Thermally assisted ordering in Mott insulators

TL;DR

This paper reveals that in the Mott insulator KCuF3, thermal expansion stabilizes the ordered phase, preventing a phase transition and suggesting a novel mechanism where order can emerge at high temperatures.

Contribution

It demonstrates that thermal expansion can energetically stabilize the ordered phase in Mott insulators, challenging the traditional energy-entropy competition model.

Findings

Thermal expansion stabilizes the orbitally-ordered phase in KCuF3.

No phase transition occurs in KCuF3 due to energetic stabilization.

Potential for inverted phase transitions in closed-shell systems.

Abstract

Ginzburg-Landau theory describes phase transitions as the competition between energy and entropy: The ordered phase has lower energy, while the disordered phase has larger entropy. When heating the system, ordering is reduced entropically until it vanishes at the critical temperature. This established picture implicitly assumes that the energy difference between ordered and disordered phase does not change with temperature. We show that for the Mott insulator KCuF3 this assumption is strongly violated: thermal expansion energetically stabilizes the orbitally-ordered phase to such and extent that no phase transition is observed. This new mechanism explains not only the absence of a phase transition in KCuF3 but even suggests the possibility of an inverted transition in closed-shell systems, where the ordered phase emerges only at high temperatures.