take a closer look at how point particles behave in a gravitational background. A covariant description of point particles in a non-trivial background is the world-line formalism, where the degrees of freedom are the worldline fields xµ(τ) and spin-operators Sµν that encodes the spin degrees of freedom. A general worldline action in non-trivial gravitational background takes the form [1, 2, 3]:

(1)

where, and Ωµν is the angular velocity. The first

two terms of the Lagrangian are universal irrespective of the details of the point-like particle, while the terms

in LSI correspond to spin-induced multipole terms that are beyond minimal coupling and depend on the inner structure of the particle.

They can be parameterized as:

(2)

where E and B are the electric and magnetic compo-nents of the Riemann tensor, and Sµ are the spin vectors extracted from the spin operators via . The coefficients C2n, C2n+1 are Wilson coefficients that describe how a spinning particle interacts with the grav-itational background, which requires detailed knowl-edge of the spinning particle at hand. In other words, not only are spinning particles in general distinct, their differences can be characterized though an infinite number of unknown parameters! For a spinning black hole, the complete Wilson coefficients were only derived recently [4].

From the discussion above we see that by considering a spinning black hole, the spin multipoles introduce an infinite number of “probes” that allow us to characterize the nature of black holes even at long distances. Now, let’s try to connect black holes with elementary particles.

the tango of Rotating Black Holes and Spinning Particles

yu-tin HuAnG nAtionAl tAiWAn university

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While the black holes and elementary particles share the same set of quantum numbers, for black holes the spins are really classical, as compared to the quantized spins for elementary particles. Thus in making the connection, what we really have in mind is an analytic extrapola-tion of spin S → ∞ with ħ → 0 and Sħ held fixed. On the face of it, this appears to be a daunting task, since we need to characterize the couplings of spinning particles with gravitational fields in such a way that an extrapola-tion can be made. At an even deeper level, the proposal doesn’t make sense physically since we know that el-ementary particles, in the sense that they are devoid of internal structure, do not exist beyond spin-2.

The above difficulty was circumvented by considering on-shell matrix elements: instead of starting with fields, Lagrangians and then deriving physical observables, we can work with the observables directly, which often times are completely determined from the kinematics and symmetry alone. A few years ago, N. Arkani-Hamed, T. C. Huang and the author introduced a novel “spinor-helicity formalism” to parameterize general scattering matrix elements in four-dimensions [5]. This formalism utilizes kinematic variables that are free of any constraint and transform covariantly under the Little group, which is the symmetry group under which physical states form representations. The simplest three-point amplitude for an electromagnetically and gravitationally coupled spin-ning particle is given by:

(3)

where h = (1,2) and, for positive photons

and gravitons respectively. Importantly, the variable x defined kinematically as:

(4)

where we’ve used the bi-spinor representation of mo-menta (p)αα. = pμ(σμ)αα. and since q2 = 0, qαα. = λαλ

~α..

When Eq. (3) was first written down, for S = , 1 it was quickly matched with QED and electroweak photon couplings, as well as gravitationally minimally coupled spinors and vectors. An immediate question was what

kind of higher spin particles can match to Eq. (3) for general S? In other words, what kind of higher spin physical state couples to electromagnetic and gravita-tional fields minimally? It turns out, in the S → ∞ limit we have a definite answer, it is a rotating black hole! In-deed, in subsequent work [8, 7], by taking the classical spin limit of Eq. (3), it was shown that Wilson coefficients for the multipole moments of a Kerr black hole were cor-rectly reproduced. Note that the simplicity of black hole couplings in such a kinematic basis can also be viewed as the on-shell avatar of the no-hair theorem.

This simplicity has led to the streamlined computation of spin effects in the scattering angle [8], linear and angular impulse [9] of rotating black holes, giving the complete 1 PM (post-Minkowskian) conservative Hamiltonian of spinning binary black holes [10], as well as new insights into the origin of shift relations between rotating and Schwarzschild black hole solutions [11].

The identification of black hole dynamics with a mini-mally coupled spinning particle leads to several con-ceptual challenges. We know that at the end of the day, elementary particles cannot exceed spin-2. On the scat-tering amplitude side, this constraint shows up at the four-point amplitude, where there is no longer a well-defined notion, even kinematically, of minimal coupling. However, the very same amplitude enters into the de-termination of the spin-dependent terms in the 2 PM Hamiltonian, which should be uniquely determined from general relativity (GR). In other words, there appears to be a tension between the scattering of elementary par-ticles and the uniqueness of classical black holes. What is the principle that ultimately resolves this? Furthermore, while we have identified minimal coupling kinematically, by simply requiring that in some suitable basis, the cou-pling contains minimal momentum dependence. Is there a more physical property that can be utilized to charac-terize such a coupling? The fact that for spin – minimal coupling appears to be favored for entanglement maxi-mization, is a tantalizing possibility to explore [12].

References

[1] W. D. Goldberger and i. z. rothstein, “An effective field theory of gravity for extended objects,” Phys. rev. D 73, 104029 (2006) [hepth/0409156].

[2] r. A. Porto, “Post-newtonian corrections to the motion of spinning bodies in nrGr,” Phys. rev. D 73, 104031 (2006) [gr-qc/0511061].

[3] M. levi and J. steinhoff, “spinning gravitating objects in the effective field theory in the post-newtonian scheme,” JHeP 1509, 219 (2015) [arXiv:1501.04956 [gr-qc]].

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[4] J. vines, “scattering of two spinning black holes in post-Minkowskian gravity, to all orders in spin, and effective-one-body mappings,” Class. Quant. Grav. 35, no. 8, 084002 (2018) doi:10.1088/1361-6382/aaa3a8 [arXiv:1709.06016 [gr-qc]].

[5] n. Arkani-Hamed, t. C. Huang and y. t. Huang, “scattering Amplitudes for All Masses and spins,” arXiv:1709.04891 [hep-th].

[6] A. Guevara, A. ochirov and J. vines, “scattering of spinning Black Holes from exponentiated soft factors,” JHeP 1909, 056 (2019) doi:10.1007/JHeP09(2019)056 [arXiv:1812.06895 [hep-th]].

[7] M. z. Chung, y. t. Huang, J. W. Kim and s. lee, “the simplest massive s-matrix: from minimal coupling to Black Holes,” JHeP 1904, 156 (2019) doi:10.1007/JHeP04(2019)156 [arXiv:1812.08752 [hep-th]].

[8] A. Guevara, A. ochirov and J. vines, “scattering of spinning Black Holes from exponentiated soft factors,” JHeP 1909 (2019) 056 [arXiv:1812.06895 [hep-th]].

[9] A. Guevara, A. ochirov and J. vines, “Black-Hole scattering with General spin Directions from Minimal-Coupling Amplitudes,” Phys. rev. D 100 (2019) no.10, 104024 [arXiv:1906.10071 [hep-th]].

[10] M. z. Chung, y. t. Huang, J. W. Kim and s. lee, “Complete Hamiltonian for spinning binary systems at first post-Minkowskian order,” arXiv:2003.06600 [hep-th].

[11] n. Arkani-Hamed, y. t. Huang and D. o'Connell, \Kerr black holes as elementary particles,” JHeP 2001, 046 (2020) doi:10.1007/JHeP01(2020)046 [arXiv:1906.10100 [hep-th]].

[12] A. Cervera-lierta, J. i. latorre, J. rojo and l. rottoli, \Maximal entanglement in High energy Physics,” sciPost Phys. 3, no. 5, 036 (2017) doi:10.21468/sciPostPhys.3.5.036 [arXiv:1703.02989 [hep-th]].

Yu-tin huang received his PhD degree in 2009 from YitP at SunY Stony Brook university. He went on to post doctoral positions at uClA and university of Michigan, and became a member at institute of Advance Studies in Princeton in 2013. in 2014 he was appointed assistant professor at ntu. He is one of the leading experts in Scattering amplitudes, and received the 2018 nishiha Asia award as well as 2018 ta-You Wu memorial award and is the Golden-Jade fellow of CtP/ntu.

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Bulletinapctp sectiON

Viruses: The Overlords of Earth’s Living Organisms

The world has been gripped with novel coronavirus (COVID-19) panic since early 2020. We are still under the dark shadow of COVID-19 even though five months have passed since the virus emergence in China. What has made it possible for a virus to totally upend the world as we know it?

Existing theories about viruses have been overturned as scientific research has found that viruses have an even broader diversity than previously known. That has ac-cordingly made it more complicated to clearly define what is—and what is not—a virus. Basically, however, vi-ruses are nothing more than “selfish genes” surrounded by proteins. Viruses are not able to create their own energy, which means they are unable to reproduce by themselves. In fact, viruses have a close relationship with their hosts and totally depend on their host cells during their entire life cycle. There is nothing more extraordi-nary than to watch a virus attach itself to a living host, break through the host’s elaborate defense mechanisms to build a stronghold, and then use that stronghold to gradually dominate the host.

Viruses target all living organisms on Earth, including bacteria—the most basic living organism—fungi, yeasts, plants, and animals. Some scientists estimate that there are around 1.6 million unknown viruses in mammals and birds alone, a figure that demonstrates the incredible di-versity of viruses that live in our world. That being said, we have discovered, at most, only 1 percent of the viruses in existence. Viruses, in the popular imagination, are just pathogens that cause infections; however, not all viruses are harmful to their hosts. In fact, more than 99 percent

of the viruses that exist on Earth are innocuous to human beings. Moreover, viruses sometimes even “help” their hosts survive. Viruses have used hosts as their habitat for many thousands of years, which means that most viruses actually have a symbiotic relationship with them.

Why are viruses considered “selfish”? Viruses used as vac-cines to prevent infections or those used to treat specific diseases may even appear altruistic. In truth, this “altru-ism” is really no more than a way for viruses to build a stronghold within their hosts and keep themselves alive by “preserving” the genes that ensure they continue to exist. Essentially, the human race has been able to clev-erly use this strategy of viruses for its own benefit. To sur-vive, viruses eliminate some parts of the genes they do not need and, if required, they even use genetic recom-bination mechanisms to acquire the genes of our viruses. Viruses even manipulate genes to make them smaller so they can more efficiently copy themselves. Viruses com-pete among themselves within the immune systems of their hosts and those that win in this survival of the fittest become the dominant species and have the opportunity to dominate their hosts. Viruses evolve as this process re-peats itself—something familiar to us because human be-ings have competed against each other for their survival.

Novel Viruses: An Ongoing Threat

It is likely that viruses came into existence after living organisms appeared on Earth. As different species be-gan to appear through the process of evolution, viruses probably co-speciated with their hosts for long period. Viruses made their habitat in specific kinds of living or-ganisms and created their own “species barriers” that allowed them to build up their host ranges. But this does

How Do novel viruses threaten Humankind?KAnG-SeuK CHoi

AviAn DiSeASeS lAB, ColleGe oF veteRinARY MeDiCine, Seoul nAtionAl univeRSitY 1 GWAnAK-Ro, GWAnAK-Gu, Seoul 08826, RePuBliC oF KoReA

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June 2020 vol. 30 no. 3 apctp sectiON

not mean that viruses have simply stayed in one place to dominate their own hosts.

Viruses become a threat to new hosts when they jump across the species barrier. This is otherwise known as “spillover,” and viruses that do this are called “novel viruses.” Spillover is just one part of the evolution pro-cess for viruses. Because novel viruses have no symbiotic relationship with their new hosts, they multiply at fero-cious speeds to build strongholds in their new habitats. Meanwhile, the immunity systems of the new hosts work overtime to fight back against these novel viruses. While most viruses end up failing to cross the species barrier, they can cause significant harm to their new hosts if they succeed.

Measles, smallpox, AIDS, and other many viruses we know about have overcome the species barrier from orig-inal natural hosts (animals) as mentioned above. In re-cent years, we have witnessed that viruses in wild animals spill over to humans. We saw this in in 2002 with severe acute respiratory syndrome (SARS), 2009 with the novel H1N1 influenza, 2012 with Middle East respiratory syn-drome (MERS) (an outbreak of which occurred in 2015 in Korea) and, in 2019, COVID-19. This spillover does not just impact human beings. There are actually many more cases of viruses spilling over from one animal spe-cies to another, but these novel viruses have remained the interest of veterinary virologists and most have fallen beneath the radar of the general public. Many people have probably ignored news of such viruses because they do not impact their health or daily lives. One prominent example of this is bird flu (avian influenza). Wild migra-tory birds (ducks and geese) originally carry avian flu viruses without any clinical signs, but recently a strain of this influenza frequently has caused fatal infection in chickens and has resulted in substantial economic loss in poultry industry.

How do novel viruses come into existence? Novel viruses are viruses that have either emerged recently or have been discovered in their hosts. In short, while the virus may be new to that host, the virus had a preexisting host (a natural host) that it used as its habitat. COVID-19, for example, only started infecting humans recently, but used bats as its host in the past. What this means is that a virus that already existed in nature went through the spillover process to enter a new host—in this case, hu-man beings. In this sense, these novel viruses are really nothing “new.”

Novel viruses that have caught the attention of humans recently (SARS, H1N1 influenza, MERS, and COVID-19, among others) originally came from wild animals. The “push and pull” effect can be used to explain the environmental factors that helped create these novel viruses. Wild animals that carry a potential novel virus have been forcibly “pushed” out of their existing habitats and “pulled” into environments where they live closer to human beings. This process has increased the opportuni-ties for wild animals to impact the habitats of new hosts (in this case, humans) and, accordingly, has increased the likelihood that the viruses will infect human beings. COVID-19 is an example of this. Phylogenetic analysis has shown that COVID-19 virus may originated from the Chinese horseshoe bat, which strongly suggests that the bat species was the natural host of the novel coronavi-rus. This may mean that human beings first caught the coronavirus while hunting the bats in caves or during the butchering process. It is tantalizing to think that if hu-man beings had never pulled bats into the human world, COVID-19 may never have spilled over to humans.

When the first outbreak of COVID-19 occurred at a wet market in the Chinese city of Wuhan, the world focused its attention on Chinese wet markets. As a matter of fact, infectious disease experts have long focused their at-tention on Chinese wet markets as a “powder keg” that could ignite the “push and pull” effect required to create a novel virus. In contrast to Korean traditional markets, Chinese wet markets allow visitors to actually eat a range of wild animals. This eating culture has provided the en-vironmental conditions for various wild animals to gather together in one place. It is unclear where all of these wild animals were caught or what kinds of viruses they hosted. In the case of COVID-19, it may have been that there was a mixing of viruses of many different wild animals and this created the new virus. Then human beings became infected during the process of butchering or touching the butchered meat of these animals. Ultimately, what this would mean is that wild animals in these wet markets were an important part of creating the new virus.

What’s Next?

The threat of novel viruses has reared its head frequently in the 21st century and led to several unpredictable virus shocks. The pandemic spread of COVID-19 has led to an omnidirectional crisis that has halted much of the world’s economic activity—something we have never experi-enced before. Infectious diseases are notoriously unpre-

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dictable, which means it is close to impossible to know when the current crisis might end. Humankind, however, will eventually beat COVID-19 given that the full weight of its resources is being placed on developing a vaccine or at least a treatment for the disease. The 21st century, however, has witnessed periodical emergence and epi-demic episodes of novel viruses fatal to human beings. Hardly anyone believes that the end of the COVID-19 pandemic means the end of the threat from novel viruses as a rule of thumb. We will continue to live with the dan-ger of future outbreaks caused by novel viruses we have never experienced before.

Does that mean that we are helpless against novel virus shocks forever and ever? The COVID-19 crisis presents us with an opportunity to reflect on the limitations of our current public health response systems against novel vi-ruses. The international community will likely begin dis-cussing new strategies and ways to shift the paradigm of how the world can preemptively and internationally pre-vent the spread of infectious diseases. This may include establishing a system to manage the risk factors posed by the wildlife ecosystem; establishing a system to control risk factors of novel viruses (including, of course, chang-

ing some of our eating cultures); improving the sharing network of information about infectious diseases between countries; and ensuring countries are transparent in sharing information about infectious disease outbreaks.

The public health crisis caused by COVID-19 has forced the general public to make handwashing, wearing masks, and practicing social distancing an integral part of daily life. These practices have led to a significant decrease in not just COVID-19 infections but also cases of influ-enza, a disease that also affects our respiratory systems. The COVID-19 pandemic has created greater awareness among the public about how to deal with viruses. Indeed, the pandemic has drastically increased awareness and interest in the general public about infectious diseases caused by viruses more than any other time in the past. Better knowledge about viruses can help correct misun-derstandings of information related with them and shore up support for evidence-based ways to handle outbreaks. There have been a diverse range of books published recently about viruses. The efforts of these authors will help diversify scientific discussions and provide a greater opportunity for the public to broaden their understand-ing of science in general.

Kang-seuk choi was an oie (World organization for Animal Health) designated expert for infectious disease and a senior research specialist in the Animal and Plant Quarantine Agency in Korea and, now a professor at the Snu college of veterinary medicine and Research institute for veterinary Science. He received DvM and MS degrees in veterinary medicine from Seoul national university and PhD degree from Chungbuk national university.His research interests include epidemiology of veterinary viral diseases and development of diagnostics and vaccine against infectious diseases.

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September 2 (Off the western coast of Africa)

“Fine day and sea like glass. Quite a gloom was cast round the cabin as soon as we were up when three deaths were reported on board. The burials took place at 11am and there were 4 burials. The Colonel read the service and it was quite a touching scene. This afternoon two more burials took place and there is another death. The other ships are also busy with burials. One of the deaths is our Clarionette player in the band and a beautiful player he was too. The strange thing about this sickness is that the big strong men seem to get it the worst and are the ones that die. One of the deaths is namesake of myself I hear. Today was mess orderly again. Was also able to eat a bit today and feel much better.”1

The above is an excerpt from a journal kept by a soldier who left New Zealand by boat in 1918 to fight with the Allies in World War I. He lived with other soldiers on the ship, “packed in like sardines.” Right before the massive outbreak of influenza in the fall of 1918, his boat suffered from many deaths. The war itself ended on November 11, 1918, before the soldiers on the ship ever saw battle, yet over 90 percent of the 1,117 on board came down with influenza and 77 died. The journal writer caught the

disease but later recovered. The soldier’s journal provides an account of epidemiological significance on densely packed spaces and demonstrates that the fear of infec-tious disease was more acute than that of the war itself.

Influenza and the human race share a long history to-gether. Doctors in the Middle Ages believed that people were infected with diseases due to influence from the po-sition of the stars and the cold. The Italians coined a term for this “influenced illness”: influenza. The word was then brought to English in the 18th century during a Europe-an pandemic. The Middle Ages had the Black Death, but 20th century world faced the threat of influenza.

The first case of the 20th century’s influenza pandemic was reported at an American military camp in Kansas, USA, in the spring of 1918. Soldiers at the camp were dispatched to Europe to fight in World War I, and when they returned home, the influenza they brought back was even more virulent than before.

How was this frightening infectious disease discovered one hundred years ago?

We Are not Prepared SunGSil Moon

MiCRoBioloGiSt

1 https://wwwnc.cdc.gov/eid/article/18/11/ad-1811_article

(illustration by Min-jeong Kim)

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Richard Pfeiffer, a student of the “father of bacteriology,” Robert Koch, isolated bacteria he found in the lungs and mucous of patients who were infected by influenza dur-ing an outbreak in 1892. Pfeiffer believed that the bacte-ria was the cause of the influenza, and people called the bacteria “Pfeiffer’s bacillus.” When an influenza pandem-ic broke out in 1918 and patients exhibited similar symp-toms to the outbreak in 1892, many in the world’s scien-tific and medical communities accepted Pfeiffer’s theory even though there was not much evidence for it. There were, of course, those who disagreed with Pfeiffer’s the-ory. Some believed that pneumococcus or streptococcus were to blame, while others argued that not all influenza patients had influenza bacillus. A paper published in the Journal of the American Medical Association (JAMA) in 1919 reported on test results that found that influenza bacillus created a toxin and that this toxin could seep through filters and was strong enough to kill rabbits within hours.

Despite the controversy over Pfeiffer’s bacillus, Dr. Wil-liam H. Park of New York City Board of Health, Division of Pathology, Bacteriology, and Disinfection was sure that Pfeiffer had pinpointed the cause of the influenza pandemic and he tried to develop a vaccine. Dr. Paul Louis in Philadelphia, who was developing a vaccine for pneumococcus, also tried to develop a new vaccine by adding influenza bacillus to his pneumococcus vaccine. On October 19, 1918, he produced a vaccine that could treat pneumococcus, streptococcus, and influenza bacil-lus. Clinical trials were conducted in several areas with around 100,000 participants, but Dr. Louis was unable to prove that his vaccine had any effect.

This was because the influenza pandemic was caused by a virus, not the bacteria called influenza bacillus. Everyone knows about this virus today, but the fact that the influ-enza was caused by a virus—not bacteria—was discovered in the 1930s. That was ten years after the 1918 pandemic had ended, killing around 50 million people worldwide. It turns out that Pfeiffer’s bacillus was actually H. influen-zae type b (Hib), the cause of cerebromeningitis.

How were the genes of the virus thought to have been bacteria in 1918 eventually discovered?

In 1951, Johan Hultin, a doctoral student at the Univer-sity of Iowa, traveled to a small village in Alaska called Brevig Mission to find the cause of the 1918 influenza virus. He went to Alaska because a large number of Alas-kan Inuit had died from the 1918 influenza outbreak

and their bodies had been buried in Alaska’s permafrost. Hultin believed that the 1918 influenza virus could still be inside the bodies because they had not decomposed, due to the cold. Hultin received permission from the vil-lage elders to collect a lung tissue sample from a young woman who had been buried in the permafrost. He took that lung tissue sample back to Iowa and injected it into chicken eggs to try to isolate the virus. Unfortunately, he failed in this endeavor.

By the late 1970s, scientists in the life sciences had de-veloped the technology to analyze gene hierarchies. Hul-tin set out again to find the influenza virus in 1997, 46 years after his first endeavor. At the time, a team led by Dr. Jeffery Taubenberger at the Armed Forces Institute of Pathology succeeded in isolating the RNA of the in-fluenza virus from a stored sample of lung tissue of an American soldier who had died of the disease in 1918. The team was able to analyze three short hierarchies of the eight fragments of the virus’s genome from the 1918 influenza virus. Using the lung tissue from the Inuit woman named “Lucy,” and the lung tissues taken from dead American soldiers, which had been stored for research purposes, Taubenberger’s team and Hultin suc-cessfully analyzed the genome of the influenza virus that had caused the 1918 pandemic.

The team found, surprisingly, that the 1918 influenza virus had not spilled over from birds to humans; rather, it was similar to an influenza virus that infected pigs. It took a great deal of effort by a lot of scientists over a period of 10 years to identify the whole genome sequenc-ing of the eight fragments of the virus.

Since 1918, humankind has faced three different strains of influenza. The 1957 avian influenza outbreak (H2N2) and the 1968 (H3N2) outbreak in Hong Kong each killed around one million people. The swine flu (H1N1) outbreak in 2009 killed around 250,000 people. While these influenza strains are genetically different from each other, the outbreaks have forced humans to interact with viruses and to even cultivate the ability to fight back and defeat them.

We know who the enemy is. With the knowledge that the enemy is a viruses, not bacteria, scientists have been able to develop vaccines and medical treatments. Scientists have also been able to quickly analyze the genomes of viruses within days of an outbreak, and this knowledge is shared across the globe. The World Health Organiza-

33


tion (WHO) has a Global Influenza Surveillance and Response System (GISRS) that monitors changes in influenza viruses and has worked to establish a defense mechanism to protect the world economy and interna-tional society.

Why, then, are we being pummeled by COVID-19?

Dr. Donald Burke, a virologist at the University of Pitts-burgh, has long warned about viruses that can cause pan-demics. He has explained that there are three types of viruses we must be aware of: 1) viruses that have already caused worldwide pandemics such as Orthomyxoviridae viruses (influenza) and retroviruses (HIV); 2) viruses that have the proven ability to cause pandemics among ani-mals, not human beings (such as Orthomyxoviridae vi-ruses, Paramyxoviridae viruses—also called Hendra and Nipah viruses—and coronaviruses, which include SARS and MERS); and, 3) viruses that have the innate ability to evolve through mutation such as those that may be able to infect human beings through rapid evolution (retrovi-ruses, Orthomyxoviridae viruses, and coronaviruses). Ac-cordingly, it is not a coincidence that Dr. Burke was able to predict something like COVID-19.

In 1918, as World War I was still being fought, the movement and mobilization of militaries across the world meant that many soldiers had to live in extremely densely populated areas. That soldiers were packed in like sardines on the military ship mentioned at the be-ginning of this article provides a hint at the conditions of closely packed hosts that allow viruses to quickly spread. Humans today live in enclosed spaces and densely populated cities. That New York City, America’s most populated city, can suffer more COVID-19 cases than the entire country of China shows how a densely populated area of hosts can serve as the optimal environment for the spread of a virus.

During World War I, soldiers from Europe, America, South America, and different parts of Asia took part in the war. They traveled the globe by land and sea ac-companied at all times by viruses. In a globalized world, COVID-19 got its start in China but has spread through-out the world through land and sea routes.

The world’s medical system in 1918 failed to respond to the influenza pandemic properly. Over 30 percent of American doctors had gone to war, and there was a shortage of civilian doctors and nurses. This forced the

country’s medical authorities to set up makeshift hospi-tals staffed by medical students. Modern health systems, which are focused on treating chronic health problems rather than infectious diseases, are less capable of de-fending against such diseases. Research funds handed out to the medical and science communities have also shifted from infectious diseases to supporting the study of chronic diseases and ailments associated with old age. Over the past one hundred years, technology and the environment have undergone major advances; however, ironically, as humankind and its society became more improved and sophisticated, the belief that humankind can overcome viruses has collapsed. The defense mecha-nisms in place to protect against infectious disease were really just a mirage. The world’s response to the novel coronavirus has been a loose collection of individual bat-tles rather than a unified response, due to the economic, political, and cultural characteristics of each country. Viruses never weaken. The way humanity has developed and the way we live our lives have created the perfect en-vironment for viruses to survive.

Nancy Messonnier, the director of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention, said the following as the US just started to face COVID-19: “It’s not so much a question of ‘if’ this will happen anymore, but rather more a question of exactly ‘when’ this will happen …” Messon-nier’s statement is applicable to really any of the viruses that Dr. Burke mentioned. In short, while “no one knows the day or hour” when virus pandemics will strike, we should know that they will at some point.

And that is the reason why the entire world, from the medical and scientific communities to politicians and businesspeople, needs to prepare for another pandemic —whenever it comes. Humankind will have to continue fighting this unseen enemy even one hundred years from now. We are still not prepared.

sungsil Moon is a mother, research microbiologist, writer, and Korean-American. She received a BS in Microbiology from Hannam university and the MS and the PhD degree in Microbiology and immu-nology from Korea university. She is working on viruses and vaccine development in the uSA and writing columns for gender equality in the science field.

34

BulletinrevieW aNd research

ABSTRACT

We have proposed one of the most powerful methods to calculate few-body problems in physics, called Gaussian Expansion Method. The method has been successfully applied to various hypernuclei such as Λ, double Λ and Ξ hypernuclei. In this paper, we describe the structure of 7ΛHe, which is a neutron-rich hypernucleus, as a success-ful example.

INTRODUCTION

Many important problems in physics can be treated by solving accurately the Schrödinger equation for three- and four-body interacting particles. By solving this equa-tion we can predict various observable, before perform-ing its measurement, and obtain new understandings by comparing the observed data and our theoretical predic-tion.

For this purpose, it is necessary to develop a method to calculate precisely the three- and four-body problems, and to apply it to various fields such as nuclear physics as well as atomic physics. Along this line, we have been developing our few-body calculation method, Gaussian Expansion method (GEM), which is a kind of variational approach. This method fist was proposed by Kamimura and collaborators [1] and has been successfully applied to various three-body systems such as muonic molecules, [1, 2] three-nucleon bound states (3H, 3He), [3] neutron-rich nuclei [4] and antiprotonic helium atoms. [5] After-wards, the present author and collaborators proposed to modify the GEM with a new-type of basis functions, called infinitesimally-shift-Gaussian Lobe basis function, [6] and it became in this way much easier to apply the GEM to the solution of the four- and five-body problems.

To show the accuracy of our method, we performed a four-body benchmark test calculation of the 4He ground state with realistic NN interaction among seven few-body groups using different approaches. The calculated bind-ing energy, radii and density distribution were found in good agreement among each other. [7]

GEM has been most extensively applied to the study of hypernuclear physics by the present author and collabo-rators. One of the important goals in the hypernuclear physics is to produce neutron-rich Λ hypernuclei and to study the dynamical change of the structure by compar-ing the core nuclei. Since the Λ hyperon is free from the nuclear Pauli principle, it is considered to be responsible for a sizeable dynamical contraction of hypernuclear sys-tems due to the addition of a Λ. This is currently known as the “glue-like” role of the Λ particle.

Using this phenomena, the present author has studied with some detail several neutronrich Λ hypernuclei. In the light nuclei sector, near the neutron drip line, some new interesting phenomena concerning the neutron halo have been observed. If a Λ particle is added to such a halo nucleus, which is a very weakly bound system, the resulting hypernucleus will become stable against the neutron decay. To investigate the structure of neutron-rich Λ hypernuclei, we have been studied several systems, like nnΛ, [12] 6ΛHe, [11] A = 7 hypernuclei, [8–10] etc.

METHOD

We shortly explain in this section the formalism of GEM for the three-body system. We consider the case of cen-tral forces only for simplicity of the expressions (see Ref.

Gaussian expansion Method and its Application to nuclear Physic with Strangeness

eMiKo HiyAMA1,2 1DePArtMent of PHysiCs, KyusHu university, fuKuoKA, 819-0395, JAPAn

2strAnGeness nuCleAr PHysiCs lABorAtory, riKen nisHinA Center, WAKo, 351-0198, JAPAn

Communicated by Tohru Motobayashi

Doi: 10.22661/AAPPsBl.2020.30.3.34

35

June 2020 vol. 30 no. 3 revieW aNd research

[6] for more complicated cases including the solutions of four-body systems). The Schrödinger equation is written as

, (2·1)

with obvious notation. The three-body total wave func-tion ΨJM is described as a sum of amplitudes of three re-arrangement channels c = 1 – 3 (Fig. 1).

(2·2)

Each amplitude is expanded in terms of the Gaussian basis functions written in Jacobi coordinates rc and Rc:

(2·3)

(2·4)

(2·5)

Nnl (NNL) denotes the normalization constant. The Gauss-ian ranges νn and λN are given by a geometric progres-sion.

The eigenenergies E and the coefficients A(c)nclc,NcLc are de-

termined by means of the Rayleigh-Ritz variational prin-ciple.

Since the GEM obtained results are reliable, our predic-tions have been in good agreement with the data. In the next section, a successful example is shown.

STRUCTURE OF NEUTRON-RICH HYPERNUCLEUS, 7ΛHE

The observed data of the core nucleus 6He reported a 0+

1 ground state and the second 2+1 state. In 2012, see

Ref. [13], a second 2+ resonant state was observed in the charge-exchange-reaction 6Li(t , 3He)6He with param-eters Ex = 2.6 ± 0.3 and Γ = 1.6 ± 0.4 MeV.

From the theoretical point of view, many authors have studied this system. [14–16] Among these studies, Myo et al. [16] calculated the 6He spectrum using the complex scaling method (CSM) to obtain the energy together with the decay width of the low-lying states. They reproduced the observed ground state and the first 2+ state. They also obtained a second 2+ state. Then, it is interesting to add a Λ particle in 6He. Due to the “glue-like” role of Λ, it is likely to have narrower 3/2+

2 and 5/2+2 resonant states

in 7ΛHe. It was also observed that other states, the 1/2+

ground and the 3/2+1 and 5/2+

1 excited states, become more stable than the corresponding states of the core nucleus 6He.

To investigate the structure of 7ΛHe, we have performed

a four-body cluster model calculation of this system de-scribed as α+Λ+n+n and using GEM. In particular, we expected to found some resonant states in this hyper-nucleus. In order to obtain these resonant states, we em-ployed the complex scaling method, in a similar way of how was applied by Myo et al. to determine the resonant states of the core nucleus 6He.

In 6He, we found a bound ground state with 0+ which is in good agreement with the observed energy. We found also two 2+ states at Er = 0.96 MeV with Γ = 0.14MeV and Er = 2.81 MeV with Γ = 4.63 MeV, respectively. The calculated energy of the first 2+ is also in good agree-ment with the data. The SPIRAL data reported an en-ergy for the second 2+ state Er = 1.63 MeV with Γ = 1.6 MeV, which is much lower than the calculated Er energy and had narrower decay width than the calculated one.

In the 7ΛHe system, and due to the “glue-like” role of Λ

particle, all the states become more stable than the cor-responding states of 6He. The calculated Λ-separation energy of BΛ = E(6He)–E(7

ΛHe) = 5.36 MeV, which is

fig. 1: three sets of Jacobi coordinates of three-body system.

36


consistent with the recent observed data of JLab E01011 experiment: BΛ

expt = 5.68±0.03(stat.)±0.25(sysm.) MeV. The recent analysis of the JLab E05-E115 experiment [17] also reports an excited-state BΛ

expt = 3.65±0.20(stat.)±0.11(sysm.) MeV, which is also in good agreement with our calculations (see Fig. 2). In addition, we predict addi-tional 5/2+ and 3/2+ states having respectively Er = 0.03 MeV with Γ = 1.13 MeV and Er = 0.07 MeV with Γ = 1.01 MeV.

To encourage future experimental activity at JLab, we propose to perform the experiments 7Li(γ, K+)7

ΛHe pro-ducing these states. The calculated cross sections are 3.4 and 4.3 nb+sr for 3/2+

2 and 5/2+2, respectively, which seem

to be an accessible value for a future experiment. This experiment is of crucial importance in order to confirm the existence of the second 2+ state in 6He.

References

[1] M. Kamimura, Phys. rev., A38, 621 (1988)[2] M. Kamimura, Muon Catal. fusion 3, 335 (1988).[3] H. Kameyama, M. Kamimura, and y. fukushima, Phy. rev. C40, 974 (1989).[4] s. funada H. Kameyama, and y. sakuragi, nucl. Phys. A 575, 93 (1994).[5] y. Kino, M. Kamimura, and H. Kudo, nucl. Phys. A 631, 649 (1998).[6] e. Hiyama, y. Kino and M. Kamimura, Prog. Part. nucl. Phys. 51, 223 (2003).[7] H. Kamada et al., Phys. rev. C 64, 044001 (2001).[8] e. Hiyama, M. Kamimura, t. Motoba, t. yamada, and y. yamamoto, Phys.

rev. C53, 2075 (1996).[9] e. Hiyama, M. Kamimura, K. Miyazaki, and t. Motoba, Phys. rev. C 59, 2351

(1999).

[10] e. Hiyama, M. isaka, M. Kamimura, t. Myo and t. Motoba, Phys. rev. C 91, 054316 (2016).

[11] e. Hiyama, s. ohnishi, M. Kamimura, and y. yamamoto, nucl. Phys. 908, 29 (2013).

[12] e. Hiyama, s. ohnishi, B.f. Gibson, and th. A. rijken, Phys. rev. C 89, 061302(r) (2014).

[13] X. Mougeot et al., Phys. lett. B718, 441 (2012).[14] s. C. Pieper, r. B. Wiringa, and J. Carlson, Phys. rev. C 70, 054325 (2004).[15] A. volya and v. zelevinsky, Phys. rev. lett. 94, 0542501 (2005).[16] t. Myo, K. Kato and K. ikeda Phys. rev. C 76, 054309 (2007).[17] t. Gogami et al., Phys. rev. C94, 021302(r) (2016).

fig. 2: the calculated energy spectra of 6He and 7ΛHe. the states of 2+1 , 2+

2 and 1+ of 6He and 3/2+

2 and 5/2+2 are obtained using CSM. the values in parentheses

are decay widths Γ in Mev. the figure is taken from Ref. [10]

37


ABSTRACT

Plasma-based technologies for material treatment repre-sent a rapidly developing field that promises significant advantages over many other technological approaches. Due to the use of electrically charged high-energy fluxes of matter, plasma-based technologies can be used for fast, energy efficient treatment of various materials, includ-ing metals, ceramics, plastics and complex compounds. In this review, we briefly outline the processes that take place at various plasma-materials interfaces, and discuss future trends and possible novel applications for plasma-based technologies in the treatment and processing of materials.

INTRODUCTION

Plasma is the fourth state of matter, and is generally a mixture of ions, electrons, neutral particles and radicals [1, 2]. Currently, plasma-based systems and techniques are used across a very broad spectrum of applications ranging from space propulsion [3–9] and plasma thrust-ers [10–12], medicine [13–15], biology [16–18], materials treatment [19, 20], agriculture [21, 22], to many other

areas [23]. For technological applications, the processes at the interface between plasma and matter are of key importance, since they determine material and energy fluxes to the surface from the plasma bulk.

Apart from technological systems, application of cold plasmas for treatment of biological tissues and living cells may be extremely important, as demonstrated by the coronavirus COVID-19 pandemic 2020 [24, 25].

In general, separation of electric charges at the interface results in the formation of a plasma-surface sheath, with a width that could be controlled by changing plasma parameters and properties of the surface. As such, fluxes of material and energy to the surface could be ef-ficiently controlled by modulating plasma and surface parameters, with the possibility of attaining sophisticated control over fluxes when the surface is micro- and nano-structured [26–29].

Since electrons in plasma feature much higher velocities than heavier ions and radicals, a surface immersed in

Processes at Plasma-Matter interfaces: An overview and Future trends

iGor levCHenKo1,2, KAterynA BAzAKA3,1,2,7, oleG BArAnov4,5, oleKsii CHerKun1,7, MiCHAel KeiDAr6 AnD sHuyAn Xu1,7 1 PlAsMA sourCes AnD APPliCAtion Center/sPACe ProPulsion Centre sinGAPore, nie,

nAnyAnG teCHnoloGiCAl university, 637616, sinGAPore 2 sCienCe AnD enGineerinG fACulty, QueenslAnD university of teCHnoloGy, BrisBAne, AustrAliA

3 reseArCH sCHool of eleCtriCAl, enerGy AnD MAteriAls enGineerinG, tHe AustrAliAn nAtionAl university, ACt 2601, AustrAliA

4 nAtionAl AerosPACe university, KHArKiv, uKrAine 5 JoŽef stefAn institute, lJuBlJAnA, sloveniA, eu

6 MeCHAniCAl AnD AerosPACe enGineerinG, sCHool of enGineerinG AnD APPlieD sCienCe, tHe GeorGe WAsHinGton university, sCienCe & enGineerinG HAll 3550, 800 22nD street, nortHWest WAsHinGton, DC 20052

7 Center for sPACe ProPulsion AnD GrAvitAtionAl universe (sPaGu), HAnGzHou institute for ADvAnCeD stuDy, tHe university of CHinese ACADeMy of sCienCes, 1 XiAnG sHAn zHi nonG, HAnG zHou, CHinA 310024

Communicated by Won Namkung

Doi: 10.22661/AAPPsBl.2020.30.3.37

38


plasma acquires a negative electric charge (the so-called floating potential ), even when no external potential is ap-plied. As a result, ions are accelerated by the electric field towards the surface, bringing additional energy to the surface. These phenomena allow for the efficient growth of small particles, such as nanotubes and graphene nano-flakes, in plasma (Fig. 1).

PLASMA SHEATHES AND MATERIAL FLUXES

Bulk plasma is usually separated from electrically charged surfaces, and a sheath is formed between them. In this sheath, a strong electric field is sustained [30, 31]. It is usually assumed that the ions from plasma enter the sheath with the Bohm velocity:

, (1)

where Te is the electron temperature, and mi is the mass of ions in plasma. The plasma behavior near the surface depends on the relation of electron temperature to sur-

face potential US. When the electron temperature is low, i.e. Te ≪ US, the sheath is thick and its thickness may be calculated as:

, (2)

where λD is the Debye length. The Debye length may be expressed as:

, (3)

where ε0 is the dielectric constant, n is the plasma density, and e is the elemental electron charge.

When the sheath is thin, i.e. when a low bias has been ap-plied to the surface, the sheath width may be estimated as:

, (4)

i.e., as several Debye lengths; here kλ is a constant which could be typically assumed in the range of 1 to 5 [32].

During the growth of nanostructures in the presence of plasma, a very complex electric field between the plasma bulk and nano-structured surface is formed. As a result, the fluxes of ions from plasma depend on the morphol-ogy of the surface, as well as the plasma parameters. Importantly, the nature of the material from which these nanostructures are made (conductive or insulating) also influence the structure of the electric field, and hence the distribution of ion fluxes over the surface [33–36].

In the electric field, the ions move along complex trajec-tories, and eventually deposit onto the nanostructures. The structure of the electric field acting onto an ion located in a point described by vector r (ion position vec-tor) can be simulated using the expression:

, (5)

where N is the number of nanostructures growing on the surface, ρi is the density of the electric charge on the sur-face, z is the distance from the surface to the ion position vector, r is the ion position vector relative to the ith nano-structure, and Si is the surface area of the nanostructure. Using this expression, the equation of ion motion can be directly integrated and the distribution of ion fluxes over the growing nanostructures can be calculated. Next, the

fig. 1: Plasma-based systems may offer a simple yet efficient means for nanofabrication. A schematic of the process for graphene flake fabrication in an arc discharge setup. (a) the apparatus for the graphene growth consists of an anode, a cathode, a graphene collecting surface and (optionally) a focusing magnet. A DC discharge is sustained in He gas. A carbon material is supplied via anode evaporation. (b) the carbonaceous deposit is removed from the surface and undergoes ultrasonication, and eventually (c) a graphene flake suspension is produced which can be used, e.g., for printing (d). Processes at the plasma-solid interface are important (e): here, we show an influx of carbon atoms from the plasma to the growing graphene flake, the evaporation of carbon atoms from the graphene surface, the surface diffusion of carbon atoms around the graphene surface, and the incorporation of carbon adatoms into the edges of flakes. Reprinted from [30] under the Creative Commons Attribution international license (CC BY).

39


growth of the nanostructure can be simulated using the equations for surface diffusion over the growing nano-structures and the surface of the substrate [26, 30, 33]. Apparently the electric field does not affect the trajec-tories of neutral particles, thus the uniform deposition of neutral species should be added to the electron field-modulated flux.

The two-dimensional flux of atoms to the edges of the growing nanostructures, along with the direct ion fluxes from plasma, are the most important parameters that de-termine the growth kinetics and morphology of plasma-grown structures. The equation for surface diffusion may be used in the form [36]:

, (6)

where DS is the diffusion coefficient, η is the density of adatoms on the surface, ψin is the flux of atoms and ions from the plasma sheath to the substrate, and ψvp is the evaporation flux from the surface.

The processes at the interface between plasma and liq-uids are much more complex than the processes at the interface between plasmas and solid surfaces, and usu-ally involve multiple plasma-initiated chemical reactions. Nevertheless, these plasma-liquid interfacial processes are very important in both material processing and med-icine [44]. Below we will briefly outline some important applications of plasma-surface interfacial systems, and the relevant processes at the interfaces.

Plasmas feature a very rich set of control parameters and control knobs that allow for sophisticated adjustment of the nucleation and growth of nanostructures in plasma-based reactive environments. In Fig. 2, various aggregate states, along with the control means and technological schemes, are shown in their interaction with the pro-cesses and various plasma-generated particles. Plasma it-self can be generated and sustained in various aggregate states.

Electric and magnetic fields are broadly used to control the plasma generation and fluxes of matter and energy from the plasma to the nanostructured surfaces. The thin green ring in Fig. 2 also lists the set of the most popular plasma-based techniques that are currently in use for technological and other tasks, namely:

DC – Direct current discharge,Arc – Arc discharges,CCP – Capacitively coupled plasma,ICP – Inductively coupled plasma,MW – Microwave plasma, andECR – Electron cyclotron resonance.

When plasmas interact with a surface, a large number of processes are initiated and sustained through the electric energy applied to the discharge. Among the most impor-tant processes are:

Ionization: Generation of ions from neutral atoms, usually via impact by ions or/and electrons, or via ultraviolet radia-tion in plasma;

Excitation: Transition of electron bounds in a neutral par-ticle shell to another state with a higher energy; and

Dissociation: Splitting of molecules or complexes to smaller

fig. 2: Solid, liquid, gas? unique benefits of using different types of media for material fabrication and processing. A transition between these states within the same technological run affords a wide diversity of possible processes at different time, energy, density and temperature scales arising from the interactions between the species: from femtoseconds for collisions to milliseconds for adsorption events; with the highest range of achieved energy (0.1 to 104 ev), and density of particles (1014 to 1022 m-3). the environment presents a large number of possible means of process control, through electrical and magnetic fields, pressure gradients, and friction forces. there is the possibility to drive the process as a non-equilibrium ballistic or as a thermal treatment. there is a large variety of physical and chemical processes in the volume and on the surface. there is the possibility for selective processing of objects from the nano- to micro scale, and there is a possibility to develop processes that meet environmental sustainability requirements. Reprinted with permission from [35]. Copyright RSC, 2018.

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fragment, also under the impact of ions or/and electrons, or via ultraviolet radiation in plasma.

Importantly, some of the above processes occur in plasma and solid/liquid matter, causing, e.g., evapora-tion or penetration of atoms into the growing structures. Plasmas can be generated and sustained in various me-dia, and their parameters can be varied over a very wide range, typically in:

• Ion and neutrals temperatures: from 100 °C to several thousand °C;

• Electron temperatures: from fractions of eV to several tens of eV;

• Electron density: up to 10 22 m –3 (for high current arc dis-charges); and

• Interaction time: from 10 –15 to 10 –3 s for thermally – driven processes.

PLASMA-LIQUID INTERFACES

Plasma-liquid interactions in biomedical applicationsThe application of plasma in medicine is a very promis-ing field, in particular for the treatment of tumours and cancers of various origin. Many different types of atmo-spheric pressure plasmas have been tested for their re-activity toward cancer cells, and significant progress has been achieved in using plasmas in oncotherapy [17, 38].

It is currently believed that reactive oxygen and nitrogen species (ROS and RNS), along with plasma-generated ultra-violet radiation, are the most active components ca-pable of selective killing of various types of cancer cells.

Importantly, in biomedical applications, plasma is not simply a source of reactive species, but it utilizes several additional phenomena that give rise to desirable syn-ergistic effects (Fig. 3). For example, when applied to tissues directly, plasmas can induce cellular responses through ion and electron fluxes, ultraviolet radiation, and plasma-synthesized chemicals in the liquid, as well as via plasma-synthesized nanoparticles, which could perform many useful functions – e.g., they could kill can-cer cells, or transport and release therapeutic agents. At present, reactive oxygen and nitrogen species (ROS and RNS) are the primary plasma-synthesized reagents used in the treatment of cancer cells; however, other promis-ing species emerge as the field continues to develop.

Currently it is assumed that the following mechanisms are responsible for the formation of H2O2, NO2

– and other RONS in plasma-treated liquids [16, 38, 39]:

O2 + e– → O2– , (1)

O2– + H+ → HO2, (2)

HO2 + e– → HO2–, (3)

HO2– + H+ → H2O2, (4)

HO2 + e → H2O* + e, (5) H2O* → *OH + H*, (6) H* + O2 → *OH + O, (7) O2 + e → O + O + e, (8) O + H2O → 2*OH, (9) *OH + O → *HO2, (10) *HO2 → H + O2*, (11) O + O2 → O3, (12)

and many others [16, 40].

The self-organization of plasmas at plasma-liquid inter-faces was recently demonstrated as a tool to boost the efficiency of plasma-based anti-cancer therapy. In this case, the plasma forms ordered structures [41] over the surface of the liquid, a process that involves an interplay between a large set of physical and chemical parameters. The self-organized patterns of plasmas over the surface of liquids was found to be more effective in killing cancer

fig. 3: the synergistic relationship between plasmas, nanomaterials and their biomedical applications. Reprinted from [37] under the Creative Commons Attribution international license (CC BY).

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cells when compared to typical non-organized plasma jets. Specifically, plasmas with self-organized patterns have better overall effectiveness in killing cancer cells than non-self-organized plasmas [17, 38]. The precise mechanisms that underpin the nature of the observed enhanced biological activity are yet to be fully elucidated, and consequently are subjects for intense research efforts [42, 43]. Moreover, the RON and ROS densities are also higher in the self-organized stratified plasmas (Fig. 4).

Plasma-liquid interactions in nanotechnologyInterfacial processes that define plasma interactions with liquid surfaces also play an important role in nanotech-nology and synthesis of nanoscale materials [44]. Similar to applications in biology and medicine, reactive oxygen species (ROS) may be useful in driving desirable plasma-based interfacial reactions in nanotechnology. Figure 5 is an example of a set of important reactions induced by plasma in liquids for SiO2-based thin film deposi-tion. Plasma-generated ROS species play a key role in

fig. 4: (a) An atmospheric pressure glow micro-discharge setup. A small (several mm) gap between the cathode and the surface of the liquid accommodates a bunch of plasma. (b) Photographs of the discharge patterns above the therapeutic media during the activation process. the self-organized patterns have complex structures that are strongly dependent on the voltage-current conditions. (c) optical emission spectra generated by the atmospheric pressure glow discharge above water. (d) Current-voltage dependency of the system with optical photographs of the self-organized stratified interface patterns. it is possible to distinguish four discharge stages. (e) A table summarizing the relationships between discharge stages, self-organized patterns and optical emissions. Reprinted from [17] under the Creative Commons Attribution international license (CC BY).

fig. 5: the use of the european Cooperation for Science and technology (CoSt) jet to study the reactions leading to Sio2-based thin films deposited by cold atmospheric pressure plasma (CAP). Reprinted from [47] under the Creative Commons Attribution international license (CC BY).

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this chain of processes. In the experiments made under the umbrella of the European Cooperation for Science and Technology (COST) funding organization, [45] the He gas with hexamethyldisiloxane (HMDSO) was used to study the effect of ROS on film formation. A reaction pathway has been proposed to form the films enriched with Si-O [46]. Specifically, addition of oxygen to the mix-ture resulted in the formation of ROS, which in turn pro-vided additional Si-O bonds. Schematics of the reactions are shown in Fig. 5 [47].

As we have mentioned in the previous sub-section, syner-gistic effects can arise when plasmas are applied directly to living tissues through the plasma-driven synthesis of nanomaterials directly in the living tissues or in the ambient liquid media affected by plasma. For these ap-plications, metallic and composite nanoparticles are of principal importance [14].

Figure 6 illustrates a typical example of the processes at the plasma-liquid interface during atmospheric plasma-based synthesis of nanoparticles [37]. This process was conducted in an open-air setting, thus resulting in a very high density of ROS. No additional catalysts or reducing agents are needed in this process. This is an essentially green technology, capable of producing nanomaterials for various applications without complex and harmful chemical processes and reagents.

Another important method for nanoparticle production is by pulsing discharge in water. While in this case, no plasma is present above the liquid to form an interface, the plasma-liquid interfaces are still formed during the discharge in the evaporated liquid media. In this case, a similar chain of reactions could be produced, result-ing in an efficient, fast formation of nanoparticles [48]. In this process, metal electrodes (cathodes and anodes) were submerged into the liquid, and the pulsing power was supplied to the electrodes. As a result, plasmas were generated directly in the liquid, in the gap of approx. 0.2 mm. The pulse duration was about 20 μs.

A vibrator was installed over one of the electrodes, to ensure continuous generation of the plasma. In this sys-tem, plasmas with extremely high temperatures reaching 2500 °C were sustained. Not surprisingly, the metals were efficiently ionized and the ions penetrated into the liquid, where they formed complex nanoparticles upon cooling (Figs. 7 and 8). Since the pulses were very short and the liquid efficiently cooled the dispersed metal, very small nanoparticles were produced.

fig. 6: examples of important reactions at the plasma-liquid interface: reduction of metal ions, diffusion and nucleation of nanoparticles. Reduction is initiated by solvated electrons, or plasma-generated reactive species. Reprinted from [37] under the Creative Commons Attribution international license (CC BY).

fig. 7: Schematic of pulsed-plasma-in-liquid method. Reprinted from [48] under the Creative Commons Attribution international license (CC BY).

fig. 8: interfacial processes involved in the formation of Pd-Fe alloy nanoparticles by pulsed plasma in water. Reprinted from [48] under the Creative Commons Attribution international license (CC BY).

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This example illustrates how very high plasma tempera-tures, together with efficient cooling in liquids, allow for the efficient control of nanosynthesis.

PLASMA-SOLID INTERFACES

Two major types of plasma-solid interactionsWe will distinguish the two major types of the plasma-solid interfaces: (i) plasma interactions with a relatively flat surface, e.g. during the formation of a thin film or multilayered structure; and (ii) plasma interactions with nanostructured surfaces or arrays of vertically aligned nanostructures on the surfaces immersed in the active plasma. In the first case, plasma chemistry and surface diffusion (partially influenced by plasma [49]) will largely determine the behavior of the system. In the second case, due to the strong influence of the nanostructure-generated electric fields, focusing of the ion fluxes at the nanostructures will take place. Importantly, when plasma interacts with nanostructures, electric fields influence significantly the shapes and distribution of nanostruc-ture sizes, including the morphology of the surface and sharpness of the nanostructures [21, 27].

Plasma-solid interactions in surface treatmentLet us examine a set of typical processes at the interface between the plasma and a flat surface with the formation

of hydrogenated amorphous silicon film as an example. The process starts from nucleation of small particles that then grow into hydrogenated amorphous silicon. There are three relatively independent processes that proceed at various levels:

(i) Processes in the plasma sheath that determine the plasma-matter interactions at the interface;

(ii) Interactions of ions and radicals with the surface, determin-ing the material influx to the growth zone; and

(iii) Growth on the surface, resulting in the formation of the hy-drogenated amorphous silicon film.

Figure 9 shows several important processes in this system. While some processes (such as the formation of radicals, atom ionization and others) occur in plasma, many other key processes take place in the plasma-surface sheath and directly on the surface. These processes are, e.g., desorp-tion and adsorption of SiH3 molecules; abstraction of H by H and SiH3 radicals; abstraction of SiH3 with the use of SiH3 radicals; attachment of H and SiH3 to dangling bonds; diffusion of SiH3 across the surface; and sputter-ing of hydrogen from the surface by the direct impact of ions from the plasma [49].

This process was conducted in the reactor with a highly reactive inductively-coupled plasma [8], which is a com-

fig. 9: Schematic representation of the basic surface reactions of SiH3 and H radicals during the α-Si:H film growth used in the model. left panel: processes in plasma. Right panel: processes on the surface and in the plasma-surface sheath: (1) adsorption of SiH3; (2) desorption of SiH3; (3) H abstraction by a H radical; (4) H abstraction by a SiH3 radical; (5) SiH3 abstraction by a SiH3 radical; (6) direct chemisorption of SiH3 into a dangling bond; (7) direct chemisorption of H into a dangling bond; (8) hopping of the adsorbed SiH3 on hydride sites; (9) chemisorption of adsorbed SiH3 into a dangling bond; and (10) ion sputtering of the mono-hydride sites. Reprinted from [49] under the Creative Commons Attribution international license (CC BY).

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mon medium for the synthesis of hydrogenated silicon layers. The key specific energies are represented in Table 1.

Plasma-solid interactions for growing vertical nanostructuresFigure 10 shows a set of chemical and physical phenom-ena that occur during the growth of vertically-aligned carbon nanostructures, such as vertical surface-attached nanotubes and graphene flakes. Importantly, hydrogen atoms are more reactive towards α-carbon, as compared to the sp2 and sp3 hybridized carbons.

This results in the removal of α-carbon and then ensures nucleation and growth of crystalline carbon nanoflakes, forming sharp, highly reactive edges. Some publications also demonstrate the important role of oxygen and ni-trogen, which help to remove α-carbon from the growing nanostructures.

Oxygen radicals O* significantly reduce the density of surface defects, and suppress the nucleation of carbon and formation of interfacial layers. As a result, the car-bon nanostructures are formed without any interfacial layers. Moreover, oxygen ensures the growth of carbon nanowalls in a vertical direction via the removal of any small nuclei that could form horizontal layers. Next, OH radicals also help to remove the α-carbon. Admixture of Ar atoms in the reactive environment helps to form C2 molecules through direct impact dissociation. Important-ly, the high density of argon atoms in the reactive envi-ronment helps to increase the density of carbon dimers, thus ensuring higher graphitization [53]. Furthermore, Ar atoms in plasma enhance the growth of carbon nano-walls due to the higher stability of plasma.

Figure 11 shows the interfacial plasma-solid processes during the growth of well-shaped carbon nanowalls in the process, where CH4 and H2 molecules are involved. Vertical graphene flakes bend when the network of sp2 hybridized carbons overcomes the relevant energy threshold. Due to the presence of ions and electrons in plasma, an electric field orthogonal to the surface is formed, which contributes to overcoming the energy threshold on sharp edges of the growing nanostructures. As a result, the vertical graphene flakes bend in the elec-

table 1. typical plasma parameters at the surface. Reprinted from [49] under the Creative Commons Attribution international license (CC BY).

parameter value

electron temperature ion temperature Plasma density neutral gas pressure Substrate potential Substrate temperature Percentage of SiH3 gas Percentage of H gas Percentage of Ar gas

1-3 ev 0.01-0.15 ev 1010 to 1012 cm-3 20-100 mtorr 0 to -300 v 300-700 K 10-40 % 5-35 % 55-85 %

fig. 10: Schematic of the processes during nucleation and the first stage of the growth of vertical nanoflakes in plasma. Dissociation of CH4 molecules and surface diffusion result in the formation of hexagonal carbon structures. the presence of H atoms in the plasma system assists in the nucleation of the carbon radical and simultaneously acts as the etchant gas for a-C [50, 51].

fig. 11: A schematic explanation of the processes during the carbon nanowall growth. e: the direction of an electric field; CHX(g): HC growth species; C(G): Graphene sheets; H: Atomic hydrogen used as an etchant. CHX(α): α-C etched along with H atoms in the form of hydrocarbon (HC); vG edge: edges of the vertically-oriented carbon nanowalls.

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tric field. Plasma also etches the dangling bonds at sharp edges, thus promoting the formation of the network of isolated, stand-alone nanostructures.

A strong electric field at the sharp edges of growing nanostructures ensures focusing of the ion current to-ward the edges, thus resulting in carbon nanowalls with greater height [54]. In addition, the electric field ensures the formation of sp2 hybridized carbons, which serve as nucleation centers. Moreover, the lateral electric field at the sharp edges of nanowalls interacts with surface plas-mons and could change the growth mode of the vertical carbon nanostructures [55].

To demonstrate the potential of plasma-based processes for materials treatment and activation, let us examine several examples (more examples of the processes of various types may be found in numerous references pro-vided). Figure 12 illustrates many types of nanostructures synthesized by pulsing plasmas. Using a short pulsing plasma discharge, quite different structures may be ob-tained in an energy efficient process. As compared to other techniques, such as thermal furnace-based meth-ods, the use of plasma-based process environments has the potential to significantly lower the energy cost of the final product [56]. While the whole set of processes

involved in the growth of vertical graphenes in plasma is very complex, it offers a multitude of opportunities to control the process (Fig. 13).

fig. 13: Summary of time–temperature growth regimes for the initial growth of different carbon nanostructures. Reprinted from [51] under the Creative Commons Attribution international license (CC BY).

fig. 12: SeM images of Moo3 nanostructures. (a–e) Deposited in the pin-to-pin electrode configuration, (f–h) deposited in the pin-to-plate electrode configuration; (i) porous networks of Moo3 deposited in the pin- to-plate configuration. More details on the processes, plasma parameters and characterization of nanostructures may be found elsewhere [56]. Reprinted under the Creative Commons Attribution international license (CC BY).

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Another example of highly controllable, energy efficient synthesis of nanostructures in plasma-based techniques, with plasma-surface interfacial processes taking the lead-ing role, is shown in Fig. 14. Free-standing perfectly crystalline silicon carbide nanocrystals were fabricated in atmospheric pressure plasmas using the low-cost, ligand-free technique.

This process resulted in the synthesis of ultra-small nano-crystals that were highly crystalline and perfectly control-lable in sizes. Moreover, this plasma-based technology ensures very low levels of surface contamination for the fabricated nanostructures. The use of atmospheric pres-sure significantly reduces the cost of the product.

FUTURE TRENDS

A deep understanding of physical and chemical process-es occurring at the plasma-surface interfaces is critically important for many plasma applications, spanning medi-cine and biology, plasma-based space propulsion [58–63], synthesis of novel materials [64–66] and sophisticated control of material activation and functionalization [67]. Despite the great progress already made in this field, further studies are needed to satisfy the growing demand from the materials sciences and plasma physics commu-nities. In our opinion, the major directions should be as follows:

More attention should be paid to the self-organized behavior of interfacial systems;

Further progress should be made in designing novel, robotic, and artificial intelligence-enabled methods for rapid testing and measurement of the interfa-cial processes, both on surfaces and in plasmas. This would ensure fast progress in understanding the com-plex processes and the interference between the pro-cesses. On the other hand, plasma-based technologies are complex and certainly, their automatization is also a complex problem. An approach for such automa-tization was developed by the authors at the plasma and schematics level [8].

Next, further progress should be made in numerical modeling and simulation of the interfacial processes with the help of super-fast supercomputers and dis-tributed computer networks. This would help to achieve faster progress without the involvement of ex-pensive, time consuming experiments and extremely expensive equipment.

Taking into account the key role that the interfacial pro-cesses play in the aforementioned systems, further studies in this area should be considered as a matter of priority.

Acknowledgements: This work was supported in part by the following funds and organizations: the Office for Space Technology and Industry – Space Research Pro-gram (OSTIn-SRP/EDB) through the National Research Foundation, Singapore, and in part by the Ministry of Education Academic Research Fund (MoE AcRF, grant No. Rp6/16 Xs), Singapore; I. Levchenko acknowledges the support from the Science and Engineering Faculty, Queensland University of Technology; O. Baranov ac-knowledges the support from the PEGASUS (Plasma En-abled and Graphene Allowed Synthesis of Unique Nano-Structures) project, funded by the European Union’s research and innovation program, Horizon, under grant agreement No. 766894; and the authors would like to express special thanks to L. Xu, S. Huang and the entire group at the Plasma Sources and Applications Centre / Space Propulsion Centre, Singapore (PSAC/SPCS) for their help.

fig. 14: (a) Schematic diagram for the atmospheric pressure plasma reactor (RF is an abbreviation for radio frequency). (b) Side-view photo showing the ignited plasma with gas flowing from the top. (c) typical transmission electron microscopic image of the nanoparticles produced in argon plasma. Reprinted from [57] with permission. Copyright RSC 2016, under the Creative Commons Attribution international license (CC BY).

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Jun'ichi Yokoyama (Japan) / PresidentRESCEU, The University of TokyoHongo Bunkyo-ku, Tokyo 113-0033, JapanE-mail: [email protected]

HyoungJoon Choi (Korea) / vice PresidentYonsei University50 Yonsei-ro, Seodaemun-gu, Seoul, KoreaE-mail: [email protected]

Nobuko Naka (Japan) / SecretaryKyoto UniversityOiwake-cho, KitashirakawaSakyo-ku, Kyoto 606-8502, JapanE-mail: [email protected]

Keun-Young Kim (Korea) / TreasurerGwangju Institute of Science and Technology123 Cheomdangwagi-ro, Buk-gu,Gwangju, 61005, KoreaE-mail: [email protected]

Gui-Lu Long (China/Beijing) / ex-offi cioTsinghua UniversityHaidian District, Beijing 100084E-mail: [email protected]

Jodie Bradby (Australia)Research School of Physics, Building 60, ANU Campus, Canberra, ACT 2601E-mail: [email protected]

Xiu-dong Sun (China/Beijing)Harbin Institute of Technology92 West Dazhi Street,Nan Gang District,HarbinE-mail: [email protected]

Tao XiANG (China/Beijing)Institute of Physics, Chinese Academy of SciencesP.O. Box 603, Beijing 100190E-mail: [email protected]

ruiqin Zhang (Hong Kong)Department of Physics and Materials ScienceCity University of Hong KongG6702, 6/F, Academic 1, Tat Chee Avenue,Kowloon, Hong Kong SARE-mail: [email protected]

Mio Murao (Japan)Department of Physics, The University of TokyoBldg.1 #234, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, JapanE-mail: [email protected]

Akira Yamada (Japan)Tokyo Institute of Technology2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8550, JapanE-mail: [email protected]

Woo-Sung Jung (Korea)POSTECH77 Cheongam-ro, Nam-gu, Pohang,Gyeongsangbuk-do, 37673, KoreaE-mail: [email protected]

Kurunathan ratnavelu (Malaysia)UCSI University Kuala Lumpur CampusNo. 1, Jalan Menara Gading, UCSI Heights (Taman Connaught), Cheras 56000 Kuala Lumpur, MalaysiaE-mail: [email protected]

rajdeep Singh rawat (Singapore)National Institute of Education, Singapore (NIE)1 Nanyang Walk, Singapore 637616E-mail: [email protected]

fu-Jen Kao (China/Taipei)National Yang-Ming UniversityNo.155, Sec.2, Linong Street, Taipei 112E-mail: fj [email protected]

Meng-fan Luo (China/Taipei)National Central UniversityNo. 101, Section 2 Kuang-Fu Road, Hsinchu, Taiwan 300, R.O.C.E-mail: mfl [email protected]

Nguyen Quang Liem (vietnam)Vietnam Academy of Science and Technology18 Hoang Quoc Viet Rd. Hanoi, VietnamE-mail: [email protected]

COUNCiL MEMBErS (2020-2022)

Member Societies

Australian Institute of PhysicsThe Chinese Physical SocietyPhysical Society of Hong KongIndian Physics AssociationIndonesian Physical SocietyPhysical Society of JapanThe Japan Society of Applied PhysicsKazakh Physical SocietyThe Korean Physical SocietyMalaysian Institute of PhysicsMongolian Physical SocietyNepal Physical SocietyNew Zealand Institute of PhysicsPhysical Society of the PhilippinesInstitute of Physics, SingaporeThe Physical Society located in TaipeiThai Physics SocietyVietnam Physical Society

All Past Presidents

Gui-Lu Long (China/Beijing)January 1, 2017 - December 31, 2019Seunghwan Kim (Korea)January 1, 2014 - December 31, 2016Shoji Nagamiya (Japan)January 1, 2011 - December 31, 2013 Jie Zhang (China/Beijing)January 1, 2008 - December 31, 2010Tien T. Tsong (China/Taipei)January 1, 2005 - December 31, 2007Won Namkung (Korea)January 1, 2001 - December 31, 2004Chen Jiaer (China/Beijing)January 1, 1998 - December 31, 2000Michiji Konuma (Japan)July 2, 1994 - December 31, 1997C. N. Yang (Hong Kong)August 11, 1990 - July 2, 1994

C. N. Yang (Hong Kong) / Honorary PresidentMichiji Konuma (Japan) / Special Advisor

PAST EDiTOrS-iN-CHiEf

Shoji Nagamiya RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, JapanE-mail: [email protected]

Won NamkungPOSTECH, 77 Cheongam-Ro, Nam-gu Pohang 790-784, KoreaE-mail: [email protected]

W-Y. Pauchy HwangNational Taiwan University, Taipei 106E-mail: [email protected]

S. C. LimFaculty of Engineering Multimedia University Cuberjaya, Selangor, MalaysiaE-mail: [email protected]

The AAPPS Bulletin is also published electronically athttp://aappsbulletin.org/

Supported by the Asia Pacific Center for Theoretical Physics (APCTP)http://apctp.org

ISSN: 0218-2203

and the Korean Government through the Science and Technology Promotion Fundand Lottery Fund

volume 30 number 3 june 2020 - aappsbulletin

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