Photoinduced valley and electron-hole symmetry breaking in $\alpha$-$T_3$ lattice: The role of a variable Berry phase

We consider $\alpha$-$T_3$ lattice illuminated by intense circularly polarized radiation in terahertz regime. We present quasienergy band structure, time-averaged energy spectrum and time-averaged density of states of $\alpha$-$T_3$ lattice by solving the Floquet Hamiltonian numerically. We obtain exact analytical expressions of the quasienergies at the Dirac points for all values of $\alpha$ and field strength. We find that the quasienergy band gaps at the Dirac point decrease with increase of $\alpha$. Approximate forms of quasienergy and band gaps at single and multi-photon resonant points are derived using rotating wave approximation. The expressions reveal a stark dependence of quasienergy on the Berry phase of the charge carrier. The quasi energy flat band remains unaltered in presence of radiation for dice lattice ($\alpha =1$). However, it acquires a dispersion in and around the Dirac and even-photon resonant points when $0<\alpha <1$. The valley degeneracy and electron-hole symmetry in the quasienergy spectrum are broken for $0<\alpha <1$. Unlike graphene, the mean energy follows closely the linear dispersion of the Dirac cones till near the single-photon resonant point in dice lattice. There are additional peaks in the time-averaged density of states at the Dirac point for $0<\alpha \le 1$.