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Description
Strong optical driving can induce transient superconducting-like behavior in materials even above their equilibrium $T_c$. Because many of these systems already exhibit short-range superconducting correlations in equilibrium, an important question is whether periodic driving can enhance coherence in the absence of true long-range order. To investigate this possibility, we study the two-dimensional XY model with a time-periodically modulated stiffness using overdamped Langevin dynamics. We show that, despite leaving the average coupling unchanged, the drive can significantly enhance long-range, time-averaged correlations, even at temperatures well above the equilibrium Berezinskii-Kosterlitz-Thouless transition. The response depends strongly on the competition between the drive frequency and the intrinsic relaxation rate. At high frequencies, the drive mainly acts as a source of heating, suppressing both correlations and conductivity. At lower frequencies, by contrast, the optical conductivity is reshaped: the real part exhibits an effectively longer Drude scattering time, while the imaginary part displays an enhanced low-frequency $1/\omega$ contribution. We map out these regimes as functions of temperature, drive frequency, and amplitude, and interpret them using simple analytical considerations together with a vortex-thermalization picture. Overall, our results point to a generic nonequilibrium route for enhancing coherence in XY-like systems, and we comment on its possible relevance to experiments on light-induced superconductivity in cuprates.
Reference: D. De Santis, M. H. Michael, S. Chattopadhyay, A. Cavalleri, G. Refael, P. A. Lee, E. A. Demler, “Enhanced coherence in the periodically driven two-dimensional XY model”, arXiv:2511.12287 (2025)