Evidence of the Coulomb gap in the density of states of MoS$_2$

TL;DR

This study provides experimental evidence of Coulomb gap formation in the density of states of MoS₂, revealing the interplay of disorder and Coulomb interactions in its insulating state through magnetotransport measurements.

Contribution

It demonstrates the presence of a Coulomb gap in MoS₂ and analyzes the effects of disorder and Coulomb interactions on its insulating state using magnetotransport experiments.

Findings

Observation of non-saturating positive magnetoresistance.

Resistance increases significantly at low electron density and high magnetic field.

Temperature and magnetic field dependence consistent with Efros-Shklovskii law.

Abstract

$\mathrm{MoS_2}$ is an emergent van der Waals material that shows promising prospects in semiconductor industry and optoelectronic applications. However, its electronic properties are not yet fully understood. In particular, the nature of the insulating state at low carrier density deserves further investigation, as it is important for fundamental research and applications. In this study, we investigate the insulating state of a dual-gated exfoliated bilayer $\mathrm{MoS_2}$ field-effect transistor by performing magnetotransport experiments. We observe positive and non-saturating magnetoresistance, in a regime where only one band contributes to electron transport. At low electron density ($\sim 1.4\times 10^{12}~\mathrm{cm^{-2}}$) and a perpendicular magnetic field of 7 Tesla, the resistance exceeds by more than one order of magnitude the zero field resistance and exponentially drops with increasing temperature. We attribute this observation to strong electron localization. Both temperature and magnetic field dependence can, at least qualitatively, be described by the Efros-Shklovskii law, predicting the formation of a Coulomb gap in the density of states due to Coulomb interactions. However, the localization length obtained from fitting the temperature dependence exceeds by more than one order of magnitude the one obtained from the magnetic field dependence. We attribute this discrepancy to the presence of a nearby metallic gate, which provides electrostatic screening and thus reduces long-range Coulomb interactions. The result of our study suggests that the insulating state of $\mathrm{MoS_2}$ originates from a combination of disorder-driven electron localization and Coulomb interactions.