Beyond mean-field corrections to the quasiparticle spectrum of superfluid Fermi gases

The notion of quasiparticles is an essential tool for the study of interacting many-body systems. In superfluid Fermi gases, two types of elementary excitations can be identified: the fermionic branch of broken pairs, and the bosonic collective mode describing the collective motion of the pairs. These can be observed in systems of ultracold fermionic atoms, where, due to Fesbach resonances, a whole range of superfluids can be studied. Measurements of the quasiparticle spectrum are already available [1], though the theoretical study of corrections to the fermionic branch remains limited [2]. Here, we investigate this quasiparticle branch in the BCS-BEC crossover and calculate the quasiparticle lifetime and energy shift due to its coupling with the collective mode. Close to the minimum of the branch the quasiparticles are undamped, allowing us to find the energy correction in a self-consistent way, that we express in experimentally relevant quantities.

[1] S. Gupta et al., Science 300, 1723–1726 (2003); J. T. Stewart et al., Nature 454, 744 (2008); A. Schirotzek et al., Phys. Rev. Lett. 101, 140403 (2008).
[2] N. Lerch et al., Phys. Rev. Lett. 100, 050403 (2008); R. Haussmann et al., Phys. Rev. A 80, 063612 (2009).