Tunneling Spectroscopy of Two-Dimensional Materials Based on Via Contacts

Abstract

We introduce a novel planar tunneling architecture for van der Waals heterostructures based on via contacts, namely metallic contacts embedded into through-holes in hexagonal boron nitride (hBN). We use the via-based tunneling method to study the single-particle density of states of two different two-dimensional (2D) materials, superconducting NbSe2 and monolayer graphene in zero magnetic field as well as in the quantum Hall regime. In NbSe2 devices, we characterize the barrier strength and interface disorder, within the Blonder-Tinkham-Klapwijk model, for tunnel barrier thicknesses of 0, 1 and 2 layers of hBN and study the area dependence of the tunneling contacts down to 30 2 nm 2 , demonstrating that the effect of interface disorder can be suppressed in the small-area limit. Additionally, we demonstrate that for 0 layers of hBN, we cross over from diffusive to point contacts in the small-area limit. In graphene, we show that reducing the tunneling area can suppress the effects of phonon-assisted tunneling and defects in the hBN barriers. These via-based tunneling devices overcome limitations of other planar tunneling designs and can be used to produce high-quality, ultra-clean tunneling structures from a variety of 2D materials.

Andreev reflection spectroscopy in strongly paired superconductors

“…This feature stands in stark contrast to the above-gap conductance G(E ∆) that is manifestly symmetric with respect to the sign of Z [41]. Possible experimental setups for probing the negative-Z regime include (i) STS measurements using a tip with an effective quantum well at its end, implemented by attaching, e.g., a quantum dot (reminiscent of single-electron transistor probes [45,46]) or an impurity atom, or else by coating the tip with a layer of material with a smaller work function and (ii) 2D electronic transport setups where the N-S interface is tunable, either by applying a local gate near the junction or by constructing "via contacts" etched into different encapsulating insulators [47].…”