Weak-Pairing Higher Order Topological Superconductors

TL;DR

This paper introduces second-order topological superconductors with gapped surfaces and gapless modes on lower-dimensional boundaries, proposing realizations via weak-pairing instabilities in specific materials and heterostructures.

Contribution

It presents two new scenarios for realizing second-order topological superconductivity through weak-pairing instabilities in Dirac semimetals and Fermi surfaces, with simple topological invariants.

Findings

Second-order topological superconductivity can be realized in doped Dirac semimetals with $p_x+ip_y$ pairing.

$p+id$ pairing on Fermi surfaces also realizes second-order topological superconductivity.

Topological invariants depend only on low-energy properties of Fermi surfaces and pairing.

Abstract

Conventional topological superconductors are fully gapped in the bulk but host gapless Majorana modes on their boundaries. We instead focus on a new class of superconductors, second-order topological superconductors, that have gapped, topological surfaces and gapless Majorana modes instead on lower-dimensional boundaries, i.e., corners of a two-dimensional system or hinges for a three-dimensional system. Here we propose two general scenarios in which second-order topological superconductivity can be realized spontaneously with weak-pairing instabilities. First, we show that $p_x+ip_y$-wave pairing in a (doped) Dirac semimetal in two dimensions with four mirror symmetric Dirac nodes realizes second-order topological superconductivity. Second, we show that $p+id$ pairing on an ordinary spin-degenerate Fermi sruface realizes second-order topological superconductivity as well. In the latter case we find that the topological invariants describing the system can be written using simple formulae involving only the low-energy properties of the Fermi surfaces and superconducting pairing. In both cases we show that these exotic superconducting states can be intrinsically realized in a metallic system with electronic interactions. For the latter case we also show it can be induced by proximity effect in a heterostructure of cuprate and topological superconductors.