Contactless cavity sensing of superfluid stiffness in atomically thin 4Hb-TaS$_2$

TL;DR

This paper presents a contactless microwave resonator method to measure superfluid stiffness in atomically thin superconductors, demonstrated on 4Hb-TaS$_2$, revealing nodeless behavior despite broken mirror symmetry.

Contribution

It introduces a novel contact-free technique for probing superfluid phase stiffness in 2D superconductors using on-chip microwave resonators.

Findings

Superfluid stiffness measured in atomically thin 4Hb-TaS$_2$.

Superconducting transition temperature comparable to bulk.

Nodeless phase stiffness behavior observed despite broken mirror symmetry.

Abstract

The exceptional tunability of two-dimensional van der Waals materials offers unique opportunities for exploring novel superconducting phases. However, in such systems, the measurement of superfluid phase stiffness, a fundamental property of a superconductor, is challenging because of the mesoscopic sample size. Here, we introduce a contact-free technique for probing the electrodynamic response, and thereby the phase stiffness, of atomically thin superconductors using on-chip superconducting microwave resonators. We demonstrate this technique on 4Hb-TaS$_2$, a van der Waals superconductor whose gap structure under broken mirror symmetry is under debate. In our cleanest few-layer device, we observe a superconducting critical temperature comparable to that of the bulk. The temperature evolution of the phase stiffness features nodeless behavior in the presence of broken mirror symmetry, inconsistent with the scenario of nodal surface superconductivity. With minimal fabrication requirements, our technique enables microwave measurements across wide ranges of two-dimensional superconductors.