Phonons, rotons, and localized Bose-Einstein condensation in liquid He4 confined in nanoporous FSM-16

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 He4 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