Dispersive nodal fermions along grain boundaries in Floquet topological crystals

TL;DR

This paper predicts the existence of topologically protected dispersive modes along grain boundaries in Floquet topological crystals, arising from band inversion and dislocation properties, with potential experimental realizations.

Contribution

It introduces a new mechanism for gapless boundary modes in Floquet topological materials linked to band inversion and dislocation Burgers vectors.

Findings

Gapless modes appear along grain boundaries in Floquet crystals.

Modes are robust and can occur near zone center or edges.

The phenomenon is demonstrated in driven time-reversal symmetry breaking insulators.

Abstract

Driven quantum materials often feature emergent topology, otherwise absent in static crystals. Dynamic bulk-boundary correspondence, encoded by nondissipative gapless modes residing near the Floquet zone center and/or boundaries, is its most prominent example. Here we show that topologically robust gapless dispersive modes appear along the grain boundaries, embedded in the interior of Floquet topological crystals, when the Floquet-Bloch band inversion occurring at a finite momentum (${\bf K}^{\rm Flq}_{\rm inv}$) and the Burgers vector (${\bf b}$) of the constituting array of dislocations satisfy ${\bf K}^{\rm Flq}_{\rm inv} \cdot {\bf b}=\pi$ (modulo $2 \pi$). Such nondissipative gapless states can be found near the center and/or edges of the Floquet Brillouin zone, irrespective of the drive protocol. We showcase these general outcomes for two-dimensional driven time-reversal symmetry breaking insulators. Promising experimental platforms hosting such dynamic topological dispersive bands in real materials are discussed.