Excitations in liquid 4He in Geltech silica and localized Bose condensation -: art. no. 224505

We have measured the elementary excitations of liquid He-4 confined in Geltech silica for temperatures between 35 mK and 2.5 K using inelastic neutron scattering techniques. Geltech is a 40-50% porous media. The pore volume diameters are concentrated at 25 Angstrom with at most a few percent of the volume consisting of larger pores of diameter less than 70 Angstrom. We observe well-defined phonon-roton excitations at all wave vectors Q investigated (0.4less than or equal toQless than or equal to2.1 Angstrom(-1)). Also observed are two-dimensional layer modes propagating in the liquid layers adjacent to the media walls. These are identified as layer modes by measuring the dynamical structure factor of liquid He-4 as a function of filling of the Geltech. At partial fillings, the energies and lifetimes of the three-dimensional phonon-roton mode differ somewhat from the bulk superfluid He-4 values. At full filling they are the same within current precision. Most interestingly, we observe a well-defined phonon-roton excitation above the superfluid-normal transition T-c=0.725 K as determined in a torsional oscillator measurement. Observation of a well defined maxon-roton above T-c in Geltech suggests, as in the case of Vycor, that there is localized Bose-Einstein condensation with phase coherence on short length scales above the macroscopic superfluid transition temperature.