HEAT-CAPACITY OF YBA2CU3O7-DELTA CRYSTALS ALONG THE HC2 LINE

We report high-resolution ac heat-capacity measurements on YBa2Cu3O7-delta crystals near the critical temperature, several of which display untwinned regions comprising at least 50% of the sample volume. A lambda-like anomaly is observed, indicative of an exceptionally short T = 0 Ginzburg-Landau coherence length in this material. If the fluctuations are assumed to be Gaussian corrections to mean-field-behavior, the data are best described by three-dimensional fluctuations of a two-component order parameter, with the underlying BCS step indicating strong-coupling behavior. However, a more complicated order parameter or a logarithmic (critical) divergence cannot be ruled out on statistical grounds. Application of a magnetic field B broadens the transition and reduces the amplitude of the anomaly with little or no apparent shift in the onset of superconductivity; the effects are more pronounced for B parallel to the c axis than for B in the ab plane. These results are not consistent with mean-field theory, but may be understood in the context of critical finite-size scaling. This approach exploits the well-known dimensional reduction for superconductors in a magnetic field.