Phonons, rotons, and localized Bose-Einstein condensation in liquid He 4 confined in nanoporous FSM-16
Résumé
We present neutron scattering measurements of the phonon-roton and layer modes of liquid helium confined
in 28 Å diameter nanopores of FSM-16. The goal is to determine the energy, lifetime, and intensity of the
modes as a function of temperature. It is particularly to determine the highest temperature, denoted TPR, at
which well-defined phonon-roton modes are observed at higher wave vector (Q > 0.8 Å−1) in the nanopores.
The temperature TPR, which can be identified with loss of Bose-Einstein condensation (BEC), can be compared
with the superfluid to normal liquid transition temperature, TO, and other transition temperatures of 4
He in the nanopores. The aim is to identify the nature of BEC in a narrow nanopore. Two pressures are investigated, saturated vapor pressure (SVP) and p = 26 bars. We find that well-defined P-R modes are observed up to temperatures much higher than the conventional superfluid to normal liquid transition temperature, TO, observed in torsional oscillator measurements, i.e., TPR > TO. At SVP, TPR = 1.8 K and TO = 0.9 K. This supports the
interpretation that BEC exists in a localized or partially localized form in the temperature range TO < T < TPR;
i.e., there is a localized BEC region lying between the superfluid and fully normal liquid phase, as observed
in some other porous media. At close to full filling, the P-R mode energies in FSM-16 are similar to those
in bulk liquid 4He. However, a substantial P-R mode width at T → 0 K and at higher temperatures is observed.
Domaines
Gaz Quantiques [cond-mat.quant-gas]
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