Superconducting fluctuations and uniaxial-pressure dependence of T-c of a Bi2Sr2CaCu2O8+x single crystal from high-resolution thermal expansion

The anisotropic thermal expansion of a Bi2Sr2CaCu2O8+x single crystal was measured between 10 and 300 K using a high-resolution capacitance dilatometer. Clear anomalies of the expansivity, which are dominated by fluctuations and have no significant mean-field character, are observed at the superconducting transition. The relatively large anomalies and small background allows us to analyze these fluctuations in detail. It is argued, from an examination of the smoothness of the fitted background, that critical rather than 2D-Gaussian fluctuations dominate the behavior over a wide temperature range. The uniaxial-pressure dependencies of T-c (dT(c)/dp(i); i = a, b, c) are calculated from the amplitudes of the expansivity anomalies using thermodynamic relations. These are found to be very anisotropic (dT(c)/dp(a) = 1.6(2) K/GPa, dT(c)/dp(b) = 2.0(2) K/GPa and dT(c)/dp(c) = -2.8(9) K/GPa) and to largely cancel for hydrostatic pressure.