Bose-Einstein condensates in optical lattices : the superfluid to Mott insulator phase transition

Abstract

87Rb Bose Einstein Condensate in 3D optical lattice was studied in the regime of weak interaction(the superfluid phase) and strong interaction(the Mott insulating phase). The stability of superfluid currents was studied using a moving optical lattice. The critical momentum for stable superfluid current varies from 0.5 recoil momentum (shallow lattice) to 0 (the Mott insulator) as the system reaches the Mott insulator transition. The phase diagram for the disappearance of superfluidity was studied as a function of momentum and lattice depth. Our phase diagram boundary extrapolates to the critical lattice depth for the superfluid-to-MI transition. When a one-dimensional gas was loaded into a moving optical lattice a sudden broadening of the transition between stable and unstable phases was observed. A new auxiliary vacuum chamber, which is called the science chamber, was designed and installed to improve optical lattice experimental performance and imaging resolution power. Atoms are transported from the main chamber to the science chamber. By further evaporation cooling, BECs with N - 2-3 x 104 atoms are produced in a combination trap of two focused IR laser beams. High-resolution imaging was obtained with a 4-lens stack providing a resolution of - 2pm. The deep Mott insulator(MI) phase was studied using clock shift spectroscopy. Individual MI phases with integer occupation numbers could be addressed through their clock shifts, and their spatial density profile could be imaged ("shell structure"). With increasing trap depth, MI shells expanded from low to high density regions of the cloud.