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Thermodynamics of the Superconducting Phase Transition in Ba_{0.6}K_{0.4}BiO_3

P. Kumar·Donavan Hall·R. G. Goodrich
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Abstract

We suggest that the transition to superconductivity in a single crystal of Ba_{0.6}K_{0.4}BiO_3 with a T_c = 32K, and having critical fields with anomalous temperature dependencies and vanishing discontinuities in specific heat and magnetic susceptibility, may well be an example of a fourth order (in Ehrenfest's sense) phase transition. We have derived a free energy functional for a fourth order transition and calculated (for the temperature range ${T_c/2 < T \simeq T_c}$) the temperature dependence of the critical fields. We find $H_{c1}(T)\propto (1-T/T_c)^3, H_0(T)\propto (1-T/T_c)^2$ and $H_{c2}(T)\propto (1-T/T_c)^{1}$ in general agreement with experiments.

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P. Kumar, Donavan Hall, R. G. Goodrich. 1999-06-02. Thermodynamics of the Superconducting Phase Transition in Ba_{0.6}K_{0.4}BiO_3. https://doi.org/10.1103/physrevlett.82.4532

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