MAGNETIC-FIELD DEPENDENCE OF THE LOW-TEMPERATURE SPECIFIC-HEAT OF SOME HIGH-T-C COPPER-OXIDE SUPERCONDUCTORS - EVIDENCE FOR AN (HT)-T-1/2 CONTRIBUTION IN THE MIXED-STATE

Measurements of the magnetic-field dependence of the low-temperature specific heat of YBa2Cu3O7 and Ca and Sr doped La2CuO4 are reviewed, with an emphasis on recent measurements on a YBa2Cu3O7 sample in a number of applied magnetic fields (H) between 0 and 7 T at temperatures (T) between similar to 0.4 and 10 K. The major field-dependent components of the specific heat of that sample are a T proportional term and a contribution attributed to Cu2+ 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 (HT)-T-1/2 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 Cu2+ moments, the ''zero-field upturn'', is the high-temperature tail of a broad anomaly 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 spin-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.