CROSSOVER ANALYSIS OF THE HEAT-CAPACITY OF YBA2CU3O7-X NEAR TC - EVIDENCE FOR XY-LIKE CRITICAL-BEHAVIOR

The heat-capacity anomaly of a largely untwinned single crystal of YBa2Cu307-x is reanalyzed by means of a crossover model. The conventional Gaussian-corrected BCS shape is shown to be inadequate to fit the data within +/-4 K of T(c), a range much larger than estimated from the Ginzburg criterion. Improved fits in the vicinity of T(c) were obtained using the approach of Chen, Albright, and Sengers, who introduced a crossover function to interpolate smoothly between critical and mean-field behavior. The best fit was obtained for the three-dimensional XY model, the expected "intermediate" critical behavior of a superconductor. From the crossover analysis, we estimate the zero-temperture coherence length to be xi0 congruent-to 12 angstrom.