PHONON-ROTON EXCITATIONS AND QUANTUM PHASE TRANSITIONS IN LIQUID 4HE IN NANOPOROUS MEDIA

Abstract

We present measurements of the elementary phonon-roton and other excitations of liquid 4 He conflned in nanoporous media using inelastic neutron scattering methods. The aim is to compare phonon-roton (P-R) and super∞uid density measurements and to explore the interdependence of Bose-Einstein Condensation (BEC), P-R modes and super∞uidity in helium at nanoscales and in disorder. Speciflcally a goal is to determine the region of temperature and pressure in which well deflned phononroton modes exist and compare this with the super∞uid phase diagram. In porous media the liquid phase is extended up to 35-40 bars. A second goal is to investigate helium at higher pressures. At low temperature and at saturated vapor pressure (SVP) (p ’ 0) liquid 4 He supports well deflned P-R modes in all porous media investigated to date (aerogel, xerogel, Vycor, MCM-41 and gelsil of several pore diameters). As temperature is increased at SVP, the P-R modes broaden but well deflned modes exist above Tc in the normal phase, up to T‚. The super∞uid to normal transition temperature, Tc, in porous media always lies below the corresponding temperature, T‚, in bulk helium. In liquid 4 He in 25 ” and 34 ” mean pore diameter gelsil under pressure and at low temperature, we observe loss of all well deflned P-R modes at p = 36.3 36.8 bars. Yamamoto et al. have observed a possible Quantum Phase Transition (QPT) (T … 0 K) from the super∞uid to normal liquid at p = 34 bars in super∞uid density measurements. The existence of P-R modes under pressure up to 36.3 bars and their subsequent loss supports the flnding of a QPT. We discuss the implications of these results for the basic concepts of BEC and super∞uidity in helium at nanoscales and in disorder.