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. , 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.
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
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
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).
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).