Beyond the Landau criterion for superfluidity

According to the Landau criterion for superfluidity, a Bose-Einstein condensate flowing with a group velocity smaller than the sound velocity is energetically stable in the presence of perturbing potentials. We found that this is strictly correct only for vanishingly small perturbations. The superfluid critical velocity strongly depends on the strength and shape of the defect. We quantitatively study, both numerically and with an approximate analytical model, the dynamical response of a condensate flowing against an instantaneously raised spatially periodic defect. We found that the critical velocity v(c) decreases by increasing the strength of the defect V-0, up to a critical value of the defect intensity where the critical velocity vanishes.