Enhanced Bose–Einstein condensation and kinetic energy of liquid⁴He near a free surface

Abstract

We present neutron scattering measurements of Bose-Einstein condensation (BEC) in liquid 4 He adsorbed in thick layers on an MgO substrate to study whether the condensate fraction, n 0 , is increased near a free surface of liquid 4 He. The data show that there is definitely a condensate in the layers adsorbed on the substrate with a condensate fraction comparable to that of bulk liquid 4 He. Two methods of analysis are employed to cross-check the results. The data indicate that the condensate fraction increases significantly when the number of adsorbed layers is reduced. This effect is independent of the analysis technique used. In addition, a significant increase in the kinetic energy of the 4 He atoms is observed when the number of adsorbed layers is reduced.

He4adsorbed in cylindrical silica nanopores: Effect of size on the single-atom mean kinetic energy

“…1,2 The confining geometry and the enhanced surface interaction due to the large surface-to-volume ratio of porous materials modify liquid-vapor and liquid-solid transitions. When atomic and molecular systems are adsorbed in nanometric pores, they experience a reduction in the number of nearest neighbors, with effects on microscopic properties as well as on phase diagram.…”

“…15 Recent experimental DINS measurements of 4 He, adsorbed in flat surface or slit-geometry porous substrates, have shown a large increase in ͗E K ͘ and n 0 , the condensate fraction of atoms with zero momentum. 1,6 This has been attributed to the strong localization effects induced by the helium-substrate interaction potential, which mainly influence the first two or three adsorbed layers. 6 At the same time, a reduction in the equilibrium pressure observed at low helium coverage has been attributed to a reduced local density of the adsorbate.…”

Localized Bose-Einstein condensation in disordered liquidHe4films

“…In contrast, at 34 bar well-defined P-R modes are observed up to T P R = 1.5 K, e.g. T BEC ≃ 1.5 K, in 25, 34 and 44Å gelsil 19,28,41 and in 47Å MCM-41 21,28 . In 28Å FSM, Taniguchi et al…”

“…Under pressure, T c is further reduced 4,19 and remarkably T c is observed to go to zero at p = 34 bar in 25 A MPD gelsil. 4 This suggests a quantum phase tran-sition (QPT) from the superfluid to the normal liquid phase at p = 34 bar 4 .…”

“…In fully filled porous media at SVP, the P-R mode energy is the same as in the bulk liquid within precision, both at low T and as a function of T in all porous media investigated. 12,[17][18][19][20][21][22] To date, the layer mode is observed 12,18,[20][21][22][23][24][25][26][27][28] at wave vectors 1.7Å −1 < Q < 2.3 A −1 only, denoted the roton region. At these wave vectors, the layer mode energy, ω L , has a roton like dispersion with an energy minimum near Q = 2.1Å −1 .…”