Grigori E. Astrakharchik

The superfluid fraction of a quantum fluid is defined in terms of the response of the system to a weak and constant drag. Notably, Leggett long ago derived two simple expressions providing a rigorous upper bound and a heuristic lower bound for . Here we study the superfluid fraction of bosonic gases in various two-dimensional potentials, such as regular optical lattices and disordered speckles, by solving the Gross-Pitaevskii equation and performing diffusion Monte Carlo simulations.

We study the Josephson junction made of a one-dimensional ultradilute quantum liquid in a double-well potential. We analyze the dynamics as a function of the interaction strength and compare the results to the standard bosonic Josephson junction. It is found that the beyond-mean-field effects alter the dynamics, particularly in the regime, where the beyond-mean-field corrections dominate over the residual mean-field interaction.

Abstract We consider the problem of unitary Bose polarons, i.e., impurities interacting via a potential with infinite scattering length with a bath of weakly interacting bosons. We provide an analytic expression for the energy of a heavy impurity whose interaction potential has a range larger than the healing length of the bath.