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 S-ord until it collapses abruptly upon entering the antiferromagnetic metallic state at p(cr) similar to 1.3GPa. In contrast, S-ord in the Ni-doped system Ba(Co1-xNix)S-2 steadily decreases with increasing x until the antiferromagnetic metallic region is reached at x(cr) similar to 0.22. In both cases, significant phase separation between regions with static magnetic order and paramagnetic/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 RNiO3 and V2O3, which are accompanied by structural transitions without the formation of an antiferromagnetic metal phase.