Spectroscopy of bulk and few-layer superconducting NbSe2 with van der Waals tunnel junctions

Abstract

Tunnel junctions, an established platform for high resolution spectroscopy of superconductors, require defect-free insulating barriers; however, oxides, the most common barrier, can only grow on a limited selection of materials. We show that van der Waals tunnel barriers, fabricated by exfoliation and transfer of layered semiconductors, sustain stable currents with strong suppression of sub-gap tunneling. This allows us to measure the spectra of bulk (20 nm) and ultrathin (3- and 4-layer) NbSe2 devices at 70 mK. These exhibit two distinct superconducting gaps, the larger of which decreases monotonically with thickness and critical temperature. The spectra are analyzed using a two-band model incorporating depairing. In the bulk, the smaller gap exhibits strong depairing in in-plane magnetic fields, consistent with high out-of-plane Fermi velocity. In the few-layer devices, the large gap exhibits negligible depairing, consistent with out-of-plane spin locking due to Ising spin–orbit coupling. In the 3-layer device, the large gap persists beyond the Pauli limit.

Magnetic-related States and Order Parameter Induced in a Conventional Superconductor by Nonmagnetic Chiral Molecules

“…The tunneling spectra before adsorption exhibit a two-band hard gap structure ( Fig. 4a), consistent with the s-wave two-band density of states of NbSe2 40 . After molecule adsorption, six in-gap peaks appeared, situated nearly symmetrically around zero bias, but showing a strong asymmetry in magnitude.…”

Zeeman Tunability of Andreev Bound States in van der Waals Tunnel Barriers

“…Normal counter electrodes were fabricated using standard e-beam lithography methods as reported earlier. 22 Typical junction dimensions are in the order of 1-2 µm 2 and barriers are 2-3 nm thick. Measurements are conducted using standard lock-in technique, where a bias voltage V is applied to the Au counter electrode and the current I and differential conductance dI/dV are measured through a current pre-amp in ohmic contact with the NbSe 2 bulk.…”

Tunneling spectroscopy of localized states of WS2 barriers in vertical van der Waals heterostructures

“…Semiconducting layered materials such as transition metal dichalcogenides (TMDCs) are promising building blocks for transistors and tunneling devices [12][13][14]. Furthermore, because of their crystallinity and absence of surface dangling bonds, they can be used as ideal substrates and barriers [15][16][17] when the Fermi level is placed inside their band gap. Earlier studies have shown that WS 2 , which has the largest band gap among the TMDCs, is a promising material for tunneling transistors [15,18].…”