Possible topological phases in quantum anomalous Hall insulator/unconventional superconductor hybrid systems

TL;DR

This paper explores topological phases in quantum anomalous Hall insulator and unconventional superconductor hybrid systems, revealing new phases like crystalline symmetry-protected modes and multiple chiral Majorana edge states.

Contribution

It introduces a classification of topological phases in QAH/unconventional SC hybrids using three topological numbers and uncovers novel phases absent in s-wave systems.

Findings

Identification of crystalline symmetry-protected helical Majorana modes

Discovery of a line node phase with Bogoliubov Fermi surface

Multiple chiral Majorana edge modes in certain regimes

Abstract

Quantum anomalous Hall insulator (QAH)/$s$-wave superconductor (SC) hybrid systems are known to be an ideal platform for realizing two-dimensional topological superconductors with chiral Majorana edge modes. In this paper we study QAH/unconventional SC hybrid systems whose pairing symmetry is $p$-wave, $d$-wave, chiral $p$-wave, or chiral $d$-wave. The hybrid systems are a generalization of the QAH/$s$-wave SC hybrid system. In view of symmetries of the QAH and pairings, we introduce three topological numbers to classify topological phases of the hybrid systems. One is the Chern number that characterizes chiral Majorana edge modes and the others are topological numbers associated with crystalline symmetries. We numerically calculate the topological numbers and associated surface states for three characteristic regimes that feature an influence of unconventional SCs on QAHs. Our calculation shows a rich variety of topological phases and unveils the following topological phases that are no counterpart of the $s$-wave case: crystalline symmetry-protected helical Majorana edge modes, a line node phase (crystalline-symmetry-protected Bogoliubov Fermi surface), and multiple chiral Majorana edge modes. The phenomena result from a nontrivial topological interplay between the QAH and unconventional SCs. Finally, we discuss tunnel conductance in a junction between a normal metal and the hybrid systems, and show that the chiral and helical Majorana edge modes are distinguishable in terms of the presence/absence of zero-bias conductance peak.