# Probing the quantum phase transition in Mott insulator BaCoS_2 tuned by pressure and Ni-substitution

## TL;DR

This study investigates the quantum phase transition in BaCoS_2 driven by pressure and Ni-substitution, revealing a first-order transition with phase separation and no critical dynamics, similar to other magnetic Mott insulators.

## Contribution

It provides the first detailed muon spin relaxation analysis of the bandwidth- and filling-controlled Mott transition in BaCoS_2, showing a first-order transition without critical fluctuations.

## Key findings

- Antiferromagnetic insulator transitions to metallic states without structural change.
- Abrupt magnetic moment collapse under pressure at p_cr ~ 1.3 GPa.
- Gradual decrease of magnetic order with Ni doping until x_cr ~ 0.22.

## Abstract

We present a muon spin relaxation study of the Mott transition in BaCoS_2 using two independent control parameters: (i) pressure p to tune the electronic bandwidth and (ii) Ni-substitution x on the Co site to tune the band filling. For both tuning parameters, the antiferromagnetic insulating state first transitions to an antiferromagnetic metal and finally to a paramagnetic metal without undergoing any structural phase transition. BaCoS_2 under pressure displays minimal change in the ordered magnetic moment S_ord until it collapses abruptly upon entering the antiferromagnetic metallic state at p_cr ~ 1.3 GPa. In contrast, S_ord in the Ni-doped system Ba(Co_{1-x}Ni_{x})S_{2} steadily decreases with increasing x until the antiferromagnetic metallic region is reached at x_cr ~ 0.22. In both cases, significant phase separation between magnetic and nonmagnetic regions develops when approaching p_cr or x_cr, and the antiferromagnetic metallic state is characterized by weak, random, static magnetism in a small volume fraction. No dynamical critical behavior is observed near the transition for either tuning parameter. These results demonstrate that the quantum evolution of both the bandwidth- and filling-controlled metal-insulator transition at zero temperature proceeds as a first-order transition. This behavior is common to magnetic Mott transitions in RENiO_3 and V_2O_3, which are accompanied by structural transitions without the formation of an antiferromagnetic metal phase.