We investigate a two-component Fermi gas with unequal spin populations along the BCS-BEC crossover. By using the extended BCS equations and the concept of off-diagonal-long-range-order we derive a formula for the condensate number of Cooper pairs as
a function of energy gap, average chemical potential, imbalance chemical potential and temperature. Then we study the zero-temperature condensate fraction of Cooper pairs by varying interaction strength and polarization, finding a depletion of the condensate fraction by increasing the population imbalance. We also consider explicitly the presence of an external harmonic confinement and we study, within the local-density approximation, the phase separation between superfluid and normal phase regions of the polarized fermionic cloud. In particular, we calculate both condensate density profiles and total density profiles from the inner superfluid core to the normal region passing for the interface, where a finite jump in the density is a clear manifestation of this phase-separated regime. Finally, we compare our theoretical results with the available experimental data on the condensate fraction of polarized 6Li atoms [Science 311, 492 (2006)]. These experimental data are in reasonable agreement with our predictions in a suitable range of polarizations, but only in the BCS side of the crossover up to unitarity.
We discuss the unitary Fermi gas made of dilute and ultracold atoms with an infinite s-wave inter-atomic scattering length. First we introduce an efficient Thomas-Fermi-von Weizsacker density functional which describes accurately various static prope
rties of the unitary Fermi gas trapped by an external potential. Then, the sound velocity and the collective frequencies of oscillations in a harmonic trap are derived from extended superfluid hydrodynamic equations which are the Euler-Lagrange equations of a Thomas-Fermi-von Weizsacker action functional. Finally, we show that this amazing Fermi gas supports supersonic and subsonic shock waves.
We study the Bose-Einstein condensation of fermionic pairs in the uniform neutron matter by using the concept of the off-diagonal long-range order of the two-body density matrix of the system. We derive explicit formulas for the condensate density $r
ho_c$ and the condensate fraction $rho_c/rho$ in terms of the scaled pairing energy gap $Delta/epsilon_F$, where $epsilon_F$ is the Fermi energy. We calculate the condensate fraction $rho_c/rho$ as a function of the density $rho$ by using previously obtained results for the pairing gap $Delta$. We find the maximum condensate fraction $(rho_c/rho)_{max}= 0.42$ at the density $rho=5.3cdot 10^{-4}$ fm$^{-3}$, which corresponds to the Fermi wave number $k_F= 0.25$ fm$^{-1}$.
We investigate the formation of Bose-Einstein condensation and population imbalance in a three-component Fermi superfluid by increasing the U(3) invariant attractive interaction. We consider the system at zero temperature in three dimensions and also
in two dimensions. Within the mean-field theory, we derive explicit formulas for number densities, gap order parameter, condensate density and condensate fraction of the uniform system, and analyze them in the crossover from the Bardeen-Cooper-Schrieffer (BCS) state of Cooper pairs to the Bose-Einstein Condensate (BEC) of strongly-bound molecular dimers. In addition, we study this Fermi mixture trapped by a harmonic potential: we calculate the density profiles of the three components and the condensate density profile of Cooper pairs in the BCS-BEC crossover.
We determine the energy density $xi (3/5) n epsilon_F$ and the gradient correction $lambda hbar^2( abla n)^2/(8m n)$ of the extended Thomas-Fermi (ETF) density functional, where $n$ is number density and $epsilon_F$ is Fermi energy, for a trapped two
-components Fermi gas with infinite scattering length (unitary Fermi gas) on the basis of recent diffusion Monte Carlo (DMC) calculations [Phys. Rev. Lett. {bf 99}, 233201 (2007)]. In particular we find that $xi=0.455$ and $lambda=0.13$ give the best fit of the DMC data with an even number $N$ of particles. We also study the odd-even splitting $gamma N^{1/9} hbar omega$ of the ground-state energy for the unitary gas in a harmonic trap of frequency $omega$ determining the constant $gamma$. Finally we investigate the effect of the gradient term in the time-dependent ETF model by introducing generalized Galilei-invariant hydrodynamics equations.
We discuss the zero-temperature hydrodynamics equations of bosonic and fermionic superfluids and their connection with generalized Gross-Pitaevskii and Ginzburg-Landau equations through a single superfluid nonlinear Schrodinger equation.
