Reflection of a rarefaction from the center with the adiabatic expansion of a gas sphere into a vacuum

Self-induced density modulations in the free expansion of Bose-Einstein condensates

“…The non-ballistic free expansion observed with 6 Li atoms has been saluted as the first signature of superfluid behavior in a ultracold fermi vapor [2]. In both cases the atomic quantum gases can be described by the hydrodynamic equations of superfluids and, because the initial density profile is an inverted parabola, the free expansion is self-similar [3,4,5,6,7].The first theoretical investigations of the free expansion into vacuum of a classical gas sphere with constant initial density dates back to the 1960's [8,9]: numerical analysis was needed to analyze in detail the formation of a depletion at the center and of a shock wave at the surface [10,11]. More recently, rarefaction waves have been produced experimentally by the free expansion of an electron plasma [12].In this Letter we show that the free expansion of a bosonic superfluid into vacuum displays intriguing nonlinear phenomena.…”

“…The first theoretical investigations of the free expansion into vacuum of a classical gas sphere with constant initial density dates back to the 1960's [8,9]: numerical analysis was needed to analyze in detail the formation of a depletion at the center and of a shock wave at the surface [10,11]. More recently, rarefaction waves have been produced experimentally by the free expansion of an electron plasma [12].…”