Delayed response of a fermion-pair condensate to a modulation of the interaction strength

The effect of a sinusoidal modulation of the interaction strength on a fermion-pair condensate is analytically studied. The system is described by a generalization of the coupled fermion-boson model that incorporates a time-dependent intermode coupling induced via a magnetic Feshbach resonance. Nontrivial effects are shown to emerge depending on the relative magnitude of the modulation period and the relaxation time of the condensate. Specifically, a nonadiabatic modulation drives the system out of thermal equilibrium: the external field induces a variation of the quasiparticle energies, and, in turn, a disequilibrium of the associated populations. The subsequent relaxation process is studied and an analytical description of the gap dynamics is obtained. Recent experimental findings are explained: the delay observed in the response to the applied field is understood as a temperature effect linked to the condensate relaxation time.Comment: 6 page

Feynman diagrams with the effective action

A derivation is given of the Feynman rules to be used in the perturbative computation of the Green's functions of a generic quantum many-body theory when the action which is being perturbed is not necessarily quadratic. Some applications are discussed.Comment: Extended revised version. RevTex, 19 pages, 10 figure

Quantum Numbers for Excitations of Bose-Einstein Condensates in 1D Optical Lattices

The excitation spectrum and the band structure of a Bose-Einstein condensate in a periodic potential are investigated. Analyses within full 3D systems, finite 1D systems, and ideal periodic 1D systems are compared. We find two branches of excitations in the spectra of the finite 1D model. The band structures for the first and (part of) the second band are compared between a finite 1D and the fully periodic 1D systems, utilizing a new definition of a effective wavenumber and a phase-slip number. The upper and lower edges of the first gap coincide well between the two cases. The remaining difference is explained by the existence of the two branches due to the finite-size effect.Comment: 5 pages, 9 figure