We study the relaxation of the center of mass or dipole oscillations in the system of interacting fermions confined spatially. With the confinement frequency ω fixed, the particles were considered to freely move along one (quasi-1D) or two (quasi-2D) spatial dimensions. We have focused on the regime of rare collisions, such that the inelastic collision rate 1/τin ω. The dipole oscillation relaxation rate 1/τ is obtained at three different levels: by direct perturbation theory, solving the integral Bethe-Salpeter equation, and applying the memory function formalism. As long as anharmonicity is weak, 1/τ 1/τin, the three methods are shown to give identical results. In the quasi-2D case, 1/τ ≠0 at zero temperature. In quasi-1D system, 1/τ T3 if the Fermi energy EF lies below the critical value EF<3ω /4. Otherwise, unless the system is close to integrability, the rate 1/τ has a temperature dependence similar to that in quasi-2D. In all cases, the relaxation results from the excitation of particle-hole pairs propagating along unconfined directions resulting in the relationship 1/τ 1/τin, with the inelastic rate 1/τin≠0 as the phase-space opens up at finite energy of excitation ω. While 1/τ τin in the hydrodynamic regime, ω 1/τin, in the regime of rare collisions, ω 1/τin, we obtain the opposite trend 1/τ 1/τin.
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We are thankful to M. Raikh for discussions that stimulated us in working on this paper. We are also grateful to D. Orgad and E. Bettelheim for discussions of separate parts of the paper. This work has been supported by the Binational Science Foundation (BSF Grant No. 2016317). M.K. acknowledges the support by the Israel Science Foundation, Grant No. 1287/15. A.L. acknowledges the support from NSF CAREER Grant No. DMR-1653661, and the Wisconsin Alumni Research Foundation.
© 2018 American Physical Society.