Topological classification of Higher-order topological phases with nested band inversion surfaces

TL;DR

This paper introduces a unified method using nested band inversion surfaces to classify and analyze higher-order topological phases across all symmetry classes, applicable in various dimensions and without relying on spatial symmetries.

Contribution

The paper develops a comprehensive nested-BIS approach for topological classification of HOTPs, applicable to all Altland-Zirnbauer classes and higher dimensions, independent of spatial symmetries.

Findings

Nested-BIS method effectively characterizes HOTPs.

The approach applies to continuous and lattice models.

It accounts for asymmetric boundary states.

Abstract

Higher-order topological phases (HOTPs) hold gapped bulk bands and topological boundary states localized in boundaries with codimension higher than one. In this paper, we provide a unified construction and topological characterization of HOTPs for the full Altland-Zirnbauer tenfold symmetry classes, based on a method known as nested band inversion surfaces (BISs). Specifically, HOTPs built on this method are decomposed into a series of subsystems, and higher-order topological boundary states emerges from the interplay of their first-order topology. Our analysis begins with a general discussion of HOTPs in continuous Hamiltonians for each symmetry class, then moves on to several lattice examples illustrating the topological characterization based on the nested-BIS method. Despite the example minimal models possessing several spatial symmetries, our method does not rely on any spatial symmetry, and can be easily extended into arbitrary orders of topology in dimensions. Furthermore, we extend our discussion to systems with asymmetric boundary states induced by two different mechanisms, namely, crossed BISs that break a $\mathcal{C}_4$ rotation symmetry, and non-Clifford operators that break certain chiral-mirror symmetries of the minimal models.