Universality and novel phases in the BCS-BEC crossover

Lianyi He, Pengfei Zhuang

We investigate the BCS-BEC (Bardeen-Cooper-Shriffer---Bose-Einstein condensation) crossover at zero temperature in the frame of a relativistic model. The universality of the BCS-BEC crossover for nonrelativistic systems breaks down in the relativistic case and the crossover can be induced by changing the density. When the effective scattering length is much less than the fermion Compton wavelength, we recover the nonrelativistic result if the gas is initially in a nonrelativistic state. At ultrastrong coupling where the scattering length is of the order of the Compton wavelength, a new BEC state appears. In this state the condensed bosons become nearly massless and antifermions are excited. The behavior of the Goldstone mode and the mixing between the amplitude and phase modes are significantly different in different condensed regions.

Yasemin Gürcan, Joseph L. Birman

Jean-Noël Fuchs, RECATI A, W. Zwerger

Sebastian Diehl, Holger Gies A, Jan Martin Pawlowski +1 more

The functional renormalization group is used for the BCS-BEC crossover in gases of ultracold fermionic atoms. In a simple truncation, we see how universality and an effective theory with composite bosonic diatom states emerge. We obtain a unified picture of the whole phase diagram. The flow reflects different effective physics at different scales. In the BEC limit as well as near the critical temperature, it describes an interacting bosonic theory.

Jonas Siegl, Wolf Weimer, Kai Morgener +3 more

Armen Sedrakian

Several novel multi-component fermionic condensates show universal behavior under imbalance in the number of fermionic species. Here I discuss their phase structure, thermodynamics, and the transition from the weak (BCS) to strong (BEC) coupling regime. The inhomogeneous superconducting phases are illustrated on the example of the Fulde–Ferrell phase which appears in the weak coupling regime, at low temperatures and large asymmetries. The inhomogeneous phases persist through the crossover up to (and possibly beyond) the transition to the strong coupling regime.