Two-dimensional fluctuations and competing phases in the stripe-like antiferromagnet BaCoS$_2$

TL;DR

This study investigates the electronic and magnetic phase transitions in BaCoS$_2$, revealing large thermal fluctuations and competing ground states near the antiferromagnetic transition, emphasizing the role of electronic interactions over lattice effects.

Contribution

It demonstrates that the stripe-like antiferromagnetic order in BaCoS$_2$ arises purely from electronic interactions, with no lattice participation, highlighting the importance of orbital and spin competition.

Findings

Evidence of a purely electronic AFM transition at 290 K.

Detection of large thermal fluctuations near $T_N$.

Identification of competing ground states within ~1 meV energy range.

Abstract

By means of a combined x-ray diffraction, magnetic susceptibility and specific heat study, we investigate the interplay between orthorhombic distortion and stripe-like antiferromagnetic (AFM) order in the Mott insulator BaCoS$_{2}$ at $T_N=290$ K. The data give evidence of a purely electronic AFM transition with no participation of the lattice. The observation of large thermal fluctuations in the vicinity of $T_N$ and a Schottky anomaly unveils competing ground states within a minute $\sim$1 meV energy range that differ in the orbital and spin configurations of the Co ions. This interpretation suggests that the stripe-like order results from a spontaneous symmetry breaking of the geometrically frustrated pristine tetragonal phase, which offers an ideal playground to study the driving force of multi-orbital Mott transitions without the participation of the lattice.