Spin-Polarized Fractional Corner Charges and Their Photonic Realization

TL;DR

This paper explores how spin degrees of freedom influence higher order topological insulators, revealing new topological invariants, phases, and robust spin-polarized corner states, with photonic systems demonstrating these phenomena.

Contribution

It introduces the concept of spin-polarized corner charges in HOTIs and establishes new bulk-boundary correspondence principles with photonic realizations.

Findings

Prediction of spin-polarized mid-gap corner states

Definition of new bulk-boundary correspondence principles

Photonic systems demonstrating spin-linked topological phases

Abstract

We demonstrate that a spin degree of freedom can introduce additional texture to higher order topological insulators (HOTIs), manifesting itself in novel topological invariants, phases, and phase transitions. Spin-polarized mid-gap corner states of various multiplicities are predicted for different HOTI phases, and novel bulk-boundary correspondence principles are defined based on quantized bulk invariants such as total and spin corner charge. Those are shown to be robust with respect to symmetry-preserving spin-flipping perturbations. Photonic realizations of spin-linked topological phases are demonstrated in engineered systems with synthetically emulated spin degree of freedom.