Electrochemically-driven insulator-metal transition in ionic-liquid-gated antiferromagnetic Mott-insulating NiS2 single crystals
Abstract
Motivated by the existence of superconductivity in pyrite-structure CuS2, we explore the possibility of ionic-liquid-gating-induced superconductivity in the proximal antiferromagnetic Mott insulator NiS2. A clear gating-induced transition from a two-dimensional insulating state to a three-dimensional metallic state is observed at positive gate bias on single crystal surfaces. No evidence for superconductivity is observed down to the lowest measured temperature of 0.45 K, however. Based on transport, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and other techniques, we deduce an electrochemical gating mechanism involving a substantial decrease in the S:Ni ratio (over hundreds of nm), which is both non-volatile and irreversible. This is in striking contrast to the reversible, volatile, surface-limited, electrostatic gate effect in pyrite FeS2. We attribute this stark difference in electrochemical vs. electrostatic gating response in NiS2 and FeS2 to the much larger S diffusion coefficient in NiS2, analogous to the different behaviors observed among electrolyte-gated oxides with differing O-vacancy diffusivities. The gating irreversibility, on the other hand, is associated with the lack of atmospheric S; this is in contrast to the better understood oxide case, where electrolysis of atmospheric H2O provides an O reservoir. This study of NiS2 thus provides new insight into electrolyte gating mechanisms in functional materials, in a previously unexplored limit.