j. j. hamlin, r. e. baumbach, d. a. zocco, t. a. sayles ... · 0.35 k. in this paper, we report the...

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Superconductivity in single crystals of LaFePO

J. J. Hamlin, R. E. Baumbach, D. A. Zocco, T. A. Sayles and M. B. MapleDepartment of Physics and Institute for Pure and Applied Physical Sciences, University ofCalifornia, San Diego, La Jolla, CA 92093

E-mail: [email protected]

Abstract. Single crystals of the compound LaFePO were prepared using aflux growthtechnique at high temperatures. Electrical resistivity measurements reveal metallic behaviorand a resistive transition to the superconducting state at acritical temperatureTc ∼ 6.6 K.Magnetization measurements also show the onset of superconductivity near 6 K. In contrast,specific heat measurements manifest no discontinuity atTc. These results lend support to theconclusion that the superconductivity is associated with oxygen vacancies that alter the carrierconcentration in a small fraction of the sample, although superconductivity characterized byan unusually small gap value can not be ruled-out. Under applied magnetic fields,Tc issuppressed anisotropically for fields perpendicular and parallel to theab-plane, suggestingthat the crystalline anisotropy strongly influences the superconducting state. Preliminary high-pressure measurements show thatTc passes through a maximum of nearly 14 K at∼ 110 kbar,demonstrating that significantly higherTc values may be achieved in the phosphorus-basedoxypnictides.

http://arxiv.org/abs/0806.1265v2

Superconductivity in single crystals of LaFePO 2

1. Introduction

There has been a flurry of research activity following the recent reports of superconductivitywith high critical temperaturesTc in the system LnFeAs[O1−xFx] where Ln is a lanthanide[1, 2, 3, 4, 5, 6]. To date, values ofTc as high as 55 K have been reported for Ln = Sm [5].These compounds belong to a general class of compounds with alayered crystal structure ofthe form LnFePnO that were first synthesized by Jeitschko andcoworkers with Pn = P [7]and As [8]. Superconductivity in this series of materials was discovered in LaFePO in 2006by Kamiharaet al. [9], for which values ofTc that range from 3 K [9] to 7 K [10] havebeen reported. However, in a recent study of polycrystalline materials, it was concluded thatstoichiometric LaFePO is metallic but non-superconducting [11] at temperatures as low as0.35 K. In this paper, we report the synthesis of single crystals of LaFePO. These crystalsexhibit superconducting transitions at 6.6 K and 6.0 K, according to electrical resistivityand magnetic susceptibility measurements, respectively.However, there is no specific heatjump at Tc, suggesting that only a small fraction of the sample is superconducting. Thesuperconductivity appears to be a property of the single crystals, since the resistively measuredupper critical field is quite anisotropic. It is possible that the superconductivity is associatedwith oxygen vacancies that dope a small fraction of the compound with charge carriers. Wepresent measurements of the electrical resistivity, magnetic susceptibility, and specific heatin the normal state. Preliminary measurements of the pressure dependence ofTc are alsoreported.

2. Experimental Details

Single crystals of LaFePO were grown from elements and elemental oxides with purities> 99.9% in a molten Sn:P flux. The growths took place over a 1 week period in quartzampoules which were sealed with 75 torr Ar at room temperature. The inner surface of eachquartz ampoule was coated with carbon by a typical pyrolysismethod. The starting materialswere La, Fe2O3, P, and Sn, which were combined in the molar ratios 9:3:6:80.5, similar toa previous synthesis reported by Krellner and Geibel [12] for CeRuPO single crystals. TheFe2O3 powder was dried for∼ 10 hours at 300C before weighing. The ampoule was heatedto 1135 C at a rate of 35C/hr, kept at this temperature for 96 hours, and then rapidlycooled to 700C. After removing the majority of the flux by spinning the ampoules in acentrifuge, LaFePO single crystal platelets of an isometric form with typical dimensions of∼ 0.5 × 0.5 × 0.05 mm3 were collected and cleaned in hydrochloric acid to remove the fluxfrom the surface of the crystals prior to measurements. After cleaning, the crystals wereobserved to have a distinct gold color. The platelets cleaved easily in theab-plane and werenotably malleable, in contrast to the cuprate superconductors. A typical platelet is shown inthe inset of Fig. 1.

X-ray powder diffraction measurements were made using a Bruker D8 diffractometerwith a non-monochromated Cu Kα source to check the purity and crystal structure of theLaFePO single crystals. Due to their malleability, the crystals were difficult to grind into a


Figure 1. Powder x-ray diffraction pattern for LaFePO. They-axis is linear. The verticallines below the diffraction pattern indicate calculated Bragg peak positions. Inset: (color)Photograph shows a typical single crystal of LaFePO.

fine powder. Thus, the powder diffraction pattern was generated from a collection of severalcrystals which were cut into small pieces using a razor bladeand then ground into a coarsepowder using a mortar and pestle.

Electrical resistivityρ(T ) measurements were performed in a four-wire configurationwith the current in theab-plane, at temperaturesT = 2-300 K and magnetic fieldsH = 0-8T using a conventional4He cryostat and a Quantum Design Physical Properties MeasurementSystem (PPMS). In order to explore the anisotropy of the superconducting properties, theresistivity was measured for 2-10 K withH both parallel and perpendicular to the crystalab-planes,ρ(T,H‖) andρ(T,H⊥), respectively. For these measurements, the geometry wasarranged such that the current flowed in theab-plane and was perpendicular toH, as illustratedin the upper panels of Fig. 3 .

DC magnetizationM(T,H) measurements were made using a Quantum Design Mag-netic Properties Measurement System (MPMS) in order to probe both the superconductingand normal state properties of the single crystal platelets. The specimens were mounted incotton-packed gelatin capsules with theab-plane perpendicular to the magnetic field. Multi-ple single crystal platelets were each individually measured for 2-10 K andH = 5 Oe underboth zero field cooled (ZFC) and field cooled (FC) conditions in order to characterize batchhomogeneity via variation inTc, which was found to be minimal. Magnetization versus tem-perature atH = 1.0 T and 7-300 K was measured for a collection of several hundredsinglecrystal specimens (m = 13.06 mg) in order to characterize the normal state.


0

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ρ (µ

Ωcm

)

Temperature (K)

LaFePO

reduced (700 oC)

as grown

0

200

400

600

800

0 100 200 300ρ

(µΩ

cm

)T (K)

1

10

100

1000

10 100ρ

- ρ 0

T (K)

Figure 2. Electrical resistivityρ versus temperatureT measured in theab-plane for an as-grown single crystal and a crystal that was reduced in flowingargon at 7000C for∼ 12 hours.Left inset: Resistivity over the entire measured temperature range showingT 2 behavior for∼10-100 K and linearT behavior for∼100-300 K. Right inset: Log-log plot ofρ− ρ0 versusT . The solid lines are fits to the data which demonstrate theT 2 behavior for 10-100 K for theas grown sample and 10-80 K for the reduced sample.

Specific heatC(T ) measurements were made for 2-300 K in a Quantum Design PPMSsemiadiabatic calorimeter using a heat-pulse technique onthe same collection of singlecrystals used for the normal state magnetization measurements. The specimens were attachedto a sapphire platform with a small amount of Apiezon N grease.

A single high-pressure experiment was performed using a mechanically loadedcommercial diamond anvil cell (DAC), manufactured by KyowaSeisakusho Ltd. Thediamond anvils were beveled from 500 to 250µm tips. One of the diamonds contains sixdeposited tungsten microprobes encapsulated in high-quality homepitaxial diamond. Thefabrication of “designer” diamonds is described in Ref. [13]. The gasket was made froma 200µm thick MP35N foil pre-indented to 40-50µm with a 130µm diameter hole drilledthrough the center of the pre-indented region using an electrical discharge machine (EDM).Two 5 µm diameter ruby spheres were loaded into the hole in the gasket and the remainingspace in the hole was filled with∼ 5 small pieces of LaFePO crystal. Because the sample isin direct contact with the metallic gasket, the measured resistance results from a combinationof the sample and gasket resistivity. Due to the geometry of the leads, the measured resistancegenerally does not drop completely to zero when the sample becomes superconducting.


Figure 3. Top panels: Electrical resistivityρ versus temperatureT data for LaFePO singlecrystal platelets for temperatures 2-10 K and magnetic fields‖ to theab-plane: 0 T, 1.0 T, 2.0T, 3.0 T, 4.0 T, 5.0 T, 6.0 T, and 8.0 T (top left panel) and⊥ to theab-plane: 0 T, 0.2 T, 0.4 T,0.6 T, 0.8 T, and 1.0 T (top right panel). Bottom panel: The upper critical field lineHc2 versusT for H ‖ and⊥ to theab-plane.

However, such a configuration is sufficient for locating theTc of the sample. Pressure wasadjusted and determined at room temperature, using the fluorescence spectrum of the rubyspheres and the calibration of Chijiokeet al. [14]. A delrin spacer is used to minimizechanges in pressure during cooling to lowT . Additional details of the DAC technique aredescribed in Ref. [15].


Figure 4. DC magnetic susceptibilityM/H versus temperatureT taken inH = 5 Oe for asingle crystal specimen under zero-field-cooled (ZFC) and field-cooled (FC) conditions. Leftinsets:M versusH exhibiting both flux expulsion (Meissner) and flux penetration (vortex)superconducting states as expected for a type-II superconductor. Right inset:M/H versustemperatureT for 7-300 K andH = 1.0 T.

3. Results and Discussion

Shown in Fig. 1 is the x-ray diffraction pattern for a collection of LaFePO single crystals.The diffraction pattern conforms to the characteristic tetragonal phase of LaFePO consistingof P-Fe2-P layers of edge sharing Fe-P octahedra separated by La-O2-La sheets in which theFe atoms form two-dimensional square nets. The diffractionpattern appears to be in goodagreement with previous reports [7, 9, 11] and exhibits no impurity contributions down to the10% level. Measurements on a mosaic of several single crystals reveal that the platelets growwith their large faces parallel to the 00l crystal planes.

The electrical resistivity data, shown in Fig. 2, reveal metallic behavior whereρ(T )decreases with decreasingT until it drops abruptly to zero near the superconducting transitiontemperatureTc ∼ 6.6 K. This transition temperature is defined as the temperaturewhereρ(T ) drops to 50% of its extrapolated normal state value. The transition width∆Tc = 1.3

K is taken as the difference in the temperatures whereρ(T ) drops to 10% and 90% of


the extrapolated normal state value. For∼100-300 K,ρ(T ) has an approximately linearTdependence which evolves into a quadratic form for∼10-100 K, as shown in the left inset toFig. 2. Fits over this temperature range show that the data are well described by the expressionρ(T ) = ρ0 + AT 2 whereρ0 ∼ 14 µΩcm andA = 9.46 × 10−3µΩcm/K2. The residualresistivity ratioRRR = ρ(300K)/ρ(0) = 32 reflects the high quality of the LaFePO singlecrystal. Also shown in Fig. 2 are results for a platelet whichwas reduced at 700C in flowingAr for 24 hours. For this specimen,RRR = 58, Tc = 7.8 K, and the∆Tc = 1.6 K, indicatingthat the superconducting state may be enhanced by reductionof oxygen concentration. Fits tothe data for the reduced sample giveρ0 ∼ 16.7 µΩcm andA = 2.44× 10−2µΩcm/K2.

Shown in Fig. 3 areρ(T,H‖) andρ(T,H⊥) data for 2-10 K and 0-8 T which revealpronounced anisotropy in the upper critical field curveHc2(T ). Again,Tc is defined as 50%of the extrapolated normal state value while the transitionwidth is taken as the differencebetween the temperatures whereρ(T ) drops to 10% and 90% of the extrapolated normal statevalue. At zeroT , the anisotropy is quantified by the ratioH‖

c2(T )/H⊥c2(T ) ∼ 5.2. This result

suggests that the layered structure strongly influences thein-plane and inter-plane transportbehavior. The Clogston-Chandrasekhar [16, 17] Pauli-paramagnetic limiting field at zerotemperature is given in units of tesla byHp0 ≡ 1.84Tc, which forTc = 6.6K givesHp0 ∼ 12.1

T, a value well above the extrapolated zero temperature critical field line illustrated in Fig. 3.Thus, the upper critical field is limited by orbital depairing for bothH‖ab andH⊥ab. TheGinzburg - Landau coherence lengths parallel and perpendicular to theab-plane,ξ‖ andξ⊥,respectively, can be estimated from the slopes ofH

‖c2 andH⊥

c2 nearTc [18], i. e.,

(dH⊥c2/dT )Tc

= −Φ0/2πTcξ2

‖ (1)

and

(dH‖c2/dT )Tc

= −Φ0/2πTcξ⊥ξ‖, (2)

whereΦ0 = hc/2e = 2.07×10−7 G·cm2 is the flux quantum. From the values(dH⊥c2/dT )Tc

=

−1700 Oe/K and(dH‖c2/dT )Tc

= −8600 Oe/K we obtainξ‖ = 170 A andξ⊥ = 34 A.In an effort to determine whether the superconductivity is abulk phenomenon, zero-

field-cooled (ZFC) and field-cooled (FC) measurements of themagnetic susceptibility weremade in a field of 5 Oe. A plot of the ZFC and FC magnetic susceptibility through thesuperconducting transition is shown in Fig. 4, where the onset temperature is near 6.0 K.The ZFC and FC measurements yield maximum signals of∼ 95% and∼ 5% of completeflux expulsion, respectively. The values of4πχ are accurate to roughly±15% due touncertainties in mass, geometry and demagnetization factor of the small crystal. The smallrecovery of the diamagnetic signal on field-cooling indicates either that the material showsstrong vortex pinning or that the superconductivity is not abulk phenomenon and is possiblyassociated with regions in the crystal that are oxygen deficient. One such possibility is thatthe superconductivity resides in a region near the surface which has a composition differentfrom that of the interior of the crystal. The left inset to Fig. 4 showsM versusH for T = 5K, where the linear response of the Meissner state up to∼ 20 Oe is followed by a decreasein the magnitude ofM with increasingH, as is typical of flux penetration in the vortex state


of a type-II superconductor. Theχ(T ) data in the normal state are shown in the right inset ofFig. 4. The susceptibilityχ(T ) increases strongly with decreasingT down to∼ 220 K, whereit partially saturates. This temperature dependence is similar to that seen for the compoundFe2P [20], which may be present as inclusions or surface impurities. We estimate that ourobserved magnetic susceptibility is consistent with 1-2% percent Fe2P impurity. It is notablethat below∼ 40 K,χ(T ) exhibits a weak upturn which persists down to 2 K. This behavior isnot typical for Fe2P, and is either intrinsic to LaFePO or due to a small concentration of someother paramagnetic impurity.

Specific heat divided by temperatureC/T versusT 2 data for2 − 20 K are shown inFig. 5. The absence of a detectable jump inC(T ) nearTc ∼ 6.6 K, strongly suggests thatthe LaFePO single crystals do not exhibit bulk superconductivity. The specific heat jump atTc for a weak-coupling conventional superconductor is given by ∆C = 1.52γTc [21]. ForTc = 6.6 K andγ = 12.7 mJ/mol·K, this relation yields∆C = 127 mJ/mol·K. Comparingthis expected specific heat jump with the scatter in theC(T ) data (upper inset of Fig. 5) wouldindicate that at most 10-20% of the sample is superconducting. Our upper limit on the specificheat jump,∆C . 13 mJ/mol·K, is consistent with a very recent measurement of Kohamaetal. [22] on polycrystalline LaFePO, who find nearTc a very small discontinuity of∆C ∼ 10

mJ/mol·K. These results support the interpretation that the superconductivity is associatedwith defects such as oxygen vacancies that act to dope the compound or possibly a differentsurface composition in comparison with the bulk. On the other hand, LaFePO may exhibitsome exotic type of superconductivity that is characterized by an unusually small energy gap.

TheC/T versusT 2 data shown in Fig. 5 can be fit, between 2 K and 8 K, by the sum ofelectronicγT and latticeβT 3 terms, yielding an electronic specific heat coefficientγ = 12.7

mJ/mol·K2 and a Debye temperatureΘD = 268 K. The value ofγ is close toγ = 12.5

mJ/mol·K2 obtained for a polycrystalline sample by McQueenet al. [11]. From the valueγ = 12.7 mJ/mol·K2 and the saturated magnetic susceptibilityχ0 ∼ 2 × 10−3 emu/mol·Oe,the Wilson - Sommerfeld ratio is calculated to be RW = 14.6. By comparison to the valuesexpected for noninteracting electrons and electrons in a bound state Kondo singlet, RW = 1

and 2, respectively, this value is unphysically large. Thisresult indicates that the value ofχ0 does not represent the Pauli paramagnetic susceptibility of the conduction electrons. Forthis reason, we consider the value ofχ0 = 3× 10−4 emu/mol·Oe reported by McQueenet al.[11], which yields RW = 2.19. This inconsistency is presumably due to the inclusion of 1-2%Fe2P, which would enhance the assumed lowT susceptibility for conduction electrons. Fromthe coefficientA = 9.46 × 10−3µΩcm/K2 obtained by fits toρ(T ) andγ = 12.7 mJ/mol·K2,the ratioR ≡ A/γ2 = 6 × 10−5 µΩ cm(mol·K/mJ)2 is calculated, in order of magnitudeagreement with the Kadowaki - Woods value of1× 10−5 µΩ cm(mol·K/mJ)2 [23]. As shownin the lower inset to Fig. 5,C(T )/T continues to increase rapidly up to∼ 120 K, where itgoes through a peak and then decreases monotonically with increasingT .

Electrical resistivityρ(T ) measurements were made under high pressures of 54, 106,158, and 204 kbar, in that order, as shown in Fig. 6. The upper panels in Fig. 6 display theresistance versus pressure in the vicinity of the superconducting transition for two differentconfigurations of the six tungsten leads. Each different configuration measures a somewhat


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(m

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ol K

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T2 (K

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ol K

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T(K)

LaFePO

Figure 5. Specific heatC divided by temperatureT , C/T , versusT 2 for 2-20 K. Nodiscontinuity is observed nearTc, strongly suggesting that the superconductivity is not a bulkphenomenon. A linear fit (dashed line) toC/T versusT 2 for 2-8 K yields an electronic specificheat coefficientγ = 12.7 mJ/mol·K and a Debye temperatureΘD = 268 K. Inset:C(T )/T

versusT from 2-300 K.

different section of the sample. The lower inset of Fig. 6 shows ρ(T ) over a broadertemperature range for the measurement at 54 kbar. TheTc values are determined graphicallyas illustrated by the dashed lines in the upper panels and averaged over the two configurationsof leads. The values for the vertical bars represent the superconducting onset temperature andare determined by the temperature at which the resistivity passes through a maximum. Thepressure is determined from an average of the pressure givenby the two different pieces ofruby in the cell and the horizontal bars represent the pressure difference determined from eachindividual piece of ruby. The point at ambient pressure was taken from the resistivity curveshown in Fig. 2. Remarkably, a rather moderate pressure of 106 kbar is sufficient to nearlydouble the onset of superconductivity from∼ 7 K to ∼ 14 K, above which pressure acts tosuppressTc.


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LaFePO

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Temperature (K)

Figure 6. (color) Top panels: Resistance curves normalized to the values at 15 K for twodifferent lead configurations where the onsetTc is defined as the intersection of the dashedlines. Main panel: Pressure dependence of the onsetTc for P = 54, 106, 158 and 204 kbar.The value of theTc onset at ambient pressure corresponds to the measurement ofR versusTshown in Fig. 2 withTc determined using the same criterion as that used for the highpressurepoints as described above. The high-pressure points correspond to the DAC measurements.Horizontal bars represent the gradient of pressure along the sample, as described in the maintext. The high temperature limit of the vertical bars represent an estimate of the first appearanceof superconductivity, inferred from the first indication ofthe deviation ofρ(T ) from normalstate behavior. The dashed line is a guide to the eye.


4. Concluding Remarks

Single crystals of the compound LaFePO prepared by a flux growth technique at hightemperatures exhibit a superconducting transition near 6.6 K, as revealed by electricalresistivity and magnetization measurements. The transition temperatureTc is suppressedanisotropically for magnetic fields perpendicular and parallel to the ab plane, suggestingthat the crystalline anisotropy strongly influences the superconducting state. Measurementsof electrical resistivity under pressure show thatTc passes through a maximum of nearly14 K at ∼ 110 kbar, indicating that significantly higherTc values may be possible inthe phosphorus - based oxypnictides. In contrast, specific heat measurements show noindication of a discontinuity atTc, suggesting that stoichiometric LaFePO does not exhibitbulk superconductivity. These results indicate that either the superconductivity is associatedwith oxygen vacancies that alter the carrier concentration, or that the superconductivityis characterized by an unusually small energy gap. Further experiments in which thecompound is doped with charge carriers through chemical substitution will hopefully shedlight on the superconductivity displayed by this new class of superconducting materials.Acknowledgements We thank S. T. Weir, D. D. Jackson, and Y. K. Vohra for providingdesigner diamonds. Thanks are also due J. R. Jeffries for setting up our diamond anvil cellfacilities and O. Shpyrko for advice concerning x-ray diffraction. Research at University ofCalifornia, San Diego, was supported by the U.S. Departmentof Energy grant number DE-FG52-06NA26205, by the National Nuclear Security Administration under the StewardshipScience Academic Alliance Program through the U.S. Department of Energy grant numberDE-FG52-06NA26205, and the National Science Foundation grant number DMR0802478.

References

[1] Kamihara Y, Watanabe T, Hirano M, and Hosono H 2008J. Am. Chem. Soc. 130 3296[2] Takahashi H, Igawa K, Arii K, Kamihara Y, Hirano M, and Hosono H 2008Nature 453 367[3] Chen X H, Wu T, Liu R H, Chen H, and Fang D F 2008Nature 453 761[4] Chen G F, Li Z, Wu D, Li G, Hu Z, Dong J, Zheng P, Luo J L, and Wang N L 2008 arXiv:cond-

mat/0803.3790[5] Ren Z, Lu W, Yang J, Yi W, Shen X, Li Z, Che G, Dong X, and Sun L 2008 arXiv:cond-mat/0804.2053[6] Sefat A S, McGuire M A, Sales B C, Jin R, Howe J Y, and MandrusD 2008Phys. Rev. B 77 174503[7] Zimmer B I, Jeitschko W, Albering J H, Glaum R, and ReehuisM 1995J. Alloys and Comp. 229 238.[8] Quebe P, Terbuchte L J, and Jeitschko W 2000J. Alloys and Comp. 302 70[9] Kamihara Y, Hiramatsu H, Hirano M, Kawamura R, Yanagi H, Kamiya T, and Hosono H 2006J. Am.

Chem. Soc. 128 10012.[10] Tegel M, Schellenberg I, Pottgen R, and Johrendt D 2008arXiv:cond-mat/0805.1208[11] McQueen T M, Regulacio M, Williams A J, Huang Q, Lynn J W, Hor Y S, West D V, Green M A, and

Cava R J 2008 arXiv:cond-mat/0805.2149[12] Krellner C and Geibel C 2008J. of Crystal Growth 310 1875[13] Weir S T, Akella J, Aracne-Ruddle C, Vohra Y K, and Catledge S A 2000Appl. Phys. Lett. 77 3400[14] Chijioke A D, Nellis W J, Soldatov A, and Silvera I F 2005J. Appl. Phys. 98 114905[15] Jackson D D, Jeffries J R, Qiu W, Griffiths J D, McCall S, Aracne C, Fluss M, Maple M B, Weir S T, and

Vohra Y K 2006Phys. Rev. B 74 174401[16] Clogston A M 1962Phys. Rev. Lett. 9 266.


[17] Chandrasekhar B S 1962Appl. Phys. Lett. 1 7[18] see, for example, Salamon M B, Chapter 2, p. 43, inPhysical Properties of Superconductors, Ginsberg D

M, ed., (World Scientific, Singapore, 1989).[19] Hake R R 1967Appl. Phys. Lett. 10 189[20] Bellavance D, Mikkelsen J, and Wold A 1970J. Solid State Chem. 2 285[21] Bardeen J, Cooper L N, and Schreiffer J R 1957Phys. Rev. 108 1175[22] Kohama Y, Kamihara Y, Kawaji H, Atake T, Hirano M, and Hosono H, 2008 arXiv:cond-mat/0806.3139[23] Kadowaki K and Woods S B, Solid State Commun.58 507

j. j. hamlin, r. e. baumbach, d. a. zocco, t. a. sayles ... · 0.35 k. in this paper, we report the...

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