Amperean Pairing at the Surface of Topological Insulators

TL;DR

This paper demonstrates that ferromagnetic fluctuations on the surface of a topological insulator induce an unconventional Amperean p-wave superconducting pairing due to gauge field-mediated interactions.

Contribution

It introduces a novel mechanism for superconductivity on topological insulator surfaces via ferromagnetic fluctuation-induced gauge fields leading to Amperean pairing.

Findings

Ferromagnetic fluctuations induce gauge fields that mediate electron interactions.

The interaction favors same-direction electron pairing, leading to Amperean pairing.

A p-wave pairing instability is predicted on the topological insulator surface.

Abstract

The surface of a 3D topological insulator is described by a helical electron state with the electron's spin and momentum locked together. We show that in the presence of ferromagnetic fluctuations the surface of a topological insulator is unstable towards a superconducting state with unusual pairing, dubbed Amperean pairing. The key idea is that the dynamical fluctuations of a ferromagnetic layer deposited on the surface of a topological insulator couple to the electrons as gauge fields. The transverse components of the magnetic gauge fields are unscreened and can mediate an effective interaction between electrons. There is an attractive interaction between electrons with momenta in the same direction which makes the pairing to be of Amperean type. We show that this attractive interaction leads to a $p$-wave pairing instability of the Fermi surface in the Cooper channel.