Temperature dependence of the physical properties of Bose-Einstein condensed gases and liquids (Q2925159)

The paper presents an approach to the theory of Bose-Einstein condensation (BEC) based on the properties of many-particle Schrödinger wave functions of BE condensed systems. This ``wave function approach'' provides relatively simple physically transparent and quantitative explanations of the first principles of all the considered phenomena, demonstrating a lot of advantages over the standard field theory. It is shown that in the presence of BEC in an \(N\)-particle system, the \(N\) particle Schrödinger wave functions of thermally occupied states are the sum of a ``localized'' component and a ``delocalized'' one. If the number \(N\) of particles is sufficiently large this implies that all physical properties can be expressed as the sum of two independent contributions from these two components. One consequence of these results is the two fluid behaviors (normal fluid and superfluid). The paper results are based on the observations that (i) inelastic neutron scattering shows that BE condensed liquid helium is the known liquid with sharp peaks in its dynamic structure factor, (ii) BE condensed liquid helium is the known physical system in which pair correlations between atomic positions reduce upon cooling. In the paper of \textit{J. Mayers} [Phys. Rev., A78, 33618 (2008)], it was shown that the assumptions (i) and (ii) are valid in the ground state of any BE condensed liquid or gas, due to the delocalization of the many particle ground state wave function in the presence of BEC and the absence of long range ordering of atomic positions. The reviewed paper extends the results to finite temperature. An algorithm is provided for calculating the time development of the macroscopic density of any BE condensed fluid at any temperature, given the superfluid fraction, the interaction between atoms and the applied constraints. This algorithm is valid for both strongly interacting systems (liquid helium) and weakly interacting systems (ultra-cold clouds of atoms). It is shown that the macroscopic quantum interference fringes in the form of density oscillations, observed in overlapping BE condensed clouds of atoms, are a necessary consequence of BEC and the Schrödinger equation for the \(N\) atoms in the two clouds. Finally, new experimentally testable predictions are made of how the visibility of the density oscillations will vary with the temperature.