Spectroscopy of bulk and few-layer superconducting NbSe$_2$ with van der Waals tunnel junctions

TL;DR

This study demonstrates the fabrication and application of van der Waals tunnel junctions for high-resolution spectroscopy of NbSe2, revealing detailed insights into its superconducting gaps and depairing effects across different thicknesses and magnetic fields.

Contribution

The paper introduces a novel vdW tunnel barrier fabrication method for NbSe2, enabling detailed spectral analysis of bulk and few-layer superconductors with insights into depairing mechanisms.

Findings

Spectra show two distinct energy gaps in NbSe2.

The larger gap decreases with thickness and temperature.

Depairing effects differ between bulk and few-layer devices.

Abstract

Tunnel junctions, a well-established platform for high-resolution spectroscopy of superconductors, require defect-free insulating barriers with clean engagement to metals on both sides. Extending the range of materials accessible to tunnel junction fabrication, beyond the limited selection which allows high-quality oxide formation, requires the development of alternative fabrication techniques. Here we show that van-der-Waals (vdW) tunnel barriers, fabricated by stacking layered semiconductors on top of the transition metal dichalcogenide (TMD) superconductor NbSe$_2$, sustain a stable, low noise tunneling current, and exhibit strong suppression of sub-gap tunneling. We utilize the technique to measure the spectra of bulk (20 nm) and ultrathin (3- and 4-layer) devices at 70 mK. The spectra exhibit two distinct energy gaps, the larger of which decreases monotonously with thickness and $T_C$, in agreement with BCS theory. The spectra are analyzed using a two-band model modified to account for depairing. We show that in the bulk, the smaller gap exhibits strong depairing in an in-plane magnetic field, consistent with a high Fermi velocity. In the few-layer devices, depairing of the large gap is negligible, consistent with out-of-plane spin-locking due to Ising spin-orbit coupling. Our results demonstrate the utility of vdW tunnel junctions in mapping the intricate spectral evolution of TMD superconductors over a range of magnetic fields.