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.
Highlights
The physics of quantum materials hosting a flatband, such as the magic-angle twisted bilayer graphene, has become a forefront area of research
In comparison with known edge states, either topological or nontopological, the states uncovered here belong to a distinct class due to the following physical reasons: three-component spinor wave function, unusual boundary conditions, and a shifted flatband induced by the external scalar potential
Our theoretical prediction is general for gapped systems of massive spin-1 particles subject to an electrostatic potential applied to a finite domain
Summary
The physics of quantum materials hosting a flatband, such as the magic-angle twisted bilayer graphene, has become a forefront area of research. These materials can generate surprising physical phenomena, such as unconventional superconductivity [1,2], orbital ferromagnetism [3,4], and the Chern insulating behavior with topological edge states. The emergence of low-dissipation or dissipationless topological surface or edge states in condensed-matter systems is a fascinating phenomenon [5,6,7] as exemplified by topological insulators (TIs) [8,9,10,11,12,13,14,15,16]. A TI has a bulk band gap, so its interior is insulating, but there are gapless surface states within the bulk band gap, which
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