Floquet second-order topological insulator in strained graphene

TL;DR

This paper demonstrates how uniaxial strain and off-resonant circularly polarized light can induce Floquet higher-order topological phases in graphene, leading to gapped edges and robust corner modes.

Contribution

It introduces a controllable method to realize Floquet second-order topological insulators in strained graphene using a high-frequency expansion and symmetry analysis.

Findings

Strain drives Dirac cones toward the semi-Dirac critical regime.

Oblique light incidence stabilizes a phase with gapped edges and in-gap corner modes.

First-principles calculations support the topological phase in nanostructures.

Abstract

Graphene provides a canonical setting for Floquet band engineering, where circularly polarized light can dynamically open topological gaps at Dirac points and generate nonequilibrium Hall responses. Here we show that uniaxial strain and off-resonant circularly polarized light with tunable incidence angle enable a controllable route to Floquet higher-order topology in graphene. Using a strained honeycomb tight-binding model with Peierls coupling and a high-frequency expansion for the effective Floquet Hamiltonian, we find that strain drives the Dirac cones toward the Dirac-merging (semi-Dirac) critical regime, where the light-induced mass becomes strongly anisotropic. For oblique incidence, the projected drive is effectively elliptically polarized and, in combination with strain, stabilizes a phase with gapped edges but robust in-gap corner modes in finite geometries, realizing a Floquet second-order topological insulator. We characterize the phase diagram via the Chern number and a crystalline-symmetry-quantized polarization invariant. Finally, first-principles-informed tight-binding calculations corroborate the predicted topological evolution in strained graphene nanostructures. Our results identify driven strained graphene as a realistic and tunable platform for realizing and diagnosing Floquet higher-order topological phases.