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Persistent URL http://purl.org/net/epubs/work/29565
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Record Id 29565
Title Microscopic connection between Bose-Einstein condensation and the two fluid model of liquid 4He
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Abstract It is shown that a function Psi(r(-->)) of the position co-ordinate r(-->) can be defined in terms of the many particle Schrodinger wave function Psi. It is shown that for the ideal Bose gas and correlated basis functions, Psi(r(-->)) can be split into two parts in the presence of Bose Einstein condensation. One part, Psi(subscript s)(r(-->)), is completely phase coherent over macroscopic length scales, while the other part, Psi(subscript N)(r(-->)), has only short range phase coherence, with a phase which varies randomly over atomic length scales. It is postulated that this is also true in superfluid 4He. It is shown that in a sufficiently large system Psi(subscript S) and Psi(subscript N) contribute separately to the fluid density, particle flow, the momentum distribution, the dynamic structure factor S (q(-->,omega) and the static structure factor S(q(-->)). This is so only in a Bose condensed system and not when BEC is absent. It is shown that the two components satisfy the standard two fluid equations of liquid 4He. The identification of Psi (subscript S) with the superfluid and Psi (subscript N) with the normal fluid provides a direct link bewteen BEC and two fluid behaviour. The additional postulate that Psi (subscript (s) (r(-->)) is identical to Psi(r(-->)) in the ground state, leads to the predictions that the condensate fraction 'f' and the intensity of sharp peaks in S(q(-->),omega) are both exactly proportional to the superfluid fraction, in good agreement with experiment. The postulates also provide a simple explanation of the anomalous decreases in density and spatial order as the temperature is lowered in superfluid 4He.
Organisation CCLRC
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Language English (EN)
Type Details URI(s) Local file(s) Year
Report RAL Technical Reports RAL-TR-2001-044. 2001. raltr-2001044.pdf 2001
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