Mixed-parity octupolar pairing and corner Majorana modes in three dimensions

TL;DR

This paper proposes a new class of three-dimensional topological superconductors with Majorana corner modes, achieved through mixed-parity pairing states that break time-reversal symmetry and are realizable in doped octupolar insulators.

Contribution

It introduces a novel third-order topological superconductor model with mixed-parity pairing, predicting Majorana corner modes in cubic crystals, and suggests experimental realization in high-pressure compounds.

Findings

Identification of mixed-parity pairing states hosting Majorana corner modes

Proposal of doped octupolar insulators as platforms for these superconductors

Potential realization in NaCl and similar compounds under high pressure

Abstract

We identify time-reversal symmetry breaking mixed-parity superconducting states that feature eight Majorana corner modes in properly cleaved three-dimensional cubic crystals. Namely, when an odd-parity isotropic $p$-wave pairing coexists with cubic symmetry preserving even-parity octupolar $d_{x^2-y^2}+i d_{3z^2-r^2}$ pairing, the gapless surface Majorana modes of the former get localized at the eight corners, thus yielding an \emph{intrinsic} third-order topological superconductor (TOTSC). A cousin $d_{xy}+id_{3z^2-r^2}$ pairing also accommodating eight corner Majorana modes, by virtue of breaking the cubic symmetry, in contrast, yields an \emph{extrinsic} TOTSC. We identify a doped octupolar (topological or trivial) Dirac insulator as a suitable platform to sustain such unconventional superconductors, realized from an intraunit cell pairing. Finally, we argue that the proposed TOTSC can be experimentally realizable in NaCl and other structurally similar compounds under high pressure.