Surface effects in the unitary Fermi gas

We study the extended Thomas-Fermi (ETF) density functional of the superfluid unitary Fermi gas. This functional includes a gradient term which is essential to describe accurately the surface effects of the system, in particular with a small number of atoms, where the Thomas-Fermi (local density) approximation fails. We find that our ETF functional gives density profiles which are in good agreement with recent Monte Carlo results and also with a more sophisticated superfluid density functional based on Bogoliubov-de Gennes equations. In addition, by using extended hydrodynamics equations of superfluids, we calculate the frequencies of collective surface oscillations of the unitary Fermi gas, showing that quadrupole and octupole modes strongly depend on the number of trapped atoms. Density profile n(r) of the unitary Fermi gas with 20 atoms under harmonic confinement. Comparison among different theoretical predictions: our extended Thomas-Fermi (ETF) density functional, superfluid local density approximation (SLDA) density functional, and fixed-node diffusion Monte Carlo (FNDMC) calculation