Nearly flat Andreev bound states in superconductor-topological insulator hybrid structures

TL;DR

This paper demonstrates that in superconductor-topological insulator hybrid structures, Andreev bound states become nearly flat at zero energy when the chemical potential is far from the Dirac point, revealing new spectral behavior.

Contribution

It introduces the phenomenon of nearly flat Andreev bound states in TI-superconductor systems and characterizes their dependence on chemical potential and system geometry.

Findings

Andreev bound states become nearly flat at zero energy away from the Dirac point.

The flat dispersion follows a power law $E o k^N$, with $N$ depending on chemical potential.

Similar spectral evolution occurs in periodic superconducting proximity structures.

Abstract

Exotic excitations arise at the interface between a three-dimensional topological insulator (TI) and superconductors. For example, Majorana fermions with a linear dispersion, $E\sim k$, exist in a short $\pi$ Josephson junction on the TI surface. We show that in these systems, the Andreev bound states spectrum becomes nearly flat at zero energy when the chemical potential is sufficiently away from the Dirac point. The flat dispersion is well approximated by $E\sim k^N$, where $N$ scales with the chemical potential. Similar evolution from linear to flat dispersion also occurs for the subgap spectrum of a periodic superconducting proximity structure, such as a TI surface in contact with a stripe superconductor.