Collective excitations of low density fermion-boson quantum-liquid mixtures

TL;DR

This paper explores the collective excitations in a low-density fermion-boson quantum liquid mixture, revealing novel zero-sound modes, damping effects, and conditions for phase separation, with implications for experimental observation.

Contribution

It introduces the analysis of BEC-mediated fermion-fermion interactions leading to new collective modes and phase separation in fermion-boson mixtures.

Findings

Identification of undamped zero-sound mode supported by repulsive mediated interactions

Discovery of a damped BEC-sound mode influenced by fermion interactions

Estimation of phase separation onset conditions and relevant scales

Abstract

We investigate the collective excitations of a low temperature dilute gas mixture that consists of a Bose-Einstein condensate and a Fermi-gas that is a normal (i.e. non-superfluid) Fermi-liquid. We find that the BEC-mediated fermion-fermion interactions, as a consequence of retardation, can become repulsive and support a zero-sound mode that is essentially undamped. In addition, we find a damped zero-sound mode that can be described as a BEC-sound mode modified by fermion mediated boson-boson interactions, and we derive its decay-rate caused by Landau damping. We study the mode structure of these excitations and find avoided crossing behavior as well as a termination point. The collective mode dynamics also reveals that phase separation sets in when the fermion-mediated boson-boson interaction destroys the stability of the homogeneous BEC. We estimate the time and length scales of the onset of the phase separation, and we discuss the feasibility of experimentally probing these consequences of mediated interactions.