Multi-topological phases of matter

TL;DR

This paper introduces multi-topological phases (MTPs), a new class of topological matter supporting multiple boundary state sets without band-gap closing, expanding the understanding of topological phenomena beyond traditional band theory.

Contribution

The authors discover and experimentally demonstrate multi-topological phases that feature multiple boundary states and can occur without band-gap closing, challenging conventional topological classifications.

Findings

Identification of MTPs with multiple boundary states

Experimental realization of MTPs in photonic lattices

MTPs can exist beyond traditional band topology regimes

Abstract

The discovery of topological phases of matter and topological boundary states had tremendous impact on condensed matter physics and photonics, where topological phases are defined via energy bands, giving rise to topological band theory. However, topological systems that cannot be described by band topology but still support non-trivial boundary states are little-known and largely unexplored. Here, we uncover a new kind of topological phase of matter named "multi-topological phase" (MTP) that features multiple sets of boundary states, where each set is associated with one distinct topological invariant. Unlike conventional topological phase transitions, the MTP transitions can occur without band-gap closing. We present typical examples of MTPs in a one-dimensional topological insulator and a two-dimensional higher-order topological insulator, where the systems are otherwise trivial according to band topology. Furthermore, MTPs can exist also in indirectly gapped Chern insulators, beyond the regime where the conventional bulk-boundary correspondence predicts the existence of boundary states. Experimentally, we demonstrate the first two examples of MTPs in laser-written photonic lattices. Our findings constitute a fundamental advance in topological physics and provide a route for designing novel topological materials.