Bose spectrum of superfluid Fermi gas

V. A. Andrianov, Victor Nikolaevich Popov

Shina Tan

Sébastien Laurent, Marion Delehaye, Shu‐Wei Jin +4 more

Christopher P. Search, Han Pu, W. Zhang +1 more

The quasiparticle excitations and dynamical stability of an atomic Bose-Einstein condensate coupled to a quantum degenerate Fermi gas of atoms at zero temperature is studied. The Fermi gas is assumed to be either in the normal state or to have undergone a phase transition to a superfluid state by forming Cooper pairs. The quasiparticle excitations of the Bose-Einstein condensate exhibit a dynamical instability due to a resonant exchange of energy and momentum with quasiparticle excitations of the Fermi gas. The stability regime for the bosons depends on whether the Fermi gas is in the normal state or in the superfluid state. We show that the energy gap in the quasiparticle spectrum for the superfluid state stabilizes the low-energy excitations of the condensate. In the stable regime, we calculate the boson quasiparticle spectrum, which is modified by the fluctuations in the density of the Fermi gas.

I. Lutzenko, Alexei S. Zhedanov

It is shown that a gas of interacting bosons with weakly nonlinear commutation relations behaves as a Fermi gas rather than a Bose gas if there is a gap in the frequency spectrum. Despite the quasifermionic behavior of such a gas (absence of Bose condensation, Fermi-like distributions, etc.) superfluidity is predicted. Some systems with such quasifermions are discussed.

J. J. Kinnunen, Georg M. Bruun

We analyze a Bose-Einstein condensate (BEC) mixed with a superfluid two-component Fermi gas in the whole BCS-BEC crossover. Using a quasiparticle random-phase approximation combined with Beliaev theory to describe the Fermi superfluid and the BEC, respectively, we show that the single-particle and collective excitations of the Fermi gas give rise to an induced interaction between the bosons, which varies strongly with momentum and frequency. It diverges at the sound mode of the Fermi superfluid, resulting in a sharp avoided crossing feature and a corresponding sign change of the interaction energy shift in the excitation spectrum of the BEC. In addition, the excitation of quasiparticles in the Fermi superfluid leads to damping of the excitations in the BEC. Besides studying induced interactions themselves, we can use these prominent effects to systematically probe the strongly interacting Fermi gas.