Condensate fraction of molecules for a spin mixture of ultracold fermionic atoms.pdfVIP

a r X i v : c o n d - m a t / 0 4 0 4 5 6 5 v 3 [ c o n d - m a t .s o f t ] 1 6 J u l 2 0 0 4 Condensate fraction of molecules for a spin mixture of ultracold fermionic atoms Hongwei Xiong?, Shujuan Liu, Min Liu, Kelin Kao, and Mingsheng Zhan State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China (Dated: February 2, 2008) The condensate fraction of molecules for ultracold Fermi gases is investigated for the magnetic field below the Feshbach resonant magnetic field. Assuming that there is no loss of particles and energy during the adiabatic magnetic-field sweep, a simple theory is used to interpret the measured condensate fraction in the experiments by JILA group (Phys. Rev. Lett. 92, 040403 (2004)) and MIT group (Phys. Rev. Lett. 92, 120403 (2004)). Our theory shows that the condensate fraction of molecules is dependent on the initial condition of the system and especially on the process of the magnetic-field sweep. PACS numbers: 03.75.Ss, 05.30.Fk, 05.30.Jp, 03.75.Hh ? Electronic address: xionghongwei@wipm.ac.cn The evidence for Bose-Einstein condensates of di- atomic molecules has been finally given in several re- markable experiments [1, 2, 3] which will obviously lead to intensive theoretical and experimental researches on the ultracold Fermi gases. The magnetic-field Feshbach resonance [4, 5] plays an important role in most of the recent experiments on ultracold Fermi gases because it can change both the strength and sign of the scattering length a between fermionic atoms with different inter- nal freedom. On the side of strongly repulsive interac- tion (BEC side), there is molecule which is short-range fermionic pairs. On the side of strongly attractive inter- action (BCS side), one expects that there are fermionic pairs analogously to the electronic Cooper pairs in super- conductor. The Feshbach resonance has given us an im- portan