Quantized Quadrupole Superconductors

TL;DR

This paper introduces a new class of superconductors called quantized quadrupole superconductors that host Majorana corner modes, with the quadrupole moment serving as a topological invariant, and demonstrates their robustness and potential for realizing Majorana physics.

Contribution

The paper proposes the concept of quantized quadrupole superconductors with Majorana corner modes and identifies the quadrupole moment as a topological invariant, expanding the understanding of higher-order topological superconductivity.

Findings

Quadrupole moment is half-quantized due to particle-hole symmetry.

Mixed pairing states induce quadrupole phases in bilayer 2D electron gases.

The nontrivial phase is robust against phase fluctuations and disorder.

Abstract

We introduce a class of superconductors termed "quantized quadrupole superconductors" that support Majorana corner modes according to the bulk-corner correspondence, distinct from previous works on the second-order topological superconductors. An intrinsic physical quantity for superconductors, i.e., the quadrupole moment serves as the topological invariant, which is always half-quantized due to the particle-hole symmetry. As examples, two types of mixed pairings, $d_{x^{2}-y^{2}}\pm id_{xy}$ and $d_{x^{2}-y^{2}}\pm is$, induced in the bilayer two-dimensional electron gases with Rashba spin-orbit coupling give the quadrupole phase. Extended discussions indicate that the nontrivial phase is robust against relative phase fluctuations in the mixed pairings and the disorders. Our schemes provide realistic platforms to implement Majorana zero modes, paving the way for studying the Majorana physics.