John E. Gordon

Magnetic-field dependence of the low-temperature specific heat of some high-Tc copper-oxide superconductors evidence for an H12T contribution in the mixed state

Abstract Measurements of the magnetic-field dependence of the low-temperature specific heat of YBa 2 Cu 3 O 7 and Ca and Sr doped La 2 CuO 4 are reviewed, with an emphasis on recent measurements on a YBa 2 Cu 3 O 7 sample in a number of applied magnetic fields ( H ) between 0 and 7 T at temperatures ( T ) between ∼ 0.4 and 10 K. The major field-dependent components of the specific heat of that sample are a T proportional term an a contribution attributed to Cu 2+ magnetic moments. In contrast with most earlier results, or their interpretation, the T proportional term is not linear in H . Within the experimental uncertainty it agrees with the H 1 2 T dependence predicted by Volovik for a superconductor with lines of nodes in the energy gap. In that respect, the results are similar to those obtained in a recent Stanford/University of British Columbia collaboration. However, there is no convincing evidence for the presence of the T 2 term in the zero-field specific heat that is also expected for lines of nodes in the gap. Possible reasons for this apparent discrepancy are discussed. Less detailed data on other samples are consistent with these findings. For H = 0, the contribution associated with the Cu 2+ moments, the “zero-field upturn”, is the high-temperature tail of a broad anomally produced by internal interactions. With increasing H it evolves into a form more closely approximated by simple Schottky functions. The H dependence of this contribution to the specific heat is consistent with that expected for a low concentration of rmspin- 1 2 moments with a distribution of internal fields that is progressively narrowed with increasing H . It is not consistent with the H proportional prefactor associated with the Schottky anomalies that has been reported elsewhere.

Dependence of the low-temperature specific heat of YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} on {delta} and magnetic field: The H{sup {1/2}}T term for H{ne}0; absence of evidence for a T{sup 2} term for H=0

Specific-heat measurements with improved precision on two new samples of YBa2Cu3O7−δ with relatively low concentrations of paramagnetic centers are reported. For one sample the measurements were made for different values of δ. The data are analyzed, and the data reported earlier for another sample reanalyzed, with an improved fitting expression. In each case, the data give a well defined H1/2T term in the mixed state. No persuasive evidence for a T2 term in the superconducting state is found.

Dependence of the low-temperature specific heat of YBa2Cu3O7−δ on δ and magnetic field: Concentrations of spin-2 and spin-1/2 paramagnetic centers

Specific heat data on two samples of YBa2Cu3O7−δ with relatively low concentrations of paramagnetic centers show the presence of both spin-2 and spin-1/2 moments. Measurements on one sample following incremental increases in δ show that for δ≤0.2 approximately one spin-1/2 moment is produced for each 100 O vacancies, with no substantial increase in the linear term of the specific heat. For several reasons it is concluded that these O-vacancy related moments are different from those that have figured in previously reported correlations with parameters characteristic of the superconductivity.

SPECIFIC HEAT OF YBA2CU3O7-DELTA AND (LA2-XSRX)CUO4 : EVIDENCE FOR D-WAVEPAIRING

New low-temperature specific-heat data for YBa2Cu3O7−δ, 0≤δ≤0.2, and (La1.85Sr0.15)CuO4 in magnetic fields (H) to 9 T are reported. They show features predicted for the lines of nodes in the energy gap that are expected for d-wave pairing; for H=0, a T2 term; for H≠0 an H1/2T term with a crossover to a more complicated T dependence at higher T and lower H. The results are compared with recent measurements in other laboratories.

Specific heat evidence for strong coupling in YBa2Cu3O7

Specific heat data for YBa2Cu3O7 are consistent with gamma =15+or-3 mJ/mole.K2 and 2 Delta o/kTc=6.8+or-0.6. These results indicate that strong-coupling effects are present but that the coupling is unlikely to be predominantly a conventional electron-phonon interaction.