J.E. Gordon

Specific heat of the high-T c oxide superconductors

We review the specific heat measurements on La2CuO4, La2−xMxCuO4 (M = Ca, Sr, and Ba), YBa2Cu3O7, and the Bi-Ca-Sr-Cu-O and Tl-Ca-Ba-Cu-O systems. Tables of properties derived from the data are presented. Results on RBa2Cu3O7 (R=rare earth elements other than Y) are summarized, as are results on YBa2(Cu3−xMx)O7 (M=Zn, Cr, Fe, or Ni). The difficulties of analyzing the specific heat data, and specifically the separation of the contributions associated with magnetic impurities, are discussed. It is tentatively concluded that the data nearTc are consistent with BCS theory, although they show evidence of fluctuation effects. It is also concluded that the low-temperature zero-field data on a majority of the high-Tc oxide superconductors provide evidence of anintrinsic term that is proportional toT, a result that is inconsistent with a gap in the electronic density of states.

Specific heat of YBa2Cu3O7

Abstract The specific heats of five YBa 2 Cu 3 O 7 samples were measured in the range 0.4–100K in fields of 0, 3.5 and 7T. For two samples Cr was partially substituted for Cu. This produced only a small decrease in T c (0.35 K/mole % Cr) but a strong suppression of the Meissner effect and †C/T c , which provided a basis for estimating the normal-state specific heat near T c . A third sample had Cr present in an impurity phase. Two undoped samples showed evidence of fluctuation effects near T c . A comparison of these results with other work is given.

Specific heat data of high-Tc superconductors: Lattice and electronic contributions

Abstract A high-temperature expansion for the harmonic portion of the lattice specific heat, of the form C 3 Nk= ∑ ∞ n= 0 B n u -n , where u=[( T T b ) 2 + 1 ] and Tb≈90 K, is used to represent the lattice specific heats of the high-Tc superconductors DyBa2Cu3O7 and YBa2Cu3O7 near and above Tc. With this expansion and published data it is possible to obtain values for the electronic specific heat coefficient (γ) of 38±3 and 40±5 mJ mol-1K-2, respectively, for DyBa2Cu3O7 and YBa2Cu3O7. These values for γ, when combined with the observed ΔC(Tc) at Tc, are consistent with the BCS prediction that ΔC(Tc)=1.43γTc for a superconductor in the weak-coupling limit, although the shape of the specific heat anomaly for the dysprosium compound gives evidence of a fluctuation contribution.

Specific heat of the high-Tc superconductors (La, M)2CuO4 and YBa2Cu3O7 in magnetic fields

Abstract The specific heats, C , of the high- T c superconductors YBa 2 Cu 3 O 7 , La 1.85 Ca 0.15 CuO 4 and La 1.85 Sr 0.15 CuO 4 have been measured from 0.4 K to above the superconducting transition temperature in magnetic fields, H , from 0 to 7 T. From measured changes in C with H , values of γ( H ) = [ C ( H )/ T ] T =0 and Δ C / T c = [( C s − C n )/ T ] T = T c have been evaluated. These results have been used to estimated the normal state γ, the fraction of the sample that is superconducting in zero field, | s , and the value of the upper critical field, [ H c2 ] T =0 .

Specific heat of (BiPb)2Sr2Ca2Cu3O10−y from 78 to 300 K

Abstract The specific heat anomaly of a polycrystalline sample of BPSCCO (2223 phase) was found to have a peak height δC(T c )/T c ≈ 24 mJ/mole K 2 at T c ≈ 107 K. The anomaly provides evidence for a fluctuation contribution above T c , whereas below T c it is not possible to separate fluctuation and strong-coupling effects. A second, considerably smaller, anomaly appears at T ≈ 101 K when the sample is cooled rapidly from 140 K to 78 K.

Lattice and electronic specific heat of YBa2Cu3O7

Abstract We report specific heat measurements on YBa 2 Cu 3 O 7 (YBCO) from 78 to 284K. The higher temperature data are well-represented as the sum of a term proportional to T and a harmonic lattice contribution. These data show no evidence of an anomaly in the temperature region 200–230K. The data near T c are consistent with the specific heat of a BCS superconductor that has a contribution from 3-dimensional Gaussian fluctuations.

Origin of the “linear” term in the specific heat of YBa2Cu3O7

Abstract We have measured the specific heat, C, of a number of samples of YBCO, and interpret the results as showing the existence of localized magnetic moments on the YBCO lattice that suppress the transition to the superconducting state. Both the extent of the transition [as measured, for example, by the “jump” in C at T c , ΔC(T c )] and the magnitude of the low-temperature “linear” term in C, γ(O)T, are correlated with the concentration of these moments in the way expected for pair-breaking centers: γ(O) increases and ΔC(T c )/T c decreases with increasing moment concentration. The contribution to γ(O) associated with these pair-breaking centers, together with that previously recognized as arising from impurity phases, accounts completely for the observed γ(O). We are not aware of other results that are inconsistent with these correlations, and conclude that there is no contribution to γ(O) that is an intrinsic property of the superconducting state in YBCO.

Strong coupling, fluctuations and the specific heat of YBa2,Cu3O7-δ

Abstract Specific-heat data on the high-temperature superconductor YBa2Cu3O7-δ provide evidence for strong coupling. The data on ceramic and single-crystal samples also indicate that, when there is a fluctuation contribution to the specific-heat anomaly at Tc the anomaly is reduced in height. These results are consistent with the idea that an oxygen deficiency reduces γ and enhances fluctuation effects.

Specific heat of high-Tc YBa2(Cu3-xMx)O7 with M=Cr or Zn

Abstract The effects of substitution of Cr and Zn for Cu in YBCO have been studied calorimetrically and by magnetic measurements. There is a qualitative correlation between the suppression of the superconducting transition and an increase in the concentration of localized moments in both cases.

The specific heat of HgBa2CuO4+σ

Abstract The specific heat of a polycrystalline sample of HgBa 2 CuO 4+σ has been measured between 0.6 and 24K in zero field and in magnetic fields to 9T between 0.6 and 120K. The anomaly near T c is small and appears to arise largely from fluctuation effects.