Expansion velocity of a one-dimensional, two-component Fermi gas during the sudden expansion in the ballistic regime

We show that in the sudden expansion of a spin-balanced, two-component Fermi gas into an empty optical lattice induced by releasing particles from a trap, over a wide parameter regime, the radius R-n of the particle cloud grows linearly in time. This allow us to define the expansion velocity V-ex from R-n = V(ex)t. The goal of this work is to clarify the dependence of the expansion velocity on the initial conditions which we establish from time-dependent density matrix renormalization group simulations, both for a box trap and a harmonic trap. As a prominent result, the presence of a Mott-insulating region leaves clear fingerprints in the expansion velocity. Our predictions can be verified in experiments with ultracold atoms.