Bragg-Williams order competes with superconductivity

TL;DR

This study demonstrates that Bragg-Williams order (BWO) in In2/3PSe3 competes with superconductivity, with disordered phases exhibiting higher Tc, highlighting BWO as an independent competing order parameter.

Contribution

It provides experimental evidence that BWO directly suppresses superconductivity, supported by phenomenological and microscopic analyses, extending understanding of competing orders.

Findings

Disordered In2/3PSe3 exhibits superconductivity at 11 K under pressure.

Ordered phase shows a lower Tc of 7 K, indicating BWO suppresses superconductivity.

BWO acts as an independent order parameter competing with superconductivity.

Abstract

Orderings in charge and spin have been extensively studied to unravel their correlation to emergent superconductivity over the past decades. Bragg-Williams order (BWO), a classical structural order parameter describing site occupancy in alloys, has long been speculated to influence superconducting behavior. Yet, its role still remains ambiguous, largely due to the difficulty of isolating BWO from concomitant charge doping or competing electronic instabilities. Here, we establish In2/3PSe3 as a platform wherein indium vacancies are reversibly configurable between ordered and disordered states via thermal treatment. We show that the disordered phase undergoes a pressure-induced superconducting transition with a Tc of 11 K, significantly higher than the 7 K observed in its ordered counterpart. This constitutes a rare instance in which pure BWO variation drives a substantial shift in Tc. By combining a Ginzburg-Landau phenomenological analysis with a BCS-McMillan microscopic description, we demonstrate that BWO naturally suppresses superconductivity through electron-phonon interactions, a mechanism supported by ultra-low-wavenumber Raman measurements. Our findings support BWO as an independent order parameter that competes directly with superconductivity, extending the concept of competing orders beyond conventional electronic and magnetic degrees of freedom.