Effective Model for Fractional Topological Corner Modes in Quasicrystals

TL;DR

This paper develops a low-energy effective model for high-order topological insulator states in 2D quasicrystals, revealing how rotational symmetries and external fields can generate fractional charge corner modes and Majorana modes.

Contribution

It introduces a novel $k \, p$ Hamiltonian framework for quasicrystals, extending topological insulator theory beyond crystalline momentum-based methods.

Findings

Zeeman field induces mass-kinks and fractional charge corner modes.

Numerical calculations confirm the existence of protected corner modes.

Proximity to s-wave superconductor enables Majorana modes in quasicrystals.

Abstract

High-order topological insulators (HOTIs), as generalized from topological crystalline insulators (TCIs), are characterized with lower-dimensional metallic boundary states protected by spatial symmetries of a crystal, whose theoretical framework based on band inversion at special $k$-points cannot be readily extended to quasicrystals because quasicrystals contain rotational symmetries that are not compatible with crystals, and momentum is no longer a good quantum number. Here, we develop a low-energy effective model underlying HOTI states in 2D quasicrystals for all possible rotational symmetries. By implementing a novel Fourier transform developed recently for quasicrystals and approximating the long-wavelength behavior by their large-scale average, we construct an effective $k \cdot p$ Hamiltonian to capture the band inversion at the center of a pseudo-Brillouin zone (PBZ). We show that an in-plane Zeeman field can induce mass-kinks at the intersection of adjacent edges of a 2D quasicrystal TI and generate corner modes (CMs) with fractional charge, protected by rotational symmetries. Our model predictions are confirmed by numerical tight-binding calculations. Furthermore, when the quasicrystal is proximitized by an \textit{s}-wave superconductor, Majorana CMs can also be created by tuning the field strength and chemical potential. Our work affords a generic approach to studying the low-energy physics of quasicrystals, in association with topological excitations and fractional statistics.