Superconducting-like response in driven systems near the Mott transition

Photo-induced superconducting-like responses have been reported in $\kappa\text{-(ET})_2\text{Cu}[\text{N(CN)}_2]\text{Br}$, a metal close to the Mott transition. A closely related material, $\kappa\text{-(ET})_2\text{Cu}_2\text{(CN)}_3$, sits just on the insulating side and is a well studied spin liquid candidate. In this work, we analyze the effect of a periodic drive on both the spin liquid and the metal close to the Mott transition. On both sides of the transition, we propose that the periodic drive parametrically couples to a fractionalized charge e boson field, producing a non-equilibrium condensate. The condensate contributes a Drude peak to the physical conductivity with a width much smaller than the equilibrium scattering rate, hence likely below the experimental resolution. On the metallic side, if the material is originally close to the Mott transition, the periodic drive can induce a charge gap, moving the sample into the Mott insulator phase. In contrast to previous proposals, our mechanism does \textit{not} require any superconducting fluctuations. We propose to test this theory in the spin liquid candidates $\kappa\text{-(ET})_2\text{Cu}_2\text{(CN)}_3$ and $\beta'\text{-EtMe}_3\text{Sb[Pd(dmit}_2)]_2$. Our general theory is also directly applicable to bosonic Mott insulators formed by cold atoms in optical lattices.