Abstract
Liquid 4He becomes superfluid and flows without resistance below temperature 2.17 K. Superfluidity has been a subject of intense studies and notable advances were made in elucidating the phenomenon by experiment and theory. Nevertheless, details of the microscopic state, including dynamic atom–atom correlations in the superfluid state, are not fully understood. Here using a technique of neutron dynamic pair-density function (DPDF) analysis we show that 4He atoms in the Bose–Einstein condensate have environment significantly different from uncondensed atoms, with the interatomic distance larger than the average by about 10%, whereas the average structure changes little through the superfluid transition. DPDF peak not seen in the snap-shot pair-density function is found at 2.3 Å, and is interpreted in terms of atomic tunnelling. The real space picture of dynamic atom–atom correlations presented here reveal characteristics of atomic dynamics not recognized so far, compelling yet another look at the phenomenon.
Highlights
Liquid 4He becomes superfluid and flows without resistance below temperature 2.17 K
The strong variation in S(Q,E) with temperature implies that the dynamic atomic correlation must be strongly dependent on temperature, even though they are almost totally masked when it is integrated over energy
The results show that atoms involved in the Bose–Einstein condensation (BEC) have environment different from the atoms in the normal state
Summary
Liquid 4He becomes superfluid and flows without resistance below temperature 2.17 K. Using a technique of neutron dynamic pair-density function (DPDF) analysis we show that 4He atoms in the Bose–Einstein condensate have environment significantly different from uncondensed atoms, with the interatomic distance larger than the average by about 10%, whereas the average structure changes little through the superfluid transition. London[3,4] first proposed that the superfluid behaviour arises from Bose–Einstein condensation (BEC) to the quantummechanical ground state. In ideal gas atoms have no spatial correlations In liquid they are strongly correlated, as described in terms of the same-time or snap-shot, pair density function (PDF), g(r), the probability of two atoms separated by distance r The strong variation in S(Q,E) with temperature implies that the dynamic atomic correlation must be strongly dependent on temperature, even though they are almost totally masked when it is integrated over energy
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