Enhanced coherence in the periodically driven two-dimensional XY model

TL;DR

This paper demonstrates that periodic driving can enhance long-range correlations in the 2D XY model above its critical temperature, revealing a route to light-induced coherence in systems with pre-existing superconducting correlations.

Contribution

It shows that external periodic modulation can increase coherence in XY systems above critical temperature, with effects depending on drive frequency and amplitude.

Findings

Long-range correlations are enhanced by slow drives.

Faster drives tend to heat the system and suppress correlations.

Modified optical conductivity indicates prolonged scattering times.

Abstract

Strong optical drives have been shown to induce transient superconducting-like response in materials above their equilibrium $T_c$. Many of these materials already exhibit short-range superconducting correlations in equilibrium. This motivates the question: can external driving enhance coherence in systems with superconducting correlations but no long-range order? We explore this scenario in the two-dimensional XY model with a periodically modulated stiffness using overdamped Langevin dynamics. We find that, even though the modulation leaves the average coupling unchanged, the drive can markedly increase long-range, time-averaged correlations in systems well above the equilibrium Berezinskii-Kosterlitz-Thouless temperature. The outcome depends on the ratio of the drive frequency to the intrinsic relaxation rate: faster drives primarily heat the system, suppressing correlations and conductivity. For slower drives, the optical conductivity is modified so that the real part exhibits a prolonged effective Drude scattering time, while the imaginary part has a strengthened low-frequency $1/\omega$ behavior. We map out these regimes across temperature, frequency, and amplitude, and rationalize them via simple analytics and vortex-thermalization arguments. Overall, we identify a generic nonequilibrium route to enhance coherence in XY-like systems, with potential relevance to experiments reporting light-induced superconductivity.