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Decoherence of Superradiant Scattering from Atoms without Confinement

We study the decoherence of superradiant Rayleigh scattering from condensed atoms without confinement, by using two same pumping pulses with an interval time. The first pulse is to establish a matter-wave grating, and the coherence between different momentum modes is measured by the second pulse after a variable interval time. Different from the case in the trap, the distruction of the grating owing to the phase perturbation is very fast, and the superradiant process is inhibited very soon afterwards for released atoms. A semi-classical model is applied to simulate this phase perturbation, and the calculation agrees with our experimental results.