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Phase diagram for a Bose-Einstein condensate moving in an optical lattice

Mott insulator (MI) physics is an important paradigm for the suppression of transport by particle correlations.  So far, with ultracold atoms, the superfluid to MI transition has been studied mostly in a stationary system.  In this work, we study the stability of superfluid currents in a moving optical lattice, as suggested in ref. [1], and extend previous  studies in two regards. First, superfluidity near the MI transition has only been indirectly inferred from coherence measurements, whereas in this work, we characterize the superfluid regime by observing a critical current for superfluid flow. Second, previous studies were not able to precisely locate the phase transition partially due to the inhomogeneous density, while the sudden onset of dissipation provides a clear distinction between the two quantum phases.

We observed that superfluid current became unstable if the momentum exceeded a critical momentum, and the critical momentum varied from 0.5 recoil momentum in a very weak lattice to zero in the Mott insulator phase.  We study the phase diagram for the stability of superfluid current as a function of momentum and lattice depth. Our phase boundary extrapolates to the critical lattice depth for the SF-to-MI transition, which was measured with high precision to be 13.5 (+/- 0.2) recoil energy [2]

Picture: Critical momentum for a condensate in a three-dimensional moving lattice. The solid line shows the theoretical prediction for the superfluid region.  The horizontal solid line is a fit to the data points in the MI phase. (Inset) Fit of critical momenta near the SF-MI phase transition.

1.        E. Altman, A. Polkovnikov, E. Demler, B.I. Halperin, and M.D. Lukin, Superfluid-Insulator Transition in a Moving System of Interacting Bosons, Phys. Rev. Lett. 95, 020402 (2005).

2.        J. Mun, P. Medley, G.K. Campbell, L.G. Marcassa, D.E. Pritchard, and W. Ketterle, Phase diagram for a Bose-Einstein condensate moving in an optical lattice, Phys. Rev. Lett. 99, 150604 (2007).


G. K. Campbell, W. Ketterle, L. G. Marcassa, P. Medley, J. Mun, D. E. Pritchard

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