Specific heat anomalies in high temperature superconductors: Critical behavior or gaussian fluctuations?

The width of critical region and 2D→3D crossover temperature for high- T c superconductors of yttrium, bismuth and thallium families are estimated. For Bi- and Tl-based compounds the Ginzburg number Gi turns out to be 0.02-0.05 while for Y-123 Gi≈0.003. In Bi-2212 and Tl-2212 phases superconducting fluctuations appear to be two-dimensional up to ¦ T– T c¦∼0.3 –0.1 K. The results of recent high resolution measurements of the specific heat of the above superconductors [1–3] are discussed in detail. The experimentally accessible range is shown to cover the region of gaussian fluctuations and, for Bi and Tl compounds, the crossover region. It is argued that the logarithmic temperature dependence of fluctuation specific heat obtained would not be considered as an evidence of true 3D critical behaviour since it is found outside 3D critical region and the critical amplitude ratios measured [2,3] turn out to be essentially non-universal.