Chiral Majoranas morphing into corner states in ordinary QAH/SC systems*

Typically higher-order topological superconductivity is a consequence of anisotropic mass gaps, e.g., d-wave or s_±-wave pairings, that are responsible for gaping the edge states and the emergence of Majorana corner states. In this work, we investigate the spectrum and linear conductance G of a square-shaped quantum anomalous Hall (QAH) insulator in proximity to an ordinary s-wave superconductor (SC) via exact diagonalization and non-equilibrium Green functions. We find that a single extended chiral Majorana mode running along the perimeter of our QAH-SC square lattice evolves into four localized zero-energy MCSs, one at each corner, as the chemical potential μ and the pairing gap Δ are varied. These MCSs unveil an unexpected 2nd-order topological phase region with peak conductance G = e²/h — within the QAH-SC bulk (μ,Δ) phase diagram of Qi et. al. [1]. Our MCSs are protected by a combination of time-reversal and in-plane mirror symmetries. We conjecture that these MCSs arise from the emergence of pairing-gap phase domains around the corners of our system. A Zeeman field with varying in-plane directions can be used to manipulate these MCSs.

[1] Qi, Xiao-Liang, Taylor L. Hughes, and Shou-Cheng Zhang. "Chiral topological superconductor from the quantum Hall state." Physical Review B 82.18 (2010): 184516.