Specific Heat and Thermal Expansion Anomalies of High Temperature Superconductors

We present high resolution measurements of the specific heat and of the thermal expansion of well characterized samples of three different classes of the high Tc superconductors (HTSC), namely of La-Sr-Cu-O, Y-Ba-Cu-O and Bi- and TI-2212 and -2223. At the Tc of all these samples we observe specific heat anomalies. In the case of La-Sr-Cu-0 the anomaly can be described by a broadened mean field step, while the anomaly of Y-Ba-Cu-0 is modified by a contribution from (gaussian) fluctuations of the superconducting order parameter. These fluctuations are threedimensional (3D) near Tc and show a crossover to 2D-behaviour ≈ 20 K from Tc. The anomalies of the Bi and Tl high Tc superconductors have very little similarity with a mean field jump. The analysis of these anomalies gives evidence for strong 2D-gaussian fluctuations further away from Tc and a changeover to critical fluctuations within ≈ ±5 K around Tc . The thermal expansion anomalies of the HTSC are strongly anisotropic. From a combination of both measurements we give estimates for the uniaxial or hydrostatic pressure dependencies dTc/dpi|T=Tc.