Sharp peaks in the momentum distribution of bosons in optical lattices in the normal state

Abstract

Cold atoms in optical lattices provide a unique laboratory for investigating quantum phase transitions between strongly correlated superfluid and Mott insulator phases. One of the major bottlenecks in the analysis of experiments is a clear set of criteria for identifying the superfluid phase. A 'sharp' interference pattern in time-of-flight experiments has been widely adopted as a signature of superfluidity. Here, we show that sharp peaks are not a reliable diagnostic of superfluidity. Using large-scale quantum Monte Carlo simulations of the Bose-Hubbard model in three dimensions with up to N=1.4×104 particles, we calculate the momentum distribution n(k) as a function of temperature T and t/U, the ratio of hopping to the onsite repulsion. We find that even above the transition temperature Tc where both superfluid density and condensate fraction vanish, the interference pattern can nevertheless have sharp peaks riding over a broad background. We identify the true signature of the superfluid and give a deeper understanding of why such sharp peaks appear in the normal state. © 2008 Macmillan Publishers Limited. All rights reserved.