Conduction via surface states in antiferromagnetic Mott-insulating NiS2 single crystals

Abstract

Pyrite structure transition-metal disulfides exhibit diverse ground states vs $d$-band filling, spanning diamagnetic semiconducting, ferromagnetic metallic, antiferromagnetic Mott insulating, and superconducting in $\mathrm{Fe}{\mathrm{S}}_{2}, \mathrm{Co}{\mathrm{S}}_{2}, \mathrm{Ni}{\mathrm{S}}_{2}$, and $\mathrm{Cu}{\mathrm{S}}_{2}$. $\mathrm{Ni}{\mathrm{S}}_{2}$ is particularly interesting and poorly understood as its Mott insulating behavior is accompanied by complex antiferromagnetic ordering below \ensuremath{\sim}38 K and perplexing weak ferromagnetism below \ensuremath{\sim}30 K. Temperature-, pressure-, and composition-dependent insulator-metal transitions also occur, particularly in bandwidth-controlled $\mathrm{Ni}{\mathrm{S}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$, hole-doped ${\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{S}}_{2}$, etc. Here, we use high-quality chemical-vapor-transport-grown $\mathrm{Ni}{\mathrm{S}}_{2}$ single crystals characterized by x-ray diffraction, energy-dispersive x-ray spectroscopy, magnetometry, and extensive transport and magnetotransport measurements, to generate new insight into this system. In particular, resistivity, magnetoresistance, and Hall effect analyses vs temperature, thickness, and surface preparation, provide unequivocal evidence of surface conduction, where the more conductive surface shunts essentially all current at low temperatures. The surface transport changes from two dimensional and insulating to three dimensional and metallic as the surface preparation is varied (also displaying intriguing sensitivity to magnetic ordering), significantly clarifying literature ambiguities with respect to the electronic ground state. These results have immediate implications. First, the temperature-, pressure-, and composition-dependent insulator-metal transitions deduced in the extensive prior work on $\mathrm{Ni}{\mathrm{S}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}, {\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{S}}_{2}$, etc., must clearly be reexamined in light of rife metallic surface conduction, not previously taken into account. Second, $\mathrm{Ni}{\mathrm{S}}_{2}$ now joins $\mathrm{Fe}{\mathrm{S}}_{2}$ and $\mathrm{Co}{\mathrm{S}}_{2}$ as systems in which bulk and surface electronic behaviors are strikingly different, suggesting that metallic surface states could be a universal feature of pyrite structure transition-metal disulfides.