After a brief introduction to the static Gross–Pitaevskii (GP) differential equation for the ground state of a Bose–Einstein condensed gas and an example of its use in describing a vortex array in a trapped condensate, a recent generalization is reviewed. By starting from the Bogoliubov–de Gennes equations for superfluid fermions, an integral equation results. This is shown to contain the static GP equation as a limiting case, by means of a low-order gradient expansion of the order parameter. The time-dependent GP approximation is then presented and illustrated by a numerical example concerning drop emission under gravity from a gaseous condensate in an optical lattice. Further selected topics of present interest relating more generally to the dynamics of trapped superfluid gases concern the effects of temperature on collective excitations in a Bose gas and the fingerprints that these excitations in a gas of paired fermions contain of the crossover from a Bardeen–Cooper–Schrieffer correlated assembly to a Bose–Einstein condensate of composite bosons. Finally, some directions are pointed in which interaction of first-principles theory with experiment should prove fruitful.
Correlation functions related to the dynamic density response of the one-dimensional Bose gas in the model of Lieb and Liniger are calculated. An exact Bose-Fermi mapping is used to work in a fermionic representation with a pseudopotential Hamiltonian. The Hartree-Fock and generalized random phase approximations are derived and the dynamic polarizability is calculated. The results are valid to first order in $1∕\ensuremath{\gamma}$, where $\ensuremath{\gamma}$ is Lieb-Liniger coupling parameter. Approximations for the dynamic and static structure factor at finite temperature are presented. The results preclude superfluidity at any finite temperature in the large-$\ensuremath{\gamma}$ regime due to the Landau criterion. Due to the exact Bose-Fermi duality, the results apply for spinless fermions with weak
p-wave interactions as well as for strongly interacting bosons.
In the first part of this chapter we analyze the contact intensity
C, which has been introduced by Tan [Ann. Phys. 323, 2952 (2008)] and appears in several physical observables of the strongly correlated two-component Fermi gas. We calculate the contact
C in the full BCS-BEC crossover for a uniform superfluid Fermi gas by using an efficient parametetrization of the ground-state energy. In the case of harmonic confinement, within the Thomas-Fermi approximation, we derive analytical formulas of
C in the three relevant limits of the crossover. In the second part of this chapter we discuss the extended superfluid hydrodynamics we have recently proposed to describe static and dynamical collective properties of the Fermi gas in the BCS-BEC crossover. In particular we show the relation with the effective theory for the Goldstone field derived by Son and Wingate [Ann. Phys. 321, 197 (2006)] on the basis of conformal invariance. By using our equations of extended hydrodynamics we determine nonlinear sound waves, static response function and structure factor of a generic superfluid at zero temperature.