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

TL;DR

This paper proposes that the superconducting transition in Ba_{0.6}K_{0.4}BiO_3 is a fourth order phase transition, supported by theoretical free energy modeling and experimental critical field behavior analysis.

Contribution

It introduces a fourth order phase transition framework for superconductivity in Ba_{0.6}K_{0.4}BiO_3 and derives temperature dependencies of critical fields consistent with experimental data.

Findings

Critical fields follow specific temperature dependencies: H_{c1}(T)∝(1-T/T_c)^3, H_0(T)∝(1-T/T_c)^2, H_{c2}(T)∝(1-T/T_c)

Superconducting transition may be a fourth order phase transition, not second order

Theoretical calculations align with experimental critical field measurements

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.