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Abstract
Driving a two-dimensional Mott insulator with circularly polarized light breaks time-reversal and inversion symmetry, which induces an optically-tunable synthetic scalar spin chirality interaction in the effective low-energy spin Hamiltonian. Here, we show that this mechanism can stabilize topological magnon excitations in honeycomb ferromagnets and in optical lattices. We find that the irradiated quantum magnet is described by a Haldane model for magnons that hosts topologically-protected edge modes. We study the evolution of the magnon spectrum in the Floquet regime and via time propagation of the magnon Hamiltonian for a slowly varying pulse envelope. Compared to similar but conceptually distinct driving schemes based on the Aharanov-Casher effect, the dimensionless light-matter coupling parameter \lambda = eEa/\hbar\omegaλ=eEa/ℏω at fixed electric field strength is enhanced by a factor \sim 10^5∼105. This increase of the coupling parameter allows to induce a topological gap of the order of \Delta \approx 2Δ≈2 meV with realistic laser pulses, bringing an experimental realization of light-induced topological magnon edge states within reach.
Highlights
The experimental realization of magnetic van der Waals materials with a thickness down to the monolayer limit has sparked a new interest in fundamental aspects of twodimensional magnetism [1,2,3,4]
We show that application of the effective spin Hamiltonian, with parameters taken from the prototypical monolayer van der Waals (vdW) magnet CrI3 [2, 28,29,30], can lead to a gap ∆ ≈ 2 meV in the magnon spectrum for a realistic field strength E = 109 V/m and photon energy ω = 1 eV, inducing non-zero Chern numbers and leading to chiral magnon edge states
We have demonstrated that non-equilibrium driving based on periodic laser fields coupling to charge degrees of freedom can induce topological magnon edge states in the spin sector of prototypical two-dimensional quantum magnets
Summary
The experimental realization of magnetic van der Waals (vdW) materials with a thickness down to the monolayer limit has sparked a new interest in fundamental aspects of twodimensional magnetism [1,2,3,4]. In recent theoretical studies it has been shown that driving a two-dimensional Mott insulator with circularly polarized light breaks both time-reversal and inversion symmetries This is reflected by an induced scalar spin chirality interaction that governs the transient dynamics of low-energy spin excitations [11, 22]. We find that the dimensionless Floquet parameter that describes the magnitude of light-matter interaction is enhanced by a factor ∼ 105 compared to similar but conceptually distinct driving schemes based on the Aharanov-Casher effect for pure spin models [27, 31], since the electric field couples to the charge instead of the magnetic moment This amplification is shown to be crucial for a potential experimental realization of a topological magnon phase in monolayer vdW magnets. Since the topological properties of honeycomb ferromagnets are determined by the lattice structure and the presence or absence of time-reversal symmetry [26], we expect the model to give a correct description of the topological features of the magnon excitations
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