Demixing of boson-fermion clouds under harmonic confinement

Abstract

A cooling strategy that is currently being pursued for a gas of fermionic atoms uses sympathetic cooling via s-wave collisions against a Bose-condensed gas. At increasing values of the scattering lengths, the boson-fermion cloud undergoes demixing. Locating the onset of incipient spatial separation is relevant to fermion cooling, since at that point the diminishing overlap between the two clouds will start reducing the effectiveness of the collisional transfer processes. With full demixing, the system enters a new quantum phase, where its properties are expected to be substantially modified. In this perspective, we have used a semiclassical model to evaluate spatial demixing in mesoscopic clouds of fermionic and bosonic atoms at high dilution inside cylindrical harmonic traps. The finite system size allows three different regimes for the equilibrium density profiles: a fully mixed state, a partially mixed state in which the overlap between the boson and fermion clouds decreases, and a fully demixed state wherein the two clouds have zero overlap. Simple analytical rules account for the crossovers between these regimes as functions of the physical system parameters and are supported by extensive numerical calculations. A universal "phase diagram" involving simple scaling parameters is valid for the transition to full demixing, inside which we identify several exotic configurations for the two phase-separated clouds in addition to the simple ones consisting of a core of bosons enveloped by fermions and vice versa.