Anomalous in-gap edge states in two-dimensional pseudospin-1 Dirac insulators

TL;DR

This paper discovers a new class of in-gap edge states in two-dimensional pseudospin-1 Dirac insulators induced solely by electrostatic potentials, revealing robust, controllable spin textures without relying on topological phase transitions.

Contribution

It introduces a novel mechanism for in-gap edge state formation in pseudospin-1 systems through electrostatic potentials, independent of topological band-inversion.

Findings

In-gap edge modes emerge via electrostatic potential in 2D Dirac systems.

Spontaneous domain-wall spin textures with out-of-plane spin-angular momentum locking.

Modes are robust against boundary deformations and impurities.

Abstract

Quantum materials that host a flat band, such as pseudospin-1 lattices and magic-angle twisted bilayer graphene, can exhibit drastically new physical phenomena including unconventional superconductivity, orbital ferromagnetism, and Chern insulating behaviors. We report a surprising class of electronic in-gap edge states in pseudospin-1 materials without the conventional need of band-inversion topological phase transitions or introducing magnetism via an external magnetic type of interactions. In particular, we find that, in two-dimensional gapped (insulating) Dirac systems of massive spin-1 quasiparticles, in-gap edge modes can emerge through only an {\em electrostatic potential} applied to a finite domain. Associated with these unconventional edge modes are spontaneous formation of pronounced domain-wall spin textures, which exhibit the feature of out-of-plane spin-angular momentum locking on both sides of the domain boundary and are quite robust against boundary deformations and impurities despite a lack of an explicit topological origin. The in-gap modes are formally three-component evanescent wave solutions, akin to the Jackiw-Rebbi type of bound states. Such modes belong to a distinct class due to the following physical reasons: three-component spinor wave function, unusual boundary conditions, and a shifted flat band induced by the external scalar potential. Not only is the finding of fundamental importance, but it also paves the way for generating highly controllable in-gap edge states with emergent spin textures using the traditional semiconductor gate technology. Results are validated using analytic calculations of a continuum Dirac-Weyl model and tight-binding simulations of realistic materials through characterizations of local density of state spectra and resonant tunneling conductance.