MIXTURE OF RUBIDIUM - LITHIUM DEGENERATE BOSE AND FERMI GASSES

Subhadeep De, Daniel L. Campbell, Ryan Price, Ian B. Spielman.

Joint Quantum Institute (JQI),

University of Maryland (UMD) and National Institute for Standards and Technology (NIST), Maryland, USA.

 

Ultracold mixtures of bosons and fermions are of interest for studying superfluid properties of coupled fermions and other many-body phenomena in the presence of impurity bosons in a fermionic bath or the reverse. Attractive interaction among spin-polarized fermions induced by density fluctuation in the degenerate bosons forms phonon-induced pairing in fermions, which is alike to Cooper pairing. Such a system might resolve the dynamics of p-wave superconductivity. Ultracold heteronuclear polar molecules at their ro-vibrational ground state are objects of interest for precision measurements of fundamental constants, parity and time reversal symmetry violation. An array of such polar molecules stored in an optical lattice opens up the possibility to serve as quantum bits. Their long range dipole-dipole interaction is particularly of interest to study novel quantum phases with atoms stored in optical lattice. As for example 87Rb -6Li (rubidium-lithium) heteronuclear molecules acquire a large permanent electric dipole moment of 4.2 Debye, and are interesting gas mixtures from a molecular point of view.    

We show a new way of coupling distant bosons due to successive scattering of spin-polarized fermions, which has R-4 spatial dependence. Unlike the dipole-dipole interaction, the strength of the fermion-mediated Bose-Bose interaction can be either attractive or repulsive with the help of interspecies Feshbach resonances. The loss process due to three-body recombination will greatly be suppressed in the Mott phase of bosons. A mixture of heavy bosons and light fermions is ideal because a lattice will localize bosons while fermions can move freely. We propose to use a spin-polarized, non-interacting, degenerate 6Li gas mixed with 87Rb atoms. The fermion mediated Bose-Bose coupling is tunable using a 87Rb-6Li Feshbach resonance. We further investigate schemes to create supersolid phase and quantum magnetism in bosons with the help of the fermion-mediated interactions.

We are engaged in building an apparatus to produce mixtures of 87Rb Bose Einstein condensates (BEC) and degenerate Fermi gasses (DFG) of 6Li simultaneously. The ultracold bosons will initially be used for sympathetic cooling of fermions to reach the degeneracy. Currently the machine is commissioning the BEC and DFG is in the pipeline. We are engaged in studying many-body dynamics associated to 87Rb spinor BEC in the F = 1 hyperfine state. I will discuss the ongoing physics project with BEC that we are working on in parallel to building systems for fermions and long term goals that will be perused with the 87Rb-6Li mixtures.