Splitting the hinge mode of higher-order topological insulators

TL;DR

This paper investigates how perturbations like magnetic fields and superconductors affect the hinge modes of higher-order topological insulators, revealing potential for gapless and split modes with implications for topological quantum states.

Contribution

It demonstrates that hinge modes in HOTIs can remain gapless and split under certain perturbations, unlike in inversion symmetric TIs, and explores their potential for topological quantum applications.

Findings

Zeeman field can split hinge modes into chiral modes.

Superconductors can induce helical Majorana modes at hinges.

Different topological states can be detected via electrical transport.

Abstract

The surface of a higher order topological insulator (HOTI) comprises a two-dimensional topological insulator (TI) with broken inversion symmetry, whose mass is determined by the microscopic details of the surface such as surface potentials and termination. It hosts a helical mode pinned to selected hinges where the surface gap changes its sign. We study the effect of perturbations that break time-reversal and particle-conservation on this helical mode, such as a Zeeman field and a proximate superconductor. We find that in contrast to the helical modes of inversion symmetric TIs, which are gapped by these couplings, the helical modes at the hinges can remain gapless and spatially split. When this happens, the Zeeman field splits the helical mode into a chiral mode surrounding the magnetized region; and a superconductor results in a helical Majorana mode surrounding the superconducting region. The combination of the two might lead to the gapping of one of the chiral Majorana modes, and leave a single one-dimensional chiral Majorana around the superconducting island. We propose that the different topological states can be measured in electrical transport.