Atomic Fermi gas in the unitary limit by quantum Monte Carlo methods: Effects of the interaction range
Abstract
We calculate the groundstate properties of an unpolarized twocomponent Fermi gas with the aid of the diffusion quantum Monte Carlo (DMC) methods. Using an extrapolation to the zero effective range of the attractive twoparticle interaction, we find E/E_{free} in the unitary limit to be 0.212(2), 0.407(2), 0.409(3), and 0.398(3) for 4, 14, 38, and 66 atoms, respectively. Our calculations indicate that the dependence of the total energy on the effective range of the interaction R_{eff} is sizable and the extrapolation to R_{eff}=0 is therefore important for reaching the true unitary limit. To test the quality of nodal surfaces and to estimate the impact of the fixednode approximation, we perform releasednode DMC calculations for 4 and 14 atoms. Analysis of the releasednode and the fixednode results suggests that the main sources of the fixednode errors are longrange correlations, which are difficult to sample in the releasednode approaches due to the fast growth of the bosonic noise. Besides energies, we evaluate the twobody density matrix and the condensate fraction. We find that the condensate fraction for the 66atom system converges to 0.56(1) after the extrapolation to the zero interaction range.
 Publication:

Physical Review A
 Pub Date:
 August 2011
 DOI:
 10.1103/PhysRevA.84.023615
 arXiv:
 arXiv:1105.1748
 Bibcode:
 2011PhRvA..84b3615L
 Keywords:

 03.75.Ss;
 67.85.d;
 Degenerate Fermi gases;
 Ultracold gases trapped gases;
 Condensed Matter  Quantum Gases
 EPrint:
 7pages, 6 figures