Nonequilibrium susceptibility in photoinduced Floquet states*

We investigate the nonequilibrium spin susceptibility in an electron system subject to a time-periodic electric field [1]. The spin susceptibility is formulated on the basis of the Floquet Green's function method, and is calculated numerically in a wide range of amplitude and frequency of light. When the frequency is larger than the bandwidth, the susceptibility is enhanced by the dynamical localization effect, and their peak positions in the momentum space are shifted due to the Fermi surface deformation. In the case of the small frequency and amplitude, a multiple-peak structure emerges in the susceptibility, originating from the electron-hole excitation in and between the Floquet sidebands. To confirm those numerical results and provide the interpretation, a perturbative expression of the susceptibility is derived for small electric-field amplitude.
[1] A. Ono and S. Ishihara, Phys. Rev. B 100, 075127 (2019).