Theory of a Higher Order Phase Transition: Superconducting Transition in BKBO

TL;DR

This paper explores the theoretical framework of a higher-order phase transition, specifically a fourth-order transition, in superconductors like Ba0.6K0.4BiO3, analyzing thermodynamic and magnetic properties and discussing experimental evidence.

Contribution

It develops a theoretical model for a fourth-order phase transition, including thermodynamic equations and free energy analysis, and compares predictions with experimental data on BiO3-based superconductors.

Findings

Derived an equation for the phase boundary based on thermodynamic discontinuities.

Developed a Ginzburg-Landau free energy for higher-order transitions.

Discussed experimental results and alternative explanations for superconducting transitions.

Abstract

We describe here the properties expected of a higher (with emphasis on the order fourth) order phase transition. The order is identified in the sense first noted by Ehrenfest, namely in terms of the temperature dependence of the ordered state free energy near the phase boundary. We have derived an equation for the phase boundary in terms of the discontinuities in thermodynamic observables, developed a Ginzburg-Landau free energy and studied the thermodynamic and magnetic properties. We also discuss the current status of experiments on $Ba_{0.6}K_{0.4}BiO_3$ and other $BiO_3$ based superconductors, the expectations for parameters and examine alternative explanations of the experimental results.