Signatures of van Hove singularities probed by the supercurrent in a graphene - hBN superlattice

TL;DR

This study investigates how van Hove singularities in a graphene-hBN superlattice affect supercurrent behavior, revealing increased density of states and edge currents linked to flat bands and Fermi velocity reduction.

Contribution

It provides experimental evidence of van Hove singularities' signatures in supercurrent measurements and identifies edge supercurrents at these singularities in a graphene superlattice.

Findings

Enhanced density of states at van Hove singularities observed.

Presence of edge supercurrents linked to flat band effects.

Supercurrent behavior aligns with theoretical predictions.

Abstract

The moir\'e superlattice induced in graphene by the hexagonal boron nitride substrate modifies strongly the bandstructure of graphene, which manifests itself by the appearance of new Dirac points, accompanied by van Hove singularities. In this work, we present supercurrent measurements in a Josephson junction made from such a graphene superlattice in the long and diffusive regime, where that the supercurrent depends on the Thouless energy. We can then estimate the specific density of states of the graphene superlattice from the combined measurement of the critical current and the normal state resistance. The result matches with theoretical predictions and highlights the strong increase of the density of states at the van Hove singularities. By measuring the magnetic field dependence of the supercurrent, we find the presence of edge currents at these singularities. We explain it by the reduction of the Fermi velocity associated with the flat band at the van Hove singularity, which suppresses the supercurrent in the bulk while the electrons at the edge remain less localized, resulting in an edge supercurrent. We attribute this different behavior of the edges to defects or chemical doping.