Critical and Gaussian fluctuation effects in the specific heat and conductivity of high-Tc, superconductors

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Abstract
We report measurements of specific heat and electrical resistivity for three high-Tc, oxides, YBa2Cu3O7-δ for 0⩽ δ ⩽ 0·18, (Ca0·8Y0·2)Sr2(Tl0·5Pb0·5)Cu2O7 and Tl1·8Ba2Ca2·2Cu3O10 for which superconducting fluctuations increase rapidly as the interplane coupling is reduced. The coherence lengths ξab and ξe, and Ginzburg temperatures τG are deduced from the data. For YBa2Cu3O7-δ we observe a factor of three decrease in ξe, and an order of magnitude increase in τG between δ = 0 and 0.1, resulting in a progression from relatively weak three-dimensional fluctuations for δ = 0 to strong two-dimensional (2D) critical fluctuations over a wide temperature range for δ⩾,0·1. Large values of τG and strong 2D critical behaviour are also observed in the other two compounds. We discuss the relevance of these results to the spin-gap behaviour observed above Tc , in other properties of oxygendepleted YBa2Cu3O7-δ. Our values of ξ are lower than those deduced from Hc2 measurements, raising the possibility that additional degrees of freedom contribute to the specific heat.