SPECIFIC-HEAT OF AN UNTWINNED YBA2CU3O7-X CRYSTAL - CROSSOVER TO CRITICAL FLUCTUATIONS

We present results for the specific heat of an untwinned YBa2Cu3O7-x single crystal. The specific heat was measured with a resolution better than 50 mK between 85 K and 110 K. The divergence in the specific heat at T(c) is extremely sharp with no indication of rounding. It is not possible to fit the divergence by simply adding Gaussian fluctuations to a BCS-like step, however, we find that the divergence does fit a logarithmic divergence very well over nearly three decades between 10(-4) < \t\ < 0.1. This can be interpreted either as providing evidence for local pair superconductivity or for a wide crossover region from Gaussian to critical fluctuations.