origin of high-temperature superconductivity revealed by...
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Youhei Yamaji Department of Applied Physics, The University of Tokyo
and PRESTO, JST
Origin of High-Temperature Superconductivity Revealed by Boltzmann Machine Learning
Collaborators: Prof. Teppei Yoshida (Kyoto U.) Prof. Atsushi Fujimori (UTokyo) Prof. Masatoshi Imada (UTokyo)
Y. Yamaji, T. Yoshida, A. Fujimori, and M. Imada, arXiv:1903.08060.
Acknowledgement: Prof. Takeshi Kondo (ISSP)
DLAP2019@YITP
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An enigmatic inverse problem: Origin of high-temperature superconductivity 1986 High-temperature superconductivity in copper oxides
J. G. Bednorz K. A. Müller
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Life of Electrons in Crystalline Solids
L. D. Landau 1956
Landau’s Fermi liquid theory strongly interacting electrons and free electrons are adiabatically connected
Coulomb
W. Pauli
E. Fermi
©Feynman
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Life of Electrons in Superconductivity
1911 Zero electric resistance of Hg
1957 BCS theory
H. Kamerlingh Onnes
J. Bardeen, L. N. Cooper, J. R. Schrieffer
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Propagation of Free Electron
For a single electron in vacuum
After Fourier transformation
Spectral weight
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Many-body Schrödinger eq.
EOM of single-particle Green’s function
Description of Many-Body Electrons
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Damascelli, Hussain, & Shen, Rev. Mod. Phys. 75, 473 (2003)
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~Scattering rate
Spectral weight
Rigorous Relation between Self-Energy and Spectral Weight
Normal self-energy
Anomalous self-energy ~Rate of anomalous scattering ~ω-distribution of attractive force
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Eliashberg eqs. for BCS SC
J. R. Schrieffer, D. J. Scalapino, and J. W. Wilkins, Phys. Rev. Lett. 10, 336 (1963). W. L. McMillan and J. M. Rowell, Phys. Rev. Lett. 14, 108 (1965).
A. B. Migdal, Sov. Phys. JETP 7, 996 (1958). G. Eliashberg, Sov. Phys. JETP 11, 696 (1960).
cf.) P. Morel, P. W. Anderson, Phys. Rev. 125, 1263 (1962).
: Coupling2 x DOS of phonon
Self-Energy in BCS Superconductors
: Scale of phonon
From STS, Σnor and Σano separately obtained
SC gap function
Ratio of DOS
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Self-Energy in Cuprate Superconductors B. Keimer, S. A. Kivelson, M. R. Norman, & S. Uchida, Nature 518, 179 (2015).
Extension of Eliashberg theory: J. M. Bok, et al., Sci. Adv. 2, e1501329 (2016).
Modeling self-energy: H. Li, et al., Nat. Commun. 9, 26 (2018).
Competing energy scales, EF and Epair, prevent us from separating Σnor and Σano
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A new approach: ✔Fewer assumptions ✔ ︎More flexible representation of Σ to reveal unexpected physics beyond biased expectation
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Photoemission Electron Spectroscopy of Crystalline Solids Intensity of photoelectron
Spectral weight
Damascelli, Hussain, & Shen, Rev. Mod. Phys. 75, 473 (2003)
Free electrons
Correlated electrons
Fermi-Dirac distribution
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Spectrum of Cuprate Superconductors for T < Tc
T=12K
Bi2Sr2CuO6+δ
Bi2Sr2CuO6+δ
T. Kondo, et al., Nat. Phys. 7, 21 (2011). Bi2Sr2CaCu2O8+δ Optimally doped sample (Tc=90K)
Angle-resolved photoemission
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Known: Single-component spectral weight
Unknown: Two components of self-energy
We need to separately obtain Σnor(ω) and Σano(ω) from a single-component spectral function A(ω)
For given wave number and frequency, how often an electron is disturbed by others
Underdetermined Non-Linear Inverse Problem
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Prior Knowledge to Solve the Underdetermined Problem
Prior knowledge about the self-energy Causality
-Krammers-Krönig relation
Structure of self-energy
Physically reasonable Initial guess: Σano is confined in a finite range of ω
-ImΣano is an odd function of ω
Fraction of electron observed: Determined afterwards self-consistently
Rigo
rous
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initial guessoptimized
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2210 L-1
Rectangular function chosen as basis
-# of paramters: (# of hidden units) x (# of visible units) = 18 x 9 < 2# of visible units = 29 = 512
Coefficients by restricted Boltzmann machine
P. Smolensky (1986)
Flexible Representation of Σnor to Solve the Underdetermined Problem
subtletysubtlety
subtlety
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Flexible Representation of Σano to Solve the Underdetermined Problem
Mixture distribution of Boltzmann Machine D. H. Ackley, G. E. Hinton, & T. J. Sejnowski (1985)
-ImΣano is an odd function of ω
-# of paramters: (# of visible units)2 + # of visible units = 92 + 9 < 2# of visible units = 29 = 512
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Optimizing Self-Energies to Solve the Underdetermined Problem
By minimizing the cost function with prior knowledge, optimize Σnor and Σano
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○ training data ○ test data
Avoiding Overfitting: Cross Validation 1. Dividing data into 2 sets
2. Test data generated by maximum likelihood approach
□ training data ー test data average
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1. Optimizing Σ with training data
Go back to 1.
Initial conditions optimized
2. Measuring cost function with test data
3. Shifting the centers of mass inΣano
Bayesian Optimization Steps to Optimize Σ
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χ2 with constrained Σano
# of optimization steps # of optimization steps
center of m
ass of BMs
Bayesian Optimization Process to Optimize Σ
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-The cost function χ2 becomes 1/3 of χ2
with constrained Σano by Li et al. (2018) -Optimization is robust against noise
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Cost function with test data
noise in the experiment =1.38x10-6
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Benchmark Strong-coupling BCS J. R. Schrieffer, Theory of Superconductivity,
(Taylor & Francis Group, Boca Raton, 1973)
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Benchmark Strong-coupling BCS J. R. Schrieffer, Theory of Superconductivity,
(Taylor & Francis Group, Boca Raton, 1973)
Learning done for
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Angle-resolved photoemission
Reproduced Spectrum of SC Cuprates for T < Tc
OP90K
Bi2Sr2CuO6+δ
Bi2Sr2CuO6+δ
T. Kondo, et al., Nat. Phys. 7, 21 (2011). Bi2Sr2CaCu2O8+δ Optimally doped sample (Tc=90K)
A(ω) by optimized Σ precisely reproduces Aexp(ω)
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Promiment peaks found in both Σnor and Σano
at the same ω (~65 meV)
Self-Energy Obtained by the Bayesian Optimization Process
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Hidden Peak Structure in Σ Revealed
Bi2Sr2CuO6+δ T. Kondo, et al., Nature 457, 296 (2009). UD (Tc=23K)
Marginal FL: C. Varma et al., PRL 63, 1996 (1990). T. Valla, et al., Science 285, 2110 (1999).
Peaks exactly canceled and invisible in total Σ: Reason why it has been overlooked for 30 years
Total Σ: M. R. Norman, et al., PRB 60, 7585 (1999)
Universal ω-linear ImΣnor
due to the peak structure: Planckian dissipation, Possible holographic fluid
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Hidden peaks in Σnor/ano indeed generate SC gap and explain large gap with small anomalies in spectra
Peak structures in Σ and SC: T. Maier, D. Poilblanc, D. Scalapino, PRL 100, 237001 (2008).
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What Determines Tc ?: Attractive Interaction Estimated from ImWPEAK
Effective attractive interaction:
Normalization function:
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1° (AN)11°21°31°42° (N)
Inelastic relaxation rate:
ARPES of Bi2Sr2CuO6+δ T. Kondo, et al., Nature 457, 296 (2009).
cf.) Marginal FL: C. Varma et al., PRL 63, 1996 (1990).
What Determines Tc ?: Planckian Dissipation from ImΣPEAK
Planckian dissipation with universal Γ: J. Zaanen, Nature 430, 512 (2004).
-Even in SC phase, electrons form fluid -ImΣPEAK generates both high-Tc & Planckian dissipation
OP35K
0
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UD23KOP35KOD29KOP90K
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What Determines Tc ?
C. C. Homes et al., Nature 430, 539 (2004).
Y. J. Uemura, et al., PRL 62, 2317 (1989).
-Uemura plot Tc ∝ superfluid density -Homes plot σ(Tc)Tc ∝ superfluid density
kBT c= g(kAN) x F(kAN) x Γ(kN) Effective interaction Dissipation
SC order parameter
Bi2201 UD23KBi2201 OP35KBi2201 OD29KBi2212 OP90K
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Bi2212
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Cost function with test data
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Im o anomaly in ImΣano
Comparison with Previous Studies Compared with J. M. Bok, et al., Sci. Adv. 2, e1501329 (2016).
Compared with H. Li, et al., Nat. Commun. 9, 26 (2018).
Normal Anomalous The present method -obtained Σnor and Σano
separately at both N and AN regions for UD, OP, and UD cuprates -revealed cancellation of peaks in both Σnor and Σano
The present method -showed χ2 is 2/3 smaller -revealed peak structure responsible for SC -revealed that the peak intensity is involved in Tc determination
χ2 with constrained Σano by Li et al.
constrained Σano φ=20°
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Summary
Hidden peaks in both Σnor and Σano
-Origin of both high-Tc SC -Cancelled each other and invisible in total Σ Possible origin of the peak structure: Dark fermion scenario S. Sakai, M. Civelli, M. Imada, PRL 116, 057003 (2016). cf.) Singularities in Σ in normal state T. D. Stanescu & G. Kotliar, PRB 74, 125110 (2006).
What determine Tc ?
Y. Yamaji, T. Yoshida, A. Fujimori, and M. Imada, arXiv:1903.08060.
-Peak intensity (~g) & dissipation (~Γ) kBT c= g(kAN) x F(kAN) x Γ(kN)
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Spectroscopy of hidden physics