Andreev Reflection in Scanning Tunneling Spectroscopy of Unconventional Superconductors

TL;DR

This paper develops an analytical scattering theory for STM measurements on 2D unconventional superconductors, highlighting how Andreev reflections at various transmissions reveal detailed gap structures and symmetries.

Contribution

It introduces a comprehensive scattering theory that incorporates Andreev reflections at arbitrary transmissions, enhancing analysis of superconducting gap structures in STM experiments.

Findings

Andreev reflections provide additional information about gap symmetry.

The theory explains recent STM results on twisted bilayer graphene.

Enhanced understanding of superconducting gap structures in 2D materials.

Abstract

We evaluate the differential conductance measured in a scanning tunneling microscopy (STM) setting at arbitrary electron transmission between an STM tip and a two-dimensional (2D) superconductor with arbitrary gap structure. Our analytical scattering theory accounts for Andreev reflections, which become prominent at larger transmissions. We show that this provides complementary information about the superconducting gap structure beyond the tunneling density of states, strongly facilitating the ability to extract the gap symmetry and its relation to the underlying crystalline lattice. We use the developed theory to discuss recent experimental results on superconductivity in twisted bilayer graphene.