Depletion of the Bose-Einstein condensate in Bose-Fermi mixtures

TL;DR

This paper investigates the quantum phase transition in Bose-Fermi mixtures across a Feshbach resonance, focusing on the depletion of Bose-Einstein condensate and the associated changes in Fermi surface constraints.

Contribution

It introduces a detailed phase diagram and Luttinger constraint analysis for Bose-Fermi mixtures near a Feshbach resonance, highlighting the BEC depletion transition and its broader implications.

Findings

Critical detuning causes complete BEC depletion when fermions outnumber bosons.

Distinct Luttinger constraints characterize phases with and without BEC.

Predictions made for narrow Feshbach resonances, applicable more generally.

Abstract

We describe the properties of a mixture of fermionic and bosonic atoms, as they are tuned across a Feshbach resonance associated with a fermionic molecular state. Provided the number of fermionic atoms exceeds the number of bosonic atoms, we argue that there is a critical detuning at which the Bose-Einstein condensate (BEC) is completely depleted. The phases on either side of this quantum phase transition can also be distinguished by the distinct Luttinger constraints on their Fermi surfaces. In both phases, the total volume enclosed by all Fermi surfaces is constrained by the total number of fermions. However, in the phase without the BEC, which has two Fermi surfaces, there is a _second_ Luttinger constraint: the volume enclosed by one of the Fermi surfaces is constrained by the total number of_bosons_, so that the volumes enclosed by the two Fermi surfaces are separately conserved. The phase with the BEC may have one or two Fermi surfaces, but only their total volume is conserved. We obtain the phase diagram as a function of atomic parameters and temperature, and describe critical fluctuations in the vicinity of all transitions. We make quantitative predictions valid for the case of a narrow Feshbach resonance, but we expect the qualitative features we describe to be more generally applicable. As an aside, we point out intriguing connections between the BEC depletion transition and the transition to the fractionalized Fermi liquid in Kondo lattice models.