Probing the quantum phase transition in Mott insulator BaCoS2 tuned by pressure and Ni substitution

We present a muon spin relaxation study of the Mott transition in BaCoS2 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. BaCoS2 under pressure displays minimal change in the ordered magnetic moment Sord until it collapses abruptly upon entering the antiferromagnetic metallic state at PCR∼1.3GPa. In contrast, Sord in the Ni-doped system Ba(Co1-xNix)S2 steadily decreases with increasing x until the antiferromagnetic metallic region is reached at xcr∼0.22. In both cases, significant phase separation between regions with static magnetic order and paramagnetic/nonmagnetic regions develops when approaching PCR or xcr, 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 RNiO3 and V2O3, which are accompanied by structural transitions without the formation of an antiferromagnetic metal phase.