Observation of dynamic atom-atom correlation in liquid helium in real space

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.

Real-Space Description of Dynamics of Liquids

“…Now the snapshot PDF, obtained by integrating the DyPDF by Equation (4), shown in Figure 5, shows a peak at 3.55 Å [29] in agreement with the earlier results. However, in the earlier neutron diffraction work [31] which is highly cited, the authors used a two-axis neutron spectrometer, thus carrying out the energy integration by not using the energy analyzer.…”

“…The result shown in Figure 5 indicates that the PDF peak height does not show the decrease below T c as suggested by the earlier work [31], raising suspicion that the reported temperature dependence may be an artifact of incorrect energy integration. Now the snapshot PDF, obtained by integrating the DyPDF by Equation (4), shown in Figure 5, shows a peak at 3.55 Å [29] in agreement with the earlier results. However, in the earlier neutron diffraction work [31] which is highly cited, the authors used a two-axis neutron spectrometer, thus carrying out the energy integration by not using the energy analyzer.…”

“…Even though the collective dynamics of helium was carefully studied, particularly by inelastic neutron scattering [28], the real-space atomic correlation was never directly observed. Our study using inelastic neutron scattering and DyPDF [29] is the first to report such observation. The INS measurements were carried out at the CNCS beamline of the Spallation Neutron Source (SNS) of Oak Ridge National Laboratory.…”

“…Three incident energies, (E i = 3.5, 15 and 50 meV) were chosen, and the composite S(Q, E) was created by joining the data sets, by using the high-resolution (low E i ) data as much as possible. The sample was liquid 4 He in the standard volume-pressure (SVP) condition, and the measurement was made at five temperature points above and below the critical temperature, T c = 2.17 K. Figure 1 [29] shows S(Q, E) at T = 1.83 K (superfluid state) and the difference in S(Q, E) between T = 1.83 K and 2.85 K (normal state). In the difference figure (Figure 1b) sharpening of the roton excitation at 0.7 meV and 2 Å −1 and opening of the energy gap below the roton are clearly seen.…”

“…This result demonstrates that the BEC atoms have different atomic correlations than the normal atoms, a conclusion not expected by the current theory. [29]. The solid line describes that of the superfluid order parameter [27].…”