Unconventional superconductivity on a topological insulator

TL;DR

This paper investigates unconventional superconductivity on topological insulator surfaces, revealing gapless spectra for triplet pairing, Majorana states for singlet pairing, and the effects of Zeeman fields on surface states, with experimental predictions.

Contribution

It demonstrates the emergence of Majorana fermions in topological insulators with specific pairing symmetries and analyzes the impact of magnetic fields on surface superconductivity.

Findings

Gapless excitation spectrum for spin-triplet pairing.

Majorana fermions appear in spin-singlet d_{xy}-wave pairing.

Zeeman field significantly alters surface state transport properties.

Abstract

We study proximity-induced superconductivity on the surface of a topological insulator (TI), focusing on unconventional pairing. We find that the excitation spectrum becomes gapless for any spin-triplet pairing, such that both subgap bound states and Andreev reflection is strongly suppressed. For spin-singlet pairing, the zero-energy surface state in the $d_{xy}$-wave case becomes a Majorana fermion, in contrast to the situation realized in the topologically trivial high-$T_c$ cuprates. We also study the influence of a Zeeman field on the surface states. Both the magnitude and direction of this field is shown to strongly influence the transport properties, in contrast to the case without TI. We predict an experimental signature of the Majorana states via conductance spectroscopy.