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Arts & Sciences
[PAST EVENT] Condensed Matter and AMO Seminar
February 9, 2015
2:30pm - 3:30pm
Abstract:
Recent advances in rapidly quenched ultracold atomic Fermi gases near a Feshbach resonance arise a number of interesting problems, in the context of observing the long-sought Stoner ferromagnetic phase transition. The possibility of experimentally obtaining a "quasirepulsive" regime in the upper branch of the energy spectrum due to the rapid quench is currently debated and theoretically, the Stoner transition has mainly been investigated by using perturbation theory or at high polarization, due to the limited theoretical approaches in the strongly repulsive regime. In this work, we present an appropriate prescription for the quasirepulsive branch and prove it by resumming the two-particle virial contributions from the scattering continuum to all orders in the fugacity. By further adopting a nonperturbative large-N expansion approach, we determine a finite-temperature phase diagram for the Stoner instability of a quasirepulsive Fermi gas near resonance. Our results agree well with the known quantum Monte-Carlo simulations at zero temperature and recover the known virial expansion prediction at high temperature for arbitrary interaction strengths. At resonance, we find that the unitary Fermi gas undergoes the Stoner transition at about one and a half Fermi temperature, around which the pair formation rate becomes vanishingly small. This suggests a feasible way to observe Stoner ferromagnetism. We also apply the same prescription to study the strongly interacting Bose gases near unitary.
Recent advances in rapidly quenched ultracold atomic Fermi gases near a Feshbach resonance arise a number of interesting problems, in the context of observing the long-sought Stoner ferromagnetic phase transition. The possibility of experimentally obtaining a "quasirepulsive" regime in the upper branch of the energy spectrum due to the rapid quench is currently debated and theoretically, the Stoner transition has mainly been investigated by using perturbation theory or at high polarization, due to the limited theoretical approaches in the strongly repulsive regime. In this work, we present an appropriate prescription for the quasirepulsive branch and prove it by resumming the two-particle virial contributions from the scattering continuum to all orders in the fugacity. By further adopting a nonperturbative large-N expansion approach, we determine a finite-temperature phase diagram for the Stoner instability of a quasirepulsive Fermi gas near resonance. Our results agree well with the known quantum Monte-Carlo simulations at zero temperature and recover the known virial expansion prediction at high temperature for arbitrary interaction strengths. At resonance, we find that the unitary Fermi gas undergoes the Stoner transition at about one and a half Fermi temperature, around which the pair formation rate becomes vanishingly small. This suggests a feasible way to observe Stoner ferromagnetism. We also apply the same prescription to study the strongly interacting Bose gases near unitary.
Contact
Host: S. Zhang