Tunneling spectroscopy of gate-induced superconductivity in MoS$_2$

TL;DR

This study uses tunneling spectroscopy to analyze gate-induced superconductivity in MoS$_2$, revealing a non-conventional, partially gapped state with unique magnetic field responses, challenging traditional BCS theory.

Contribution

It provides the first detailed tunneling spectroscopy analysis of gate-induced superconductivity in MoS$_2$, uncovering a non-fully gapped state and unusual magnetic field effects.

Findings

Superconducting gap  meV at optimal doping

DOS suppression persists above critical field

Linear energy dependence of DOS near zero energy

Abstract

The ability to gate-induce superconductivity by electrostatic charge accumulation is a recent breakthrough in physics and nano-electronics. With the exception of LaAlO$_3$/SrTiO$_3$ interfaces, experiments on gate-induced superconductors have been largely confined to resistance measurements, which provide very limited information about the superconducting state. Here, we explore gate-induced superconductivity in MoS$_2$ by performing tunneling spectroscopy to determine the energy-dependent density of states (DOS) for different levels of electron density $\textit{n}$. In the superconducting state, the DOS is strongly suppressed at energy smaller than the gap, \Delta, which is maximum (\Delta ~ 2 meV) for $\textit{n}$ of ~ 10$^{14}$ cm$^{-2}$ and decreases monotonously for larger $\textit{n}$. A perpendicular magnetic field $\textit{B}$ generates states at $E<\Delta$ that fill the gap, but a 20% DOS suppression of superconducting origin unexpectedly persists much above the transport critical field. Conversely, an in-plane field up to 10 T leaves the DOS entirely unchanged. Our measurements exclude that the superconducting state in MoS$_2$ is fully gapped and reveal the presence of a DOS that vanishes linearly with energy, the explanation of which requires going beyond a conventional, purely phonon-driven Bardeen-Cooper-Schrieffer mechanism.