Unconventional gate-induced superconductivity in transition-metal dichalcogenides

TL;DR

This paper develops a comprehensive theoretical framework combining first-principles simulations and BCS gap equations to explain gate-induced superconductivity in transition-metal dichalcogenides, accounting for multi-valley effects and Coulomb interactions.

Contribution

It introduces a realistic, multi-valley model that explains experimental observations of superconductivity in TMDs, including the role of electron density and Coulomb interactions.

Findings

Superconductivity occurs when Q valleys are occupied due to electron density increase.

Electron-phonon interactions are significantly enhanced at high electron densities.

Coulomb interactions can induce a sign change in the superconducting gap between valleys.

Abstract

Superconductivity in few-layer semiconducting transition metal dichalcogenides (TMDs) can be induced by field-effect doping through ionic-liquid gating. While several experimental observations have been collected over the years, a fully-consistent theoretical picture is still missing. Here we develop a realistic framework that combines the predictive power of first-principles simulations with the versatility and insight of Bardeen-Cooper-Schrieffer gap equations to rationalize such experiments. The multi-valley nature of semiconducting TMDs is taken into account, together with the doping- and momentum-dependent electron-phonon and Coulomb interactions. Consistently with experiments, we find that superconductivity occurs when the electron density is large enough that the Q valleys get occupied, as a result of a large enhancement of electron-phonon interactions. Despite being phonon-driven, the superconducting state is predicted to be sensitive to Coulomb interactions, which can lead to the appearance of a relative sign difference between valleys and thus to a $s_{+-}$ character. We discuss qualitatively how such scenario may account for many of the observed physical phenomena for which no microscopic explanation has been found so far, including in particular the presence of a large subgap density of states, and the sample-dependent dome-shaped dependence of $T_c$ on accumulated electron density. Our results provide a comprehensive analysis of gate-induced superconductivity in semiconducting TMDs, and introduce an approach that will likely be valuable for other multivalley electronic systems, in which superconductivity occurs at relatively low electron density.