Abstract
The quenched dynamics of an ultracold homogeneous atomic two-dimensional Bose gas subjected to periodic quenches across the Berezinskii-Kosterlitz-Thouless (BKT) phase transition are discussed. Specifically, we address the effect of periodic cycling of the effective atomic interaction strength between a thermal disordered state above, and a highly ordered state below the critical BKT interaction strength, by means of numerical simulations of the stochastic projected Gross-Pitaevskii equation. Probing the emerging dynamics as a function of the frequency of sinusoidal driving from low to high frequencies reveals diverse dynamical features, including phase-lagged quasi adiabatic reversible condensate formation, resonant excitation consistent with an intrinsic system relaxation timescale, and gradual establishment of dynamically-recurring or time-averaged non-equilibrium states with enhanced coherence which are neither condensed, nor thermal. Our study paves the way for experimental observation of such driven non-equilibrium ultracold superfluid states.
Highlights
The quench dynamics of a quantum system across a phase transition are an exciting subject of active ongoing research [1,2,3,4,5]
The system is initialized in an equilibrated purely incoherent state above the critical region, with g = gmax > gc, with such a state generated numerically by dynamical relaxation to the desired state through the stochastic projected Gross-Pitaevskii equation (SPGPE). It is well-known that above the BKT phase transition the phase of the system is random, corresponding to an exponentially decaying phase correlation function: this incoherent state can be thought of as a system with a large number of “free” vortices, whose exact number is fixed by the system size, grid resolution, atomic mass, temperature and interaction strength
As we scan the system across different equilibrium configurations, we find that the number of vortices at equilibrium decreases abruptly across the BKT critical region, reaching a very low, or even zero value as the system approaches its pure superfluid limit (g gc) [26,69,75]
Summary
The quench dynamics of a quantum system across a phase transition are an exciting subject of active ongoing research [1,2,3,4,5]. Beyond the expected regimes of extremely slow pumping (which allows the system to proceed adiabatically through instantaneous equilibrium states) and extremely rapid pumping which only mildly perturbs the initial state, we find the periodic driving to be intrinsically resonant with a characteristic relaxation time, at which the periodic modulation of the scattering length causes strongly nonequilibrium features to emerge This resonance separates two interesting distinct and experimentally relevant physical regimes: Driving frequencies lower than the resonant value lead to reproducible dynamics which are independent of the quench cycle, and— while not fully equilibrated—resemble certain features of the corresponding-parameter equilibrium states.
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