fe superconductors pressure

8/3/2019 Fe Superconductors Pressure

1/14

IOP PUBLISHING REPORTS ON PROGRESS IN PHYSICS

Rep. Prog. Phys. 74 (2011) 124502 (14pp) doi:10.1088/0034-4885/74/12/124502

Pressure effects on two superconductingiron-based families

Athena S Sefat

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

Received 16 April 2011, in final form 7 July 2011Published 12 September 2011Online at stacks.iop.org/RoPP/74/124502

Abstract

Insight into the mechanism of high-temperature superconductivity can be gained bypressure-dependent studies of structural, thermodynamics and transport data. The role of

pressure may be complicated by the level of hydrostaticity. High-pressure studies on twoiron-based families ofRFeAsO (R = rare-earth metals) and AFe2As2 (A = alkaline-earthmetals) are reviewed here.

(Some figures in this article are in colour only in the electronic version)

Contents

1. Introduction 12. Ambient-pressure studies 23. High-pressure studies 3

3.1. Pressure-induced properties of RFeAsO 6

3.2. Pressure-induced properties of AFe2As2 7

3.3. Pressure-induced collapsed-tetragonal structure 9

4. Concluding remarks 11

Acknowledgments 12

References 12

1. Introduction

In spite of great progress in the expansion of bothfundamental and practical knowledge about superconductors,the underlying microscopic mechanism responsible for high-temperature superconductivity (HTS) remains elusive. Astate of superconductivity is defined by the existence of zeroresistance and a Meissner effect below a superconductingtransition temperature (TC) and critical magnetic field. Thusfar, HTS has been achieved in two classes of copper-basedoxides(cuprates)[1] andiron-basedsuperconductingmaterials(FeSCs) [2]. HTS emerges in cuprates and FeSCs through thesuppression of magnetism in the undoped materials, parents,which are in a Mott-insulating or a semi-metallic spin-density-wave form, respectively.

The superconducting state is produced primarily throughchemical substitutions (x), but may also be invoked by highpressure (P). The elemental substitution can modify theelectronic band structure and the density of state at the Fermilevel through crystal lattice size and the carrier concentrationchanges (additions of holes or electrons). With the useof chemical dopants in the two HTS classes, TC values

as high as 133K in HgBa2Ca2Cu3O8+ [3], and 55K inSmFeAsO0.9F0.1 [4], Gd1.8Th0.2FeAsO[5] orSmFeAsO0.85 [6]

have been reached. For these, the superconducting region hasa dome shape where TC first increases with x (underdoped),reaches a maximum value at the optimum x (optimaldoped or xC) and then decreases with further increase in x(overdoped); see figure 1. The pressure alters the latticeconstants through bond lengths and angles that inevitablyaffect the electronic and magnetic correlations, influencingparameters such as electron-transfer integral and exchangecoupling. Remarkably, the application of high pressure onthe parents of FeSCs has also induced superconductivity. Thepressure at which the superconducting transition temperatureis highest is defined as critical pressure (PC); see figure 1.For example, TC 20 K in LaFeAsO at 120 kbar [7], TC 30K in BaFe2As2 at 38kbar [8], TC > 20K in FeSe at15 kbar [9] and TC 46 K in Sr4V2O6Fe2As2 at 38 kbar [10].Although the pressure-induced superconductivity is commonin the parents of FeSCs, it is unusual in the parents of cuprates;Bi1.98Sr2.06Y0.68CaCu2O8+ (Bi2212) is the only example thatgives signs of superconductivity at TC 10 K in 30 kbar [11].Alternatively, superconductivity may be improved by pressurethrough compression of a superconducting material. TheTC improves for HgBa2Ca2Cu3O8+y (Hg-1223) at 150 kbar

to TC = 153K [12], for LaFeAsO0.89F0.11 at 40kbar toTC = 43K [13], and for FeSex at 70 kbar to TC 37K [14].

0034-4885/11/124502+14$88.00 1 2011 IOP Publishing Ltd Printed in the UK & the USA
http://dx.doi.org/10.1088/0034-4885/74/12/124502http://stacks.iop.org/RoPP/74/124502http://dx.doi.org/10.1088/0034-4885/74/12/124502


2/14

Rep. Prog. Phys. 74 (2011) 124502 A S Sefat

The dome-shaped pressure dependence of TC has been seenin both the cuprates and the FeSCs, where pressure initiallyenhances TC to an optimum value, but TC then deteriorates[13, 1518]. This suggests that the strength of the Cooper-pair coupling changes, and the electronic state having themaximum TC under high pressure is regarded as the optimum

doped [13, 18]. A large value of the pressure derivative ofTC,dTC/dP, is a good indication that higher TC may be possiblethrough chemical means, via doping of smaller atom sizes anddifferent valences. Theoretical calculations have predicted theenhancement of density of stateswith pressure for a few FeSCs[19] and the superconducting dome with modest pressures forcuprates [20].

This report summarizes the studies on two RFeAsO[2] and AFe2As2 [21] iron-based superconducting families,with a focus on high-pressure studies. The families aretypically referred to as 1111, and 122, and have quasi-two dimensional tetragonal crystal structures of ZrCuSiAs-and ThCr2Si2-types [22, 23], respectively (figure 2). In bothstructures, the iron atoms are coordinated by four arsenic

Figure 1. Schematic phase diagram of copper-oxide-based oriron-based superconductors. The antiferromagnetic order issuppressed by chemical doping (x) or pressure, producing asuperconducting region. The doping fraction (xC) or pressure (PC)at which the superconducting transition temperature is optimized(Max TC) is shown.

Figure 2. Schematic ofRFeAsO (R = rare-earth element) and AFe2As2 (A = alkaline-earth metal) crystal structures in P4/nmm andI4/mmm, respectively. These families are called 1111 and 122, respectively. The common FeAs layers are highlighted in gray.

atoms resulting in layers of edge-sharing FeAs4 tetrahedralin the ab-plane. In the crystal structure, the FeAs layersalternatewithinter-layersconsistingofrare-earthoxygen( RO)or alkaline-earth metal (A); see figure 2. Although there isbonding between Fe and As (d = 2.32.4), the states nearthe Fermi level are dominated by the five Fe 3dorbitals lightly

mixed with As p states [24, 25]. The FeFe distances are2.802.85 . At room temperature, the lattice parametersfor LaFeAsO are a = 4.0345 and c = 8.7387 [26], andfor BaFe2As2 are a = 3.9635 and c = 13.022 [27]. Theapplication of high pressure may vary the carrier concentrationby increasing the charge transfer between the inter- and intra-planes through the lattice compressions, induce an anisotropicshrinkage, improve sample connectivity through compactingof grains, and change the electronic density of states at theFermi level; these effects can be similarly created by chemicalsubstitutions.

2. Ambient-pressure studies

The RFeAsO and AFe2As2 parents have been the subject ofelemental-substitution studies since HTS at TC = 28K influorine-doped LaFeAsO1x Fx with x = 0.11 was reported[2]. The antiferromagnetic order in these parents, a result ofitinerant electrons nesting at the Fermi surface [24, 28, 29],may give way to superconductivity through the modificationof theelectronic andphonon structures, by theadditionof holesor electrons at ambient pressures.

The LaFeAsO parent undergoes a continuous or weaklyfirst-order structural phase transition from tetragonal toorthorhombic (Cmme) upon cooling below 155160K; thisis followed by a commensurate antiferromagnetic order justbelow 135150K [30, 31]. The lattice parameters at 4 Kare a = 5.6823, b = 5.7103 and c = 8.7117and the FeFe distances are 2.8412 and 2.8551 [26].The neutron diffraction and Mossbauer spectra indicate amoment in the range 0.250.35B/Fe [3032]. Carrier dopingplays a major role in the appearance of superconductivityby suppressing the structural phase transition and magneticorder. For example, substitutions of O2 with F inLaFeAsO1xFx [2, 3335] or Co2+ with Fe2+ in LaFe1xCoxO

2


3/14


Figure 3. The doping-dependent phase diagram of LaFeAsO1x Fxshowing the structural, magnetic (SDW) and superconductingtransition temperatures. Reprinted by permission from Macmillian

Publishers Ltd: Nature Mater. [36], copyright 2009.

[26] add electrons in the FeAs layers. With substitution ofthe smaller cobalt or fluorine, the size of the unit cell isreduced, thecarrier concentration is changed, thestructuralandmagnetic transitions are suppressed, and a superconductingregion appears; see figure 3 [24]. For optimal-dopedLaFeAsO0.89F0.11, a- and c-lattice constants are found toshrink by 0.17% and 0.30%, respectively, at room temperature[33]. With incorporation of the smaller cobalt in the Fesite, the cell volume is similarly reduced; for optimal-dopedBaFe0.89Co0.11AsO with TC 14K, cell volume shrinks

by 0.26% primarily due to contraction of the c-axis [26].Substitution of La for other rare earths (R) has also resulted infinding F-doped RFeAsO1xFx superconductors for R = Ce,Pr,Nd,Sm,Gd,TbandDy[3743],givinga TC ashighas55Kfor SmFeAsO0.9F0.1 [41]. The substitution of lanthanum withlater rare earths causes the expected lanthanide contraction ofthe unit cells, while fluorine provides electrons and change incarrier concentration.

Chemical dopingon theR-sitewithThorSralsoproducessuperconductors [5, 44]. In addition, superconductivity can beinduced by introducing oxygen vacancies into the RFeAsO1system, giving highest TC = 55KforSmFeAsO0.85 [6, 4548].Density functional calculations have shown that both oxygenvacanciesandF-dopingactinasimilarwaytointroducechargecarriers into the FeAs layer [49].

The AFe2As2 materialsexhibitan antiferromagnetic, spin-density-wave ordering of Fe spins below 170K in A =Ca [50, 51], 205K in A = Sr [52], 132K or 140K inA = Ba [21, 53], and at 200K in A = Eu [54]. Thesetransitions are likely coupled with a structural change fromthe tetragonal to an orthorhombic Fmmm phase, where thesquarenetsofFebecomerectangular[21]. For thesematerials,neutron scattering gives moments of 0.8 to 1.01B/Fe[5557],while the derived moments from Mossbauer are smaller (0.4to 0.6B) [5860]. For A

=Eu compound, there is an

additional magnetic moment of 7.5B that is carried by Eu2+below TN = 20K [54]. Chemical substitutions may be

Figure 4. The cobalt doping-dependent phase diagram forBa(Fe1x Cox )2As2, shows antiferromagnetic (AF) and structuraltransition (O orthorhombic to T tetragonal), and the ordered

magnetic moment found by neutron diffraction (inset). Thesuperconducting dome (SC) is shown. This figure is reprintedfrom [64], copyright 2009 by the American Physical Society.

used to suppress the magnetic and structural phase transitions,stabilizing a superconducting dome [2, 27, 61, 62] or anothermagnetic phase [63]. For example, the Tx phase diagram forelectron doping by means of cobalt, in Ba(Fe1x Cox )2As2,is shown in figure 4 [64]; for x 0.06 a small decrease(0.26%) in the c-axis is observed (c = 12.980) and themagnetic and structural transitions are suppressed, achievinga maximum TC at 22 K [27]. Superconductivity is also found

in isovalently doped materials with Ru for Fe [65] and P forAs [66]. Such isovalent-doping primarily tunes the structuralparameter similar to pressure-induced superconductivity.

3. High-pressure studies

With the application of pressure in place of chemicalsubstitution, some of the undesirable effects, such as latticedisorder, impurity phases and phase separations, can beeliminated. Pressure can be applied with great precision and isreproducible. In HP studies, kilobarsor gigapascals are usuallyused as pressure units (10 kbar = 1GPa 100kgmm2);

both will be used throughout this review. The four HPtechniques commonly used in studies of superconductingphenomena, in the order of decreasing pressure quality,are gas-pressure, cubic-anvil, piston-cylinder and opposed-anvil. The non-hydrostatic stress transmitted to the samplein various high-pressure devices will depend on a variety offactors including loading of the sample (uniaxial versus multi-axial loading), sample geometry and shear strength of thepressure-transmitting medium as well as the shear strengthof the high-pressure sample. In general, a combination ofmulti-axial loading with the use of a gas or liquid pressure-transmitting medium and a soft sample will result under anear hydrostatic condition. In the gas-pressured cells, low

but isotropic pressures from gases are reached below 8 kbar.In the cubic-anvil press (CAP), a sample is immersed in an

3


4/14


Table 1. A summary of pressure studies on the 1111-type families of FeSCs. Studies using a pressure Technique of piston-cylinder cell(PCC), cubic-anvil press (CAP) or diamond anvil cell (DAC), in association with a particular pressure Medium are listed. Highest TCvalues (Max TC in K units) at the critical pressure (PC in GPa) are given. For each study, the maximum pressure measured is indicated(Pmax in GPa units). The evidence of zero resistance below TC (0 R) or diamagnetic signal (Diamag.) are shown by a check-mark (

);

the negative results are shown by a cross (). The dashed symbol () indicates not reported. All the materials are polycrystalline samples.The acronym FC is used for Fluorinert.

Material Technique Medium Max TC [PC] 0 R Diamag. Pmax Ref.PCC FC70 : 77=1 : 1 3

CAP FC70 : 77=1 : 1 21 [12] 12 [13]

DAC NaCl 30

LaFeAsO CAP FC70 : 77=1 : 1 21 [12]

12 [7]

DAC NaCl 29DAC NaCl 21 [12] 35 [72]PCC FC70 : 77=1 : 1 3

SmFeAsO CAP FC70 : 77=1 : 1 11 [9] 12 [13]

DAC NaCl 30

PCC FC70 : 77=1 : 1 40 [3] 3PCC FC70 : 77=1 : 1 28 [1.1]

1.1 [73]

DAC NaCl 43 [4] 30LaFeAsO0.89F0.11 PCC n-pentane: isoamyl alcohol = 1: 1 31 [1]

9.4

[74]DAC 32 [7] 18DAC 31 [3] 13

LaFeAsO0.86F0.14 PCC FC70 : 77=1 : 1 43 [5]

1.5[18]

DAC NaCl 17LaFeAsO0.5F0.5 DAC 45 [2.7] 20

LaFeAsO1 DAC 50 [1.5] 13

oil-liquid medium encapsulated in a Teflon cell, surroundedwith a pyrophyllite block. Pressures up to 110 kbar canbe generated from six directions using tungsten carbideanvils. In the piston-cylinder cell (PCC), measurementsbelow 30 kbar are carried out on a sample that is immersed,usually in a liquid, and encapsulated in a cylindrical Tefloncell. A closely related indenter cell (IC) can achieve 45 kbar.Finally, in theopposed-anvil technique, also known as alumina(AAC), diamond (DAC) or Bridgman (BAC) anvil cells,two oppositely faced surfaces enclose a sample that may besurrounded by a medium. The ultra-high pressure suppliedby this technique may as high as several hundred kilobars,although it is expected to provide the most deviation fromhydrostaticity with the largest uniaxial component. Thepressure measurements in high-pressure devices can be carriedout using a variety of methods which can be adapted tothe specific high-pressure device being employed. In thecase where an optical access is available, ruby fluorescenceis a convenient method for pressure calibration at lowtemperatures. Theruby is a secondary pressure scale which hasbeen calibrated against known equation of state of materials.In cases where x-ray measurements can be performed, the

measured lattice parameters or volume of metals (copper,platinum, etc) at high pressures and low temperatures can

be used in pressure calibration. In the CAP, pressure valuesare obtained from the calibration curve using fixed pointsfor well established structural phase transitions in Bi, Te,Sn or ZnS.

Hydrostaticity is the ability to maintain a homogeneoushydrostatic pressure within the media, since it governs thepressure distribution inside the sample chamber. In orderto control hydrostaticity and to homogenize the pressure, theuse of a pressure-transmitting medium is crucial. Condensedgases (e.g. He, Ne, N

2, Ar), liquids (e.g. alcohol or

mixtures, oils), soft solids (e.g. alkali halides) or hard solids(e.g. MgO, Al2O3, NaCl, BN) can be the media. Atpressures above their solidifications, a blend of hydrostaticand uniaxial pressures can develop. For example, liquidsof Fluorinert70/77, Daphne7373, and Daphne7474 solidifyabove 1.0 GPa, 2.2 GPa and 3.7GPa, respectively. Themost common methanolethanolwater pressure-transmittingmedium used in DAC devices provide hydrostatic conditionsup to 16 GPa at ambient temperature. The response of liquidsand gaseous media at low temperatures and high pressures iscurrently being investigated for their suitability as hydrostaticmedia for superconducting materials.

The effects of high pressure on specific materials in thefamilies of RFeAsO and AFe2As2 are summarized in the

4


5/14


Table 2. A summary of pressure studies of 122-type parents of FeSCs. The pressure Technique of alumina anvil cell (AAC), Bridgmancell (BC), cubic-anvil press (CAP), diamond anvil cell (DAC), piston-cylinder cell (PCC) or indenter cell (IC), along with a specific pressureMedium are listed. Maximum superconducting transition temperatures (Max TC in K units) at the critical pressure ( PC in GPa) arelisted. Below TC, zero resistance (0 R) or negative susceptibility (Diamag.) may be observed, indicated by check-marks (

); negative

results are shown by a cross (). Dashed symbol () indicates not reported. All samples are single crystals, except those indicated by *,which are polycrystals. FC represents Fluorinert.

Material Technique Media Max TC

[PC

] 0 R Diamag. Pmax

Ref.

PCC Pentaneisopentane 1: 1 3 [83]BaFe2As2 AAC Daphne oil 7373 25 [2.9] 5.5

BC Steatite 33 [1.1] 6.2BC FC 70/77 1 : 1 30 [5.5]

8 [82]

BC Steatite 35 [1.5] 7.2 [85]* BC 25 [4] 4

BC FC 70/77 31 [5.5]

5.5 [105]

BC FC 70/77 35 [3]

9 [88]

CAP FC 70/77 1 : 1 25 [3]

13 [84]

CAP Glycerin 8 [86]DAC Daphne Oil 7373 29 [4]

6 [8]

IC Daphne 7373 4.5 [95]PCC Daphne oil 7373 2.4 [91]BC FC 70/77 1 : 1 38 [3.5]

7.5 [82]

CAP FC 70/77 1 : 1 30 [6]

8 [86]

SrFe2As2 * PCC glycerine 1.5

* CAP FC 70/77 1 : 1 38 [1.8] 8 [94]* DAC NaCl 14

DAC Non 32 [1.3] 12.8 [106]DAC Daphne Oil 7373 27 [3]

5.3 [8]

Daphne 7474 30 [4.4]

4.5

IC Daphne7373 30 [3.6]

4.3 [95]

Stycast 1266 30 [3.4]

3.6

PCC Silicone oil 40 [2.5] 3 [93]PCC n-pentane mineral oil or FC-75 35 [1.9] 2 [92]PCC FC-75 12 [0.5]

2 [81, 97]

CaFe2As2 PCC Helium 0.65 [100]

PCC Silicone oil 13 [7] 0.69 [96]PCC Silicone oil 12 [5]

15 [98]

DAC Non 41 [10] 70 [107]EuFe2As2 PCC Silicone oil 30 [2] 2.3 [102]

PCC Daphne 7474 30 [2.8]

3 [103]

BaFe1.92Co0.08As2 PCC Daphne oil 7373 21 [2.5]

2.4 [91]

BaFe1.80Co0.20As2 24 [2.5]

FC-75 orFC 72/84 1: 1

BaFe1.91Co0.09As2 PCC 25 [2.0]

2.0 [108]

BaFe1.85Co0.15As2 25 [0.2]

1.9

5


6/14


sections below. Tables 1 and 2 summarize the maximumTC values (Max TC) achieved at a critical pressure (PC)for a pressure technique and a pressure-transmitting medium.Comparisons between pressure studies can be made byassuming negligible discrepancies in material stoichiometryor purity. A combination of techniques may have been used to

cover various pressure regions in a report; these are groupedtogether for the same references. The failure to observe

Figure 5. The effect of pressure on the band structure and density ofstates of LaFeAsO; Vo represents the cell volume. The figure isreprinted from [67] with permission from Institute of PhysicsPublishing.

Figure 6. For LaFeAsO1x Fx series, the maximum TC values obtained under atmospheric and also under high pressure (left), and thedome-shaped feature of superconducting transition temperature versus pressure (right). With kind permission from SpringerScience + Business Media: [13].

Figure 7. Pressure dependence of the superconducting transition-temperature (TSC

) and superconducting shielding volume-fraction insingle crystals [8]. For A = Sr and Ba, DAC was used with Daphne Oil 7373 medium. For A = Ca, PCC was used with Fluorinert FC-75medium. The figure is reprinted with permission from Institute of Physics Publishing: [8], copyright 2008.

zero resistance (0 R) may be caused by a distribution inthe non-hydrostatic compressive stress or may suggest thatthe apparent superconductivity is not bulk. Only a fewreport magnetic susceptibility results and the existence of theMeissner effect (diamag.). The pressure-induced propertiesof RFeAsO and AFe2As2 families are summarized below,

offering a more extensive summary of the effects of pressurein Fe-based materials, compared with the early review papers;see [6871].

3.1. Pressure-induced properties of RFeAsO

Pressure-induced properties of RFeAsO are dependent onthe rare-earth R; see table 1. Theoretical calculations onLaFeAsO show the effects of pressure on the band structurein figure 5 [67], suggesting that pressure can induce changesin the electronic properties. With a shrinking cell volume(Vo), the band positions change and the density of statesmove. For example, the first peak above the Fermi level,

Ef, is moved towards it with pressure. Experimental datafor LaFeAsO indicate that pressure-induced superconductivityis caused when there is weak magnetic order. Transportresults show a decrease in structural and magnetic transitionsat a rate of 13.7KGPa1 [13], while 57Fe Mossbauerspectroscopy suggests a drastic decrease in the saturatedhyperfine field [72]. LaFeAsO becomes a superconductorwith maximum TC of 21K at 12GPa [7]. For the closely

6


7/14


Figure 8. Temperature-dependence of electrical resistance under applied pressures for BaFe2As2 polycrystalline samples (left panel) andsingle crystals (right two panels), in steatite medium and BC. The figure is reprinted with permission from [ 85].

related material of CeFeAsO, however,no superconductivity isobservedunderpressuresof 50 GPa, with FeandCe magnetismcoexisting below 15 GPa [73]. The NdCoAsO also remainsnon-superconducting down to 10 K and 53 GPa [74]. But,for SmFeAsO, the structural and magnetic transitions decreasewith pressure at a rate of6KGPa1, giving a maximum TCof 11K at 9GPa [13]. Generally, the magnetic rare earthsmay prevent the formation of superconducting electron pairsby hybridization of the f- and the conduction electrons.

Forthe1111superconductors,thebehaviorunderpressurecan vary; see table 1. The ambient-pressure TC valuesfor LaFeAsO1xFx improve with the application of pressure(figure 6) [13]. The pressure dependence of TC is domeshaped and the rate of dTC/dP is dependent on chemicalcomposition [13, 18, 75, 76]. For x = 0.05, the rate is only12KGPa1 and a maximum TC of29 K is reached, whilefor x = 0.11, the rate is steep at 5KGPa1 and a TC of 43 Kis achieved at 4GPa [75, 76]. However, CeFeAsO0.88F0.12shows remarkable pressure-induced suppression of TC from44K at ambient pressure to less than 1K at 265kbar [76].For NdFeAsO0.85 and NdFeAsO0.6, the monotonic rate of

TC decrease is 3KGPa1

[77, 78]. For SmFeAsO1xFx ,the pressure is reported to suppress the superconductingstate, with varying dTC/dP rates for different x [79, 80]. Adecrease in TC was also observed in the optimal oxygen-doped SmFeAsO0.85 at 2.0KGPa1 [78]. The negativepressure effects on TC for these compounds are probably dueto a decrease in the electron density of states at the Fermilevel [79].

3.2. Pressure-induced properties of AFe2As2

The pressure-induced results on the AFe2As2 parents aresummarized in table 2. One of the first pressure-induced

superconductivity reports for A = Ba, Sr and Ca compoundsshow that the region of superconductivity is found to decrease

withthesmallerA ionic size; seefigure7 [8, 81]. TheMeissnereffects for A = Ba and Sr are observed in the pressurerange 2855kbar, having bulk superconductivity within anarrow pressure range around 40 kbar [8]. In addition to suchreports, a few studies have failed to reproduce these resultson AFe2As2. The observation of superconductivity in 122 ishighly sensitive to pressure conditions, and perhaps samplequality. The results on a batch of crystals with the use ofthe same pressure cell and medium is, however, found to be

reproducible [82].For BaFe2As2 and in all reported cases, pressure

suppresses the coupled magnetic and structural transitions(TN, To) to lower temperatures with rates in the rangeof7 to 24KGPa1 [8, 8387]. For a pressure condition,the temperature dependence of electrical resistance for apolycrystalline sample is different from that for a single crystalsample, in that zero resistance is not observed for the former;see figure 8 and table 2 [86]. In addition to the sample quality,the particular level of hydrostaticity seems to matter. Thestudy of the same batch of single crystals showed that differentpressure media can have different effects on the PT phase

diagram [83]. In fact, no superconductivity is seen up to 8 GPafor a crystal in glycerin in a CAP [86] and up to 3.1GPafor a crystal floating in pentaneisopentane in a PCC [83];see figure 9. But, superconductivity in BaFe2As2 with zeroresistance can be achieved at 3 GPa for a crystal pressurizedin Fluorinert 70/77 in a BC [88], while bulk or filamentarysuperconductivity appears at 1.1 GPa for a crystal embeddedin steatite with its c-axis normal to the BC anvil faces; seefigures 8 and 9 [85, 88]. It seems that an increasing uniaxialpressure favors the appearance of a superconducting transitionin BaFe2As2. There are different pressure effects for variouscompositions in Ba(Fe1x Cox )As2 [89, 90] (figure 10). Forx

=0.04 and under 2.4 GPa, TN decreases from the ambient-

pressure value of 66K to 52 K while TC increases by a factorof two to 21 K [82]. The changes in the ambient TC values are

7


8/14


Figure 9. Resistivity measurements on BaFe2As2 single crystals using glycerin in a CAP [86] (top left), pentaneisopentane in a PCC(bottom left) [83], Fluorinert 70/77 in a BC (top right) [88], and steatite in a BC (bottom right) [83]. The figure from [86] is reprinted withpermission from the Physical Society of Japan. The figure from [88] is reprinted with permission, copyright 2011 by the American PhysicalSociety. The figures from [83] are reprinted with permission from Institute of Physics Publishing.

negligible for the optimum and overdoped compositions (x >0.06) [8991]. For the cobalt underdoped compositions, themaximum TC can be large at higher applied pressures through

the suppression of magnetic and structural transitions [90].Also, for optimum-doped Ba0.55K0.45Fe2As2, TC decreasesunder pressure at a rate of2KGPa1 [92]. These resultssuggest that proximity to magnetic instability is important inobserving superconductivity.

For SrFe2As2, the structural and magnetic transitionswere reported to decrease at a rate of 10 to 13KGPa1[93, 94]. At a critical pressure, the magnetic susceptibilityresults have shown superconducting volume fraction [8, 86]and zero resistance below TC [82, 86, 95]; see figure 11and table 2. High-pressure x-ray diffraction and resistivitydata show that the temperature of the structural transition(tetragonal to orthorhombic) overlaps with the anomaly in

transport data, suggesting that both TN and To occur at thesame temperature [93]. The pressure dependence of these

transitions is shown to be different above 3GPa for thedifferent media [95]; see figure 12. The PC is shown tobe lower for a medium that solidifies at a lower pressure,

suggesting that theuniaxialstressmay be important to promotethe suppression of the antiferromagnetic and orthorhombicphase. The pressuretemperature phase diagram for the useof three different pressure-transmitting media is shown infigure 12 [95].

For CaFe2As2, superconductivity is observed at TC 10 K underquasi-hydrostatic pressuresof5kbar[81, 9699],and not in hydrostatic pressures [100] (table 2). The schematicof the typical temperaturepressure phase diagrams are shownin figure 13. In this figure, the results from the use ofa liquid-medium clamp cell and a helium pressure cell areshown [98, 100]. With pressure, the magnetic and structuraltransitions are suppressed, a region of superconductivity

is induced, and a non-magnetic collapsed-tetragonal phase(see section 3.3) begins to develop [81, 98, 99]. The

8


9/14


Figure 10. The temperaturecomposition (x) phase diagram for Ba(Fe1x Cox )As2, with results of spin-density-wave (SDW) andsuperconducting (SC) transitions under ambient condition and 2.4 GPa [89] (left). The maximum TC achieved under pressure for various x

in Ba(Fe1x Cox )As2 (right). The figure is reprinted from [90] with permission from Institute of Physics Publishing.

Figure 11. For SrFe2As2 crystals under pressure, the bulk superconductivity is shown by a combination of zero resistivity (left) and negativemagnetic susceptibility, (right). In this study [86], pressure is generated using a CAP technique and glycerin as a pressure-transmitting

medium. The figure from [86] is reprinted with permission from the Physical Society of Japan.

antiferromagnetic phase is mainly associated with theorthorhombic phase; the coexistence between orthorhombicand collapsed-tetragonal phases may occur due to non-hydrostatic pressure components [99, 101].

For EuFe2As2, the antiferromagnetic transition associatedwith Fe is suppressed with pressure, but that associated withEu2+ is insensitive to pressure; a superconducting featureis observed above 2GPa at TC 30 K in resistivity data[102] (see figure 14). Another report gives a combination ofzero resistivity and full superconducting shielding at 2.8 GPa[103]. For the similar EuCo2As2 material, a tetragonal to

a collapsed-tetragonal phase is reported at 4.7GPa [104],without superconductivity.

3.3. Pressure-induced collapsed-tetragonal structure

The compression behavior of a couple of members of the1111 superconductors has been studied using high-pressurex-ray diffraction; see table 3. Because the studies are done atroom temperature, a correlation between the superconductingphase and the changes in low-temperature structures is yet tobe determined. For NdFeAsO0.85, both a- and c-axes, andhence the NdO and FeAs bond distances, decrease withpressures up to 8 GPa. The interlayer spacing decreases ata rate of 0.7%GPa1 while the AsFeAs and NdONd

angles stay constant across the pressure region [109]. ForNdFeAsO0.88F0.12, an isostructural (same space group of

9


10/14


P4/nmm) transformation occurs from the tetragonal (T) to ahigh-pressure collapsed-tetragonal (CT) phase at 13.5GPa[110]. The c-axis shrinks continuously in the range of033 GPa; however, the a-axis shrinks from 0 to 9.9GPa,increases from 10 GPa to 13.5 GPa (PC), then shrinks againbeyond that point. Therefore, the CT structure has a larger

a-axis and smaller c-axis compared with the T structure below9.9 GPa. The pressure dependences of the FeAs distanceand AsFeAs angle of the CT contrast those in the T phase.The FeAs distance decreases in T phase, but increases withpressure in the CT phase. The AsFeAs angle increases with

Figure 12. The temperature-pressure phase diagram for SrFe2As2parent, showing antiferromagnetic (AFM), paramagnetic (PM), andsuperconducting (SC) phase space, and their dependence on threepressure-transmitting media [95], used in association with IC. Thefigure from [95] is reprinted with permission from the PhysicalSociety of Japan.

Figure 13. Left: temperaturepressure phase diagram for CaFe2As2 under a non-hydrostatic condition, showing a superconducting dome.Solid and open squares are measurements made upon increasing and decreasing pressure, respectively. The vertical hashed lines separate theregions of orthorhombic (O) phase, O and collapsed-tetragonal (CT) phase, and CT phase. The phase transitions found in variousmeasurements are indicated by different symbols. Right: temperaturepressure phase diagram for CaFe2As2 under a hydrostatic condition,showing no superconductivity. The different symbols correspond to the transition temperatures found in magnetic susceptibility or electricalresistance data. The hollow diamonds correspond to a transition in magnetic susceptibility and the green straight line near these symbols isthe reference of helium solidification temperature. Reprinted with permission from [98] and [100]. Copyright 2011 by the AmericanPhysical Society.

pressure in the T from the ambient 110.5(1) value, whileit decreases with pressure in the CT phase to 112.2(4) at17.7 GPa, lowering distortion of the FeAs4 tetrahedron. Atambient pressure, TC is near 50K for cell volume 134.7(1)3

and it is proposed to reach its maximum at 126.2(3) 3 beforethe phase transition at 9.9GPa [111].

The transition to a CT phase is found to be commonamong materials with ThCr2Si2 structure [104, 110]; Table 3summarizes the work on these. For BaFe2As2, a reportgives an anisotropic compression of the tetragonal unitcell (a- and c-axes reducing by 2% and 11%) up to17 GPa at room temperature, at which point there is atransition to an orthorhombic phase [112]. However,another report finds the CT structure at 26 GPa underhydrostatic helium pressure [113], and at 17 GPa under non-hydrostatic conditions [114]. Figure 15 shows the measuredlattice parameters for the tetragonal structure with non-hydrostatic pressure. While the c-axis shows a decrease withpressure, a negative compressibility is observed for the a-axis

between 5 and 16 GPa. The pressure range between 17and 26 GPa presents the region at which TCT transition mayoccur below 10 K, along with a superconducting transitiontemperature [114]. High-pressure x-ray diffraction onSrFe2As2 between 140and200Kestimatethatthetetragonalto orthorhombic phase distortion may be suppressed to 0 K at4 to 5GPa [93]. At ambient temperature, a phase transitionfrom the T to a CT phase is observed above 10GPa [114](figure 15), and superconductivity may be stable below 10 Kin the pressure range of 1018GPa [98]. For CaFe2As2,the antiferromagnetic orthorhombic phase transforms to aCT non-magnetic phase at 0.35 GPa and 50 K; this transition

causes a dramatic decrease in the unit cell volume (5%), thec-lattice parameter (9.5%), and the c/a ratio (11%) [99]. Thephase transition to the CT phase is reported also at 0.3 GPa

10


11/14


at 40 K, and at 1.7GPa at 300 K [113]. The transition to theCT phase occurs in the two Ba and Ca compounds at nearlythe same values of unit cell volume and c/a ratio [113]. ForEuFe2As2, room-temperature data (figure 16) show an initialdecrease in a-lattice parameter with pressure, then an increasewith increasing pressure with a peak at 8.5 GPa, followed by

a compression behavior [107]. For the c-lattice parameter, arapid decrease is observed up to 8 GPa, followed by a smallerdecrease in pressure. This anomalous compression seems tobe correlated with therapid rise in TC to41Kataround10GPa,then a decrease in the CT phase (figure 16) [107].

4. Concluding remarks

In the simple conventional metals, the pressure dependenceof TC is negative, for example in Sn (0.482KGPa1), In(0.381KGPa1) and Pb (0.365KGPa1). Moreover,

Figure 14. The antiferromagnetic transition temperature associatedwith Fe (To), Eu(TN), and superconducting transition temperature(TC) as a function of pressure. Reprinted with permissionfrom [102]. Copyright 2011 by the American Physical Society.

Table 3. A summary of the structural x-ray diffraction performed under pressure. PC is the critical pressure, in GPa, for the transition to thecollapsed-tetragonal (CT) phase. Pmax is the maximum pressure measured in GPa. Dashed symbol () indicates not reported. Each studyused diamond anvil cell (DAC) in association with a pressure-transmitting medium.

Material Temp (K) Medium PC (GPa) Pmax (GPa) Ref.NdFeAsO0.85 Daphne oil 7373 8 [109]

NdFeAsO0.88F0.12 300 Silicone oil 13.5 33 [111]

4 : 1 ethanol/methanol 22 [107]

BaFe2As2 17 35 [114]

300, 33 Helium 26, 29 56 [113]

SrFe2As2 180, 142 Helium 4.4 [93]

300 10 23 [106]

50 Helium 0.35 0.63 [99]CaFe2As2

300, 40 Helium 1.7, 0.3

50 [113]

EuFe2As2 300 8.5 70 [107]

the application of pressure has led to several elementalsuperconductors such yttrium (TC 2K for 11 P 16GPa) and barium (TC 1 to 5.4K under 5.5 P 19 GPa). There is no association between properties ofelements and the compounds they can form. For example,neither copper (a non-magnet) nor iron (a ferromagnet)

superconduct but each belongs to one of the two classes ofHTSs. In cuprates and FeSCs, the pressure may inducesuperconductivity in theparents andthedTC/dP behavior maydiffer depending on the composition. The direction normal tothe CuO2 or FeAs layer (c-axis) is more compressible thanthe direction parallel to it. Such anisotropic compression canchange the charge distribution, leading to a modulation of theelectronic state in the superconducting layers.

In FeSCs, uniaxial pressure seems to be importantfor suppressing magnetic order. Superconductivity existswhen there are sufficient fluctuations to allow for Cooper-pair formation. Superconductivity may be induced in theparents, dTC/dP is positive for the underdoped, while it isapproximately constant for the optimal doped and negative inthe overdoped region. For different compounds in the 1111and the 122 families, results of the transport data, magneticdata and structural distortions are given in tables 1 to 3. Theexistence of superconductivity, the value of maximum TC, andthe pressure under which it occurs can vary considerably. Thepressure range across which superconductivity is observed canalso show variation. Studies show that such differences aredependent on the level of hydrostaticity due to differencesin the employed pressure technique and the medium, ratherthan sample quality. Also, the pressure results scatter moregreatly than the results of chemical substitution, on the 122 for

example, suggesting that the effects of sample quality may benegligible. Numerous reports of the tetragonal to collapsed-tetragonal isostructural phase transition suggest that it may bea common occurrence in FeSCs. Additional evidence for therelationship between superconductivity and pressure regionswhere such anomalous compressibility effects occur may be aclue to the mechanism of superconductivity.

11


12/14


Figure 15. The a- and c-lattice parameters of BaFe2As2 (left) [114] and SrFe2As2 (right) [106], refined in I4/mmm space group, as afunction of pressure, at room temperature. Diamond anvil cell was the pressure technique; no pressure-transmitting media were used. Thefigure from [114] is reprinted with permission. Copyright 2011 by the American Physical Society. The figure from [106] is reprinted with

permission from Institute of Physics Publishing.

Figure 16. For EuFe2As2 the pressure dependence behavior ofa- and c-lattice parameters (left), superconducting transition temperature(right). Diamond anvil cell was the pressure technique; no pressure-transmitting medium was used. Reprinted from [ 107] with permissionfrom Institute of Physics Publishing.

The dome-shaped pressure dependence of superconduc-tivity is often found, suggesting that external pressure can con-trol the doping level and that there is an optimum doping valuein FeSCs. The enhancement ofT

Cunder pressure prompts

chemical substitutions of the smaller ions to general latticeshrink at ambient pressure. The highest TC reached underpressure is 50 K, suggesting that a TC exceeding this may befound in the yet to be discovered systems, perhaps differentfrom the 1111 or the 122 families.

Acknowledgments

This work was supported by the US Department of Energy,Basic Energy Sciences, Materials Sciences and Engineering.The authors appreciate discussions with Balazs Sipos and

Yogesh K Vohra. They also thank Teresa Roe for submissionof permissions for the figures.

References

[1] Bednorz G and Muller K A 1986 Z. Phys. B 64 189[2] Kamihara Y, Watanabe T, Hirano M and Hosono H 2008

J. Am. Chem. Soc. 130 3296[3] Schilling A, Cantoni M, Guo J D and Ott H R 1993 Nature

363 56[4] Ren Z A et al 2008 Chin. Phys. Lett. 25 2215[5] Wang C et al 2008 Europhys. Lett. 83 67006[6] Ren Z A et al 2008 Europhys. Lett. 83 17002[7] Okada H, Igawa K, Takahashi H, Kamihara Y, Hirano M,

Hosono H, Matsubayashi K and Uwatoko Y 2008 J. Phys.Soc. Japan 77 113712

[8] Alireza P L, Ko Y T C, Gillett J, Petrone C M, Cole J M,Lonzarich G G and Sebastian S E 2008 J. Phys.: Condens.Matter21 012208

[9] Mizuguchi Y, Tomioka F, Tsuda S, Yamaguchi T andTakano Y 2008 Appl. Phys. Lett. 93 152505

[10] Kotegawa H, Kawazoe T, Tou H, Murata K, Ogino H,Kishio K and Shimoyama J 2009 J. Phys. Soc. Japan78 123707

12
http://dx.doi.org/10.1007/BF01303701http://dx.doi.org/10.1007/BF01303701http://dx.doi.org/10.1021/ja800073mhttp://dx.doi.org/10.1021/ja800073mhttp://dx.doi.org/10.1038/363056a0http://dx.doi.org/10.1038/363056a0http://dx.doi.org/10.1088/0256-307X/25/6/080http://dx.doi.org/10.1088/0256-307X/25/6/080http://dx.doi.org/10.1209/0295-5075/83/67006http://dx.doi.org/10.1209/0295-5075/83/67006http://dx.doi.org/10.1209/0295-5075/83/17002http://dx.doi.org/10.1209/0295-5075/83/17002http://dx.doi.org/10.1143/JPSJ.77.113712http://dx.doi.org/10.1143/JPSJ.77.113712http://dx.doi.org/10.1088/0953-8984/21/1/012208http://dx.doi.org/10.1088/0953-8984/21/1/012208http://dx.doi.org/10.1063/1.3000616http://dx.doi.org/10.1063/1.3000616http://dx.doi.org/10.1143/JPSJ.78.123707http://dx.doi.org/10.1143/JPSJ.78.123707http://dx.doi.org/10.1143/JPSJ.78.123707http://dx.doi.org/10.1063/1.3000616http://dx.doi.org/10.1088/0953-8984/21/1/012208http://dx.doi.org/10.1143/JPSJ.77.113712http://dx.doi.org/10.1209/0295-5075/83/17002http://dx.doi.org/10.1209/0295-5075/83/67006http://dx.doi.org/10.1088/0256-307X/25/6/080http://dx.doi.org/10.1038/363056a0http://dx.doi.org/10.1021/ja800073mhttp://dx.doi.org/10.1007/BF01303701


13/14


[11] Cuk T, Zocco D A, Eisaki H, Struzhkin V, Gosche F M,Maple M B and Shen Z X 2010 Phys. Rev. B 81 184509

[12] Chu C W, Gao L, Chen F, Huang Z J, Meng R L and Xue Y Y1993 Nature 365 323

[13] Takahashi T, Okada H, Igawa K, Kamihara Y, Hirano M,Hosono H, Matsubayashi K and Uwatoko Y 2009J. Supercond. Novel Magn. 22 595

[14] Margadonna S, Takabayashi Y, Ohishi Y, Mizuguchi T,Takano Y, Kagayama Y, Nakagawa T, Takata M andPrassides K 2009 Phys. Rev. B 80 064506

[15] Mori N et al 1991 Physica C 185189 40[16] Lin J G, Matsuishi M, Wang Y Q, Xue Y Y, Hor P H and

Chu C W 1991 Physica C 175 627[17] Mori N, Takahashi H, Shimakawa Y, Manako T and Kubo Y

1990 J. Phys. Soc. Japan 59 L3839[18] Yi W et al 2008 Europhys. Lett. 84 67009[19] Zhang H, Xu G, Dai X and Fang Z 2009 Chin. Phys. Lett.

26 017401[20] Jorgensen D 1990 Physica C 171 93[21] Rotter M, Tegel M, Schellenberg I, Hermes W, Pottgen R and

Johrendt D 2008 Phys. Rev. B 78 020503[22] Quebe P, Terbuchte L J and Jeitschko W 2000 J. Alloys

Compounds 302 70[23] Rozsa S and Schuster H U 1981 Z. Naturf. b 36 1668[24] Singh D J and Du M H 2008 Phys. Rev. Lett. 100 237003[25] Singh D J 2008 Phys. Rev. B 78 094511[26] Sefat A S, Huq A, McGuire M A, Jin R, Sales B C,

Mandrus D, Cranswick L M D, Stephens P W andStone K H 2008 Phys. Rev. B 78 104505

[27] Sefat A S, Jin R Y, McGuire M A, Sales B C, Singh D J andMandrus D 2008 Phys. Rev. Lett. 101 117004

[28] Dong J et al 2008 Europhys. Lett. 83 27006[29] Mazin I I, Singh D J, Joannes M D and Du M H 2008 Phys.

Rev. Lett. 101 057003[30] McGuire M A et al 2008 Phys. Rev. B 78 094517[31] de la Cruz C et al 2008 Nature 453 899[32] Klauss H-H et al 2008 Phys. Rev. Lett. 101 077005

[33] Sefat A S, McGuire M A, Sales B C, Jin R, Howe J Y andMandrus D 2008 Phys. Rev. B 77 174503

[34] Lu W, Yang J, Dong X L, Ren Z A, Che G C and Zhao Z X2008 New J. Phys. 10 063026

[35] Kitao S, Kobayashi Y, Higashitaniguchi S, Saito M,Kamihara Y, Hirano M, Mitsui T, Hosono H and Seto M2008 J. Phys. Soc. Japan 77 103706

[36] Luetkens H et al 2009 Nature Mater. 8 305[37] Chen G F, Li Z, Wu D, Li G, Hu W Z, Dong J, Zheng P,

Luo J L and Wang N L 2008 Phys. Rev. Lett. 100 247002[38] Ren Z, Yang J, Lu W, Yi W, Che G, Dong X, Sun L and

Zhao Z 2008 Mater. Res. Innov. 12 1[39] Ren Z et al 2008 Europhys. Lett. 82 57002[40] Chen X H, Wu T, Wu G, Liu R H, Chen H and Fang D F

2008 Nature 453 761

[41] Ren Z et al 2008 Chin. Phys. Lett. 25 2215[42] Cheng P, Fang L, Yang H, Zhu X, Mu G, Luo H,

Wang Z and Wen H 2008 Sci. China Ser. G51 719

[43] Bos J G, Penny G B S, Rodgers J A, Sokolov D A,Huxley A D and Attfield J P 2008 Chem. Commun.2008 3634

[44] Wen H, Mu G, Fang L, Yang H and Zhu X 2008 Europhys.Lett. 82 17009

[45] Kito H, Eisaki H and Iyo A 2008 J. Phys. Soc. Japan77 063707

[46] Yang J et al 2008 Supercond. Sci. Technol. 21 082001[47] Yang J et al 2009 New J. Phys. 11 025005[48] Shi Y G et al 2009 Phys. Rev. B 80 104501

[49] Kurmaev E Z, Wilks R G, Moewes A, Skorikov N A,Izyumov Y A, Finkelstein L D, Li R H and Chen X H2008 Phys. Rev. B 78 220503

[50] Ronning F, Klimczuk T, Bauer E D, Volz H andThompson J D 2008 J. Phys.: Condens. Matter20 322201

[51] Ni N, Nandi S, Kressig A, Goldman A I, Mun E D,Budko S L and Canfield P C 2008 Phys. Rev. B78 014523

[52] Krellner C, Caroca-Canales N, Jesche A, Rosner H,Ormeci A and Geibel C 2008 Phys. Rev. B 78 100504

[53] Sefat A S, McGuire M A, Jin R, Sales B C, Mandrus D,Ronning F, Bauer E D and Mozharivskyj Y 2009 Phys.Rev. B 79 094508

[54] Ren Z, Zhu Z W, Jiang S A, Xu X F, Tao Q, Wang C,Feng C M, Cao G H and Xu Z A 2008 Phys. Rev. B78 052501

[55] Huang Q, Qiu Y, Bao W, Green M A, Lynn J W,Gasparovic Y C, Wu T, Wu G and Chen X H 2008 Phys.Rev. Lett. 101 257003

[56] Kaneko K, Hoser A, Caroca-Canales N, Jesche A, Krellner C,Stockert O and Geibel C 2008 Phys. Rev. B 78 212502

[57] Goldman I A, Argyriou D N, Ouladdiaf B, Chatterji T andMcQueeney R J 2008 Phys. Rev. B 78 100506

[58] Aczel A A et al 2008 Phys. Rev. B 78 214503

[59] Tegel M, Rotter M, Weiss V, Schappacher F, Pottgen R andJohrendt D 2008 J. Phys.: Condens. Matter20 452201[60] Raffius H, Morsen E, Mosel B D, Muller-Warmuth W,

Jeitschko W, Terbuchte L and Vomhof T 1993 J. Phys.Chem. Solids 54 135

[61] Mandrus D, Sefat A S, McGuire M A and Sales B C 2010Chem. Mater. 22 715

[62] Rotter M, Tegel M and Johrendt D 2008 Phys. Rev. Lett.101 107006

[63] Sefat A S, Singh D J, VanBebber L H, Mozharivskyj Y,McGuire M A, Jin R Y, Sales B C, Keppens V andMandrus D 2009 Phys. Rev. B 79 224524

[64] Lester C, Chu J H, Analytis J G, Capelli S C, Erickson A S,Condron C L, Toney M F, Fisher I R and Hayden S M2009 Phys. Rev. B 79 144523

[65] Sharma S, Bharathi A, Chandra S, Reddy R, Paulraj S,Satya A T, Sastry V S, Gupta A and Sundar C S 2010Phys. Rev. B 81 174512

[66] Jiang S, Xing H, Wang C, Ren Z, Feng C M, Dai J,Xu Z A and Cao G H J 2009 J. Phys.: Condens. Matter21 382203

[67] Lebgue S, Yin Z P and Pickett W E 2009 New J. Phys.11 025004

[68] Chu C W and Lorenz B 2009 Physica C 469 385[69] Ronning F, Bauer E D, Park T, Kurita N, Klimczuk T,

Movshovich R, Sefat A S, Mandrus D and Thompson J D2009 Physica C 469 396

[70] Canfield P C, Budko S L, Ni N, Kreyssig A, Goldman A I,McQueeney R J, Torikachvili M S, Argyriou D N, Luke Gand Yu W 2009 Physica C 469 404

[71] Takahashi H, Okada H, Igawa K, Kamihara Y, Hirano M andHosono H 2009 Physica C 469 413

[72] Kawakami T, Kamatani T, Okada H, Takahashi H, Nasu S,Kamihara Y, Hirano M and Hosono H 2009 J. Phys. Soc.Japan 78 123703

[73] Zocco D A et al 2011 Phys. Rev. B 83 094528[74] Uhoya W, Tsoi G M, Vohra Y K, McGuire M A, Sefat A S,

Sales B C, Mandrus D and Weir S T 2010 J. Phys.:Condens. Matter22 185702

[75] Takahashi H, Igawa K, Arii K, Kamihara Y, Hirano M andHosono H 2008 Nature 453 376

[76] Zocco D A et al 2008 Physica C 468 2229[77] Takeshita N, Iyo A, Eisaki H, Kito H and Ito T 2008 J. Phys.

Soc. Japan 77 075003[78] Yi W et al 2008 Europhys. Lett. 83 57002[79] Lorenz B, Sasmal K, Chaudhury R P, Chen X H, Liu R H,

Wu T and Chu C W 2008 Phys. Rev. B 78 012505

13
http://dx.doi.org/10.1103/PhysRevB.81.184509http://dx.doi.org/10.1103/PhysRevB.81.184509http://dx.doi.org/10.1038/365323a0http://dx.doi.org/10.1038/365323a0http://dx.doi.org/10.1007/s10948-009-0475-8http://dx.doi.org/10.1007/s10948-009-0475-8http://dx.doi.org/10.1103/PhysRevB.80.064506http://dx.doi.org/10.1103/PhysRevB.80.064506http://dx.doi.org/10.1016/0921-4534(91)91947-3http://dx.doi.org/10.1016/0921-4534(91)91947-3http://dx.doi.org/10.1016/0921-4534(91)90276-5http://dx.doi.org/10.1016/0921-4534(91)90276-5http://dx.doi.org/10.1209/0295-5075/84/67009http://dx.doi.org/10.1209/0295-5075/84/67009http://dx.doi.org/10.1088/0256-307X/26/1/017401http://dx.doi.org/10.1088/0256-307X/26/1/017401http://dx.doi.org/10.1016/0921-4534(90)90460-Vhttp://dx.doi.org/10.1016/0921-4534(90)90460-Vhttp://dx.doi.org/10.1103/PhysRevB.78.020503http://dx.doi.org/10.1103/PhysRevB.78.020503http://dx.doi.org/10.1016/S0925-8388(99)00802-6http://dx.doi.org/10.1016/S0925-8388(99)00802-6http://dx.doi.org/10.1103/PhysRevLett.100.237003http://dx.doi.org/10.1103/PhysRevLett.100.237003http://dx.doi.org/10.1103/PhysRevB.78.094511http://dx.doi.org/10.1103/PhysRevB.78.094511http://dx.doi.org/10.1103/PhysRevB.78.104505http://dx.doi.org/10.1103/PhysRevB.78.104505http://dx.doi.org/10.1103/PhysRevLett.101.117004http://dx.doi.org/10.1103/PhysRevLett.101.117004http://dx.doi.org/10.1209/0295-5075/83/27006http://dx.doi.org/10.1209/0295-5075/83/27006http://dx.doi.org/10.1103/PhysRevLett.101.057003http://dx.doi.org/10.1103/PhysRevLett.101.057003http://dx.doi.org/10.1103/PhysRevB.78.094517http://dx.doi.org/10.1103/PhysRevB.78.094517http://dx.doi.org/10.1038/nature07057http://dx.doi.org/10.1038/nature07057http://dx.doi.org/10.1103/PhysRevLett.101.077005http://dx.doi.org/10.1103/PhysRevLett.101.077005http://dx.doi.org/10.1103/PhysRevB.77.174503http://dx.doi.org/10.1103/PhysRevB.77.174503http://dx.doi.org/10.1088/1367-2630/10/6/063026http://dx.doi.org/10.1088/1367-2630/10/6/063026http://dx.doi.org/10.1143/JPSJ.77.103706http://dx.doi.org/10.1143/JPSJ.77.103706http://dx.doi.org/10.1038/nmat2397http://dx.doi.org/10.1038/nmat2397http://dx.doi.org/10.1103/PhysRevLett.100.247002http://dx.doi.org/10.1103/PhysRevLett.100.247002http://dx.doi.org/10.1179/143307508X270730http://dx.doi.org/10.1179/143307508X270730http://dx.doi.org/10.1209/0295-5075/82/57002http://dx.doi.org/10.1209/0295-5075/82/57002http://dx.doi.org/10.1038/nature07045http://dx.doi.org/10.1038/nature07045http://dx.doi.org/10.1088/0256-307X/25/6/080http://dx.doi.org/10.1088/0256-307X/25/6/080http://dx.doi.org/10.1007/s11433-008-0075-9http://dx.doi.org/10.1007/s11433-008-0075-9http://dx.doi.org/10.1039/b808474bhttp://dx.doi.org/10.1039/b808474bhttp://dx.doi.org/10.1209/0295-5075/82/17009http://dx.doi.org/10.1209/0295-5075/82/17009http://dx.doi.org/10.1143/JPSJ.77.063707http://dx.doi.org/10.1143/JPSJ.77.063707http://dx.doi.org/10.1088/0953-2048/21/8/082001http://dx.doi.org/10.1088/0953-2048/21/8/082001http://dx.doi.org/10.1088/1367-2630/11/2/025005http://dx.doi.org/10.1088/1367-2630/11/2/025005http://dx.doi.org/10.1103/PhysRevB.80.104501http://dx.doi.org/10.1103/PhysRevB.80.104501http://dx.doi.org/10.1103/PhysRevB.78.220503http://dx.doi.org/10.1103/PhysRevB.78.220503http://dx.doi.org/10.1088/0953-8984/20/32/322201http://dx.doi.org/10.1088/0953-8984/20/32/322201http://dx.doi.org/10.1103/PhysRevB.78.014523http://dx.doi.org/10.1103/PhysRevB.78.014523http://dx.doi.org/10.1103/PhysRevB.78.100504http://dx.doi.org/10.1103/PhysRevB.78.100504http://dx.doi.org/10.1103/PhysRevB.79.094508http://dx.doi.org/10.1103/PhysRevB.79.094508http://dx.doi.org/10.1103/PhysRevB.78.052501http://dx.doi.org/10.1103/PhysRevB.78.052501http://dx.doi.org/10.1103/PhysRevLett.101.257003http://dx.doi.org/10.1103/PhysRevLett.101.257003http://dx.doi.org/10.1103/PhysRevB.78.212502http://dx.doi.org/10.1103/PhysRevB.78.212502http://dx.doi.org/10.1103/PhysRevB.78.100506http://dx.doi.org/10.1103/PhysRevB.78.100506http://dx.doi.org/10.1103/PhysRevB.78.214503http://dx.doi.org/10.1103/PhysRevB.78.214503http://dx.doi.org/10.1088/0953-8984/20/45/452201http://dx.doi.org/10.1088/0953-8984/20/45/452201http://dx.doi.org/10.1016/0022-3697(93)90301-7http://dx.doi.org/10.1016/0022-3697(93)90301-7http://dx.doi.org/10.1021/cm9027397http://dx.doi.org/10.1021/cm9027397http://dx.doi.org/10.1103/PhysRevLett.101.107006http://dx.doi.org/10.1103/PhysRevLett.101.107006http://dx.doi.org/10.1103/PhysRevB.79.224524http://dx.doi.org/10.1103/PhysRevB.79.224524http://dx.doi.org/10.1103/PhysRevB.79.144523http://dx.doi.org/10.1103/PhysRevB.79.144523http://dx.doi.org/10.1103/PhysRevB.81.174512http://dx.doi.org/10.1103/PhysRevB.81.174512http://dx.doi.org/10.1088/0953-8984/21/38/382203http://dx.doi.org/10.1088/0953-8984/21/38/382203http://dx.doi.org/10.1088/1367-2630/11/2/025004http://dx.doi.org/10.1088/1367-2630/11/2/025004http://dx.doi.org/10.1016/j.physc.2009.03.030http://dx.doi.org/10.1016/j.physc.2009.03.030http://dx.doi.org/10.1016/j.physc.2009.03.031http://dx.doi.org/10.1016/j.physc.2009.03.031http://dx.doi.org/10.1016/j.physc.2009.03.033http://dx.doi.org/10.1016/j.physc.2009.03.033http://dx.doi.org/10.1016/j.physc.2009.03.051http://dx.doi.org/10.1016/j.physc.2009.03.051http://dx.doi.org/10.1143/JPSJ.78.123703http://dx.doi.org/10.1143/JPSJ.78.123703http://dx.doi.org/10.1103/PhysRevB.83.094528http://dx.doi.org/10.1103/PhysRevB.83.094528http://dx.doi.org/10.1088/0953-8984/22/18/185702http://dx.doi.org/10.1088/0953-8984/22/18/185702http://dx.doi.org/10.1038/nature06972http://dx.doi.org/10.1038/nature06972http://dx.doi.org/10.1016/j.physc.2008.06.010http://dx.doi.org/10.1016/j.physc.2008.06.010http://dx.doi.org/10.1143/JPSJ.77.075003http://dx.doi.org/10.1143/JPSJ.77.075003http://dx.doi.org/10.1209/0295-5075/83/57002http://dx.doi.org/10.1209/0295-5075/83/57002http://dx.doi.org/10.1103/PhysRevB.78.012505http://dx.doi.org/10.1103/PhysRevB.78.012505http://dx.doi.org/10.1103/PhysRevB.78.012505http://dx.doi.org/10.1209/0295-5075/83/57002http://dx.doi.org/10.1143/JPSJ.77.075003http://dx.doi.org/10.1016/j.physc.2008.06.010http://dx.doi.org/10.1038/nature06972http://dx.doi.org/10.1088/0953-8984/22/18/185702http://dx.doi.org/10.1103/PhysRevB.83.094528http://dx.doi.org/10.1143/JPSJ.78.123703http://dx.doi.org/10.1016/j.physc.2009.03.051http://dx.doi.org/10.1016/j.physc.2009.03.033http://dx.doi.org/10.1016/j.physc.2009.03.031http://dx.doi.org/10.1016/j.physc.2009.03.030http://dx.doi.org/10.1088/1367-2630/11/2/025004http://dx.doi.org/10.1088/0953-8984/21/38/382203http://dx.doi.org/10.1103/PhysRevB.81.174512http://dx.doi.org/10.1103/PhysRevB.79.144523http://dx.doi.org/10.1103/PhysRevB.79.224524http://dx.doi.org/10.1103/PhysRevLett.101.107006http://dx.doi.org/10.1021/cm9027397http://dx.doi.org/10.1016/0022-3697(93)90301-7http://dx.doi.org/10.1088/0953-8984/20/45/452201http://dx.doi.org/10.1103/PhysRevB.78.214503http://dx.doi.org/10.1103/PhysRevB.78.100506http://dx.doi.org/10.1103/PhysRevB.78.212502http://dx.doi.org/10.1103/PhysRevLett.101.257003http://dx.doi.org/10.1103/PhysRevB.78.052501http://dx.doi.org/10.1103/PhysRevB.79.094508http://dx.doi.org/10.1103/PhysRevB.78.100504http://dx.doi.org/10.1103/PhysRevB.78.014523http://dx.doi.org/10.1088/0953-8984/20/32/322201http://dx.doi.org/10.1103/PhysRevB.78.220503http://dx.doi.org/10.1103/PhysRevB.80.104501http://dx.doi.org/10.1088/1367-2630/11/2/025005http://dx.doi.org/10.1088/0953-2048/21/8/082001http://dx.doi.org/10.1143/JPSJ.77.063707http://dx.doi.org/10.1209/0295-5075/82/17009http://dx.doi.org/10.1039/b808474bhttp://dx.doi.org/10.1007/s11433-008-0075-9http://dx.doi.org/10.1088/0256-307X/25/6/080http://dx.doi.org/10.1038/nature07045http://dx.doi.org/10.1209/0295-5075/82/57002http://dx.doi.org/10.1179/143307508X270730http://dx.doi.org/10.1103/PhysRevLett.100.247002http://dx.doi.org/10.1038/nmat2397http://dx.doi.org/10.1143/JPSJ.77.103706http://dx.doi.org/10.1088/1367-2630/10/6/063026http://dx.doi.org/10.1103/PhysRevB.77.174503http://dx.doi.org/10.1103/PhysRevLett.101.077005http://dx.doi.org/10.1038/nature07057http://dx.doi.org/10.1103/PhysRevB.78.094517http://dx.doi.org/10.1103/PhysRevLett.101.057003http://dx.doi.org/10.1209/0295-5075/83/27006http://dx.doi.org/10.1103/PhysRevLett.101.117004http://dx.doi.org/10.1103/PhysRevB.78.104505http://dx.doi.org/10.1103/PhysRevB.78.094511http://dx.doi.org/10.1103/PhysRevLett.100.237003http://dx.doi.org/10.1016/S0925-8388(99)00802-6http://dx.doi.org/10.1103/PhysRevB.78.020503http://dx.doi.org/10.1016/0921-4534(90)90460-Vhttp://dx.doi.org/10.1088/0256-307X/26/1/017401http://dx.doi.org/10.1209/0295-5075/84/67009http://dx.doi.org/10.1016/0921-4534(91)90276-5http://dx.doi.org/10.1016/0921-4534(91)91947-3http://dx.doi.org/10.1103/PhysRevB.80.064506http://dx.doi.org/10.1007/s10948-009-0475-8http://dx.doi.org/10.1038/365323a0http://dx.doi.org/10.1103/PhysRevB.81.184509


14/14


[80] Takabayashi Y, McDonald M T, Papanikolaou D,Margadonna S, Wu G, Liu R H, Chen X H and Prassides K2008 J. Am. Chem. Soc. 130 9242

[81] Torikachvili M S, Budko S L, Ni N and Canfield P C 2008Phys. Rev. Lett. 101 057006

[82] Colombier E, Budko S L, Ni N and Canfield P C 2009 Phys.Rev. B 79 224518

[83] Duncan W J, Welzel O P, Harrison C, Wang X F, Chen X H,Grosche F M and Niklowitz P G 2010 J. Phys.: Condens.Matter22 052201

[84] Fukazawa H, Takeshita N, Yamazaki T, Kondo K, Hirayaa K,Kohori Y, Miyazawa K, Kito H, Eisaki H and Iyo A 2008J. Phys. Soc. Japan 77 105004

[85] Mani A, Ghoshi N, Paulraj S, Bharathi A and Sundar C S2009 Europhys. Lett. 87 17004

[86] Matsubayashi K, Katayama N, Ohgushi K, Yamada A,Munakata K, Matsumoto T and Uwatoko Y 2009 J. Phys.Soc. Japan 78 073706

[87] Ahilan K, Balasubramaniam J, Ning F L, Imai Y, Sefat A S,Jin R, McGuire M A, Sales B C and Mandrus D 2009J. Phys. Soc. Japan 20 472201

[88] Ishikawa F, Eguchi N, Kodama M, Jujimaki K, Einaga M,Ohmura A, Nakayama A, Mitsuda A and Yamada Y 2009Phys. Rev. B 79 172506

[89] Ahilan K, Balasubramaniam J, Ning F L, Imai T, Sefat A S,Jin R, McGuire M A, Sales B C and Mandrus D 2008J. Phys.: Condens. Matter20 472201

[90] Ni N, Thaler A, Yan J Q, Kracher A, Colombier E,Budko S L and Canfield P C 2010 Phys. Rev. B 82 024519

[91] Ahilan K, Ning F L, Imai T, Sefat A S, McGuire M A,Sales B C and Mandrus D 2009 Phys. Rev. B 79 214520

[92] Torikachvili M S, Budko S L, Ni N and Canfield P C 2008Phys. Rev. B 78 104527

[93] Kumar M, Nicklas M, Jesche A, Caroca-Canales N,Schmitt M, Hanfland M, Kasinathan D, Schwarz U,Rosner H and Geibel C 2008 Phys. Rev. B 78 184516

[94] Igawa K, Okada H, Takahashi H, Matsuishi S, Kamihara Y,

Hirano M, Hosono H, Matsubayashi K and Uwatoko Y2009 J. Phys. Soc. Japan 78 025001[95] Kotegawa H, Kawazoe T, Sugawara H, Murata K and Tou H

2009 J. Phys. Soc. Japan 78 083702

[96] Park T, Park E, Lee H, Klimczuk T, Bauer E D, Ronning Fand Thomspon J D 2008 J. Phys.: Condens. Matter20 322204

[97] Torikachvili M S, Budko S L, Ni N, Canfield P C andHannahs S T 2009 Phys. Rev. B 80 014521

[98] Lee H, Park E, Park T, Sidorov V A, Ronning F, Bauer E Dand Thompson J D 2009 Phys. Rev. B 80 24519

[99] Kreyssig A et al 2008 Phys. Rev. B 78 184517[100] Yu W, Aczel A A, Williams T J, Budko S L, Ni N,

Canfield P C and Luke G M 2009 Phys. Rev. B79 020511

[101] Goldman A I et al 2009 Phys. Rev. B 79 24513[102] Miclea C F, Nicklas M, Jeevan H S, Kasinathan D,

Hossain Z, Rosner H, Gegenwart P, Geibel C andSteglich F 2009 Phys. Rev. B 79 212509

[103] Terashima T, Kimata M, Satsukawa H, Harada A, Hazama K,Uji S, Suzuki H S, Matsumoto T and Murata K 2009J. Phys. Soc. Japan 78 083701

[104] Bishop M, Uhoya W, Tsoi G, Vohra Y K, Sefat A S andSales B C 2010 J. Phys.: Condens. Matter22 42501

[105] Kimber S A J et al 2009 Nature Mater. 8 471[106] Uhoya W O, Montgomery J M, Tsoi G M, Vohra Y K,

McGuire M A, Sefat A S, Sales B C and Weir S T 2011J. Phys.: Condens. Matter23 122201

[107] Uhoya W, Tsoi G, Vohra Y K, McGuire M A, Sefat A S,Sales B C, Mandrus D and Weir S T 2010 J. Phys.:Condens. Matter22 292202

[108] Colombier E, Torikachvili M S, Ni N, Thaler A,Budko S L and Canfield P C 2010 Supercond. Sci.Technol. 23 054003

[109] Kumai R, Takeshita N, Ito T, Kito H, Iyo A and Eisaki H2009 J. Phys. Soc. Japan 78 013705

[110] Huhnt C, Schlabitz W, Wurth A, Mewis A and Reehuis M1997 Phys. Rev. B 56 13796

[111] Zhao J et al 2008 J. Am. Chem. Soc. 130 13828[112] Jorgensen J E, Staun Olsen J and Gerward I 2009 Solid State

Commun. 149 1161

[113] Mittal R et al 2011 Phys. Rev. B 83 054503[114] Uhoya W, Stemshorn A, Tsoi G, Vohra Y K, Sefat A S,Sales B C, Hope K M and Weir S T 2010 Phys. Rev. B82 144118

14
http://dx.doi.org/10.1021/ja8036838http://dx.doi.org/10.1021/ja8036838http://dx.doi.org/10.1103/PhysRevLett.101.057006http://dx.doi.org/10.1103/PhysRevLett.101.057006http://dx.doi.org/10.1103/PhysRevB.79.224518http://dx.doi.org/10.1103/PhysRevB.79.224518http://dx.doi.org/10.1088/0953-8984/22/5/052201http://dx.doi.org/10.1088/0953-8984/22/5/052201http://dx.doi.org/10.1143/JPSJ.77.105004http://dx.doi.org/10.1143/JPSJ.77.105004http://dx.doi.org/10.1209/0295-5075/87/17004http://dx.doi.org/10.1209/0295-5075/87/17004http://dx.doi.org/10.1143/JPSJ.78.073706http://dx.doi.org/10.1143/JPSJ.78.073706http://dx.doi.org/10.1103/PhysRevB.79.172506http://dx.doi.org/10.1103/PhysRevB.79.172506http://dx.doi.org/10.1088/0953-8984/20/47/472201http://dx.doi.org/10.1088/0953-8984/20/47/472201http://dx.doi.org/10.1103/PhysRevB.82.024519http://dx.doi.org/10.1103/PhysRevB.82.024519http://dx.doi.org/10.1103/PhysRevB.79.214520http://dx.doi.org/10.1103/PhysRevB.79.214520http://dx.doi.org/10.1103/PhysRevB.78.104527http://dx.doi.org/10.1103/PhysRevB.78.104527http://dx.doi.org/10.1103/PhysRevB.78.184516http://dx.doi.org/10.1103/PhysRevB.78.184516http://dx.doi.org/10.1143/JPSJ.78.025001http://dx.doi.org/10.1143/JPSJ.78.025001http://dx.doi.org/10.1143/JPSJ.78.083702http://dx.doi.org/10.1143/JPSJ.78.083702http://dx.doi.org/10.1088/0953-8984/20/32/322204http://dx.doi.org/10.1088/0953-8984/20/32/322204http://dx.doi.org/10.1103/PhysRevB.80.014521http://dx.doi.org/10.1103/PhysRevB.80.014521http://dx.doi.org/10.1103/PhysRevB.80.024519http://dx.doi.org/10.1103/PhysRevB.80.024519http://dx.doi.org/10.1103/PhysRevB.78.184517http://dx.doi.org/10.1103/PhysRevB.78.184517http://dx.doi.org/10.1103/PhysRevB.79.020511http://dx.doi.org/10.1103/PhysRevB.79.020511http://dx.doi.org/10.1103/PhysRevB.79.024513http://dx.doi.org/10.1103/PhysRevB.79.024513http://dx.doi.org/10.1103/PhysRevB.79.212509http://dx.doi.org/10.1103/PhysRevB.79.212509http://dx.doi.org/10.1143/JPSJ.78.083701http://dx.doi.org/10.1143/JPSJ.78.083701http://dx.doi.org/10.1038/nmat2443http://dx.doi.org/10.1038/nmat2443http://dx.doi.org/10.1088/0953-8984/23/12/122201http://dx.doi.org/10.1088/0953-8984/23/12/122201http://dx.doi.org/10.1088/0953-8984/22/29/292202http://dx.doi.org/10.1088/0953-8984/22/29/292202http://dx.doi.org/10.1088/0953-2048/23/5/054003http://dx.doi.org/10.1088/0953-2048/23/5/054003http://dx.doi.org/10.1103/PhysRevB.56.13796http://dx.doi.org/10.1103/PhysRevB.56.13796http://dx.doi.org/10.1021/ja804229khttp://dx.doi.org/10.1021/ja804229khttp://dx.doi.org/10.1016/j.ssc.2009.05.026http://dx.doi.org/10.1016/j.ssc.2009.05.026http://dx.doi.org/10.1103/PhysRevB.83.054503http://dx.doi.org/10.1103/PhysRevB.83.054503http://dx.doi.org/10.1103/PhysRevB.82.144118http://dx.doi.org/10.1103/PhysRevB.82.144118http://dx.doi.org/10.1103/PhysRevB.82.144118http://dx.doi.org/10.1103/PhysRevB.83.054503http://dx.doi.org/10.1016/j.ssc.2009.05.026http://dx.doi.org/10.1021/ja804229khttp://dx.doi.org/10.1103/PhysRevB.56.13796http://dx.doi.org/10.1088/0953-2048/23/5/054003http://dx.doi.org/10.1088/0953-8984/22/29/292202http://dx.doi.org/10.1088/0953-8984/23/12/122201http://dx.doi.org/10.1038/nmat2443http://dx.doi.org/10.1143/JPSJ.78.083701http://dx.doi.org/10.1103/PhysRevB.79.212509http://dx.doi.org/10.1103/PhysRevB.79.024513http://dx.doi.org/10.1103/PhysRevB.79.020511http://dx.doi.org/10.1103/PhysRevB.78.184517http://dx.doi.org/10.1103/PhysRevB.80.024519http://dx.doi.org/10.1103/PhysRevB.80.014521http://dx.doi.org/10.1088/0953-8984/20/32/322204http://dx.doi.org/10.1143/JPSJ.78.083702http://dx.doi.org/10.1143/JPSJ.78.025001http://dx.doi.org/10.1103/PhysRevB.78.184516http://dx.doi.org/10.1103/PhysRevB.78.104527http://dx.doi.org/10.1103/PhysRevB.79.214520http://dx.doi.org/10.1103/PhysRevB.82.024519http://dx.doi.org/10.1088/0953-8984/20/47/472201http://dx.doi.org/10.1103/PhysRevB.79.172506http://dx.doi.org/10.1143/JPSJ.78.073706http://dx.doi.org/10.1209/0295-5075/87/17004http://dx.doi.org/10.1143/JPSJ.77.105004http://dx.doi.org/10.1088/0953-8984/22/5/052201http://dx.doi.org/10.1103/PhysRevB.79.224518http://dx.doi.org/10.1103/PhysRevLett.101.057006http://dx.doi.org/10.1021/ja8036838

fe superconductors pressure

Documents