Dirac point formation revealed by Andreev tunneling in superlattice-graphene/superconductor junctions

Abstract

A graphene superlattice is formed by a one-dimensional periodic potential and is characterized by the emergence of new Dirac points in the electronic structure. The group velocity of graphene's massless Dirac fermions at the new points is drastically reduced, resulting in a measurable effect in the conductance spectroscopy. We show here that tunnel spectroscopy using a superconducting hybrid junction is more sensitive to the formation of Dirac points in the spectrum of graphene superlattices due to the additional contribution of Andreev processes. We examine the transport properties of a graphene-based superlattice-superconductor hybrid junction and demonstrate that a superlattice potential can coexist with proximity-induced superconducting correlations. Both effects contribute to change graphene's spectrum for subgap energies, and as a result, the normalized tunneling conductance features sharp changes for voltages proportional to the energy separation between the original and newly generated Dirac points. Consequently, the superconducting differential conductance provides an excellent tool to reveal how the new Dirac points emerge from the original band. This result is robust against asymmetries and finite-size effects in the superlattice potential and is improved by an effective doping comparable to the superconducting gap.