Floquet topological phase transition in $α$-$\mathcal{T}_3$ lattice

Abstract: We investigate topological characteristics of the photon-dressed band structure of $\alpha$-$\mathcal{T}_3$ lattice on being driven by off-resonant circularly polarized radiation. We obtain exact analytical expressions of the quasienergy bands over the first Brillouin zone. The broken time-reversal symmetry caused by the circularly polarized light lifts the triple point degeneracy completely at both the Dirac points. The gaps become unequal at ${\bf K}$ and ${\bf K}<sup>{\prime}$ (except at $\alpha=0$ and 1), which reveals the absence of inversion symmetry in the system. At $\alpha=1/\sqrt{2}$, the gap between flat and valence bands closes at ${\bf K}$, while that between conduction and flat bands closes at ${\bf K}<sup>{\prime}$, thereby restoring a semimetalic phase. At the gap closing point ($\alpha=1/\sqrt{2}$) which is independent of the radiation amplitude, there is a reappearance of low-energy Dirac cones around ${\bf K}$ and ${\bf K}<sup>{\prime}$ points. Under the influence of the circularly polarized radiation, the $\alpha$-$\mathcal{T}_3$ lattice is transformed from semimetal to a Haldane-like Chern insulator characterized by non-zero Chern number. The system undergoes a topological phase transition from $\mathcal{C} = 1 (-1)$ to $\mathcal{C}=2 (-2)$ at $\alpha =1/\sqrt{2}$, where $\mathcal{C}$ is the Chern number of the valence (conduction) band. This sets an example of a multiband system having larger Chern number. These results are supported by the appearance of chiral edge states in irradiated $\alpha$-$\mathcal{T}_3$ nanoribbon.