Localized Collective Excitations in Superfluid Helium in Vycor

Abstract

Inelastic neutron scattering has been used to probe the collective excitations of superfluid helium in porous Vycor glass for 0.5 # T͑K͒ # 1.9. The confined roton displays a temperature dependence similar to that of the bulk roton but with larger widths. Additional scattering has been observed near the roton minimum but with a substantially lower energy than the bulk roton. This scattering is consistent with a "two-dimensional roton," predicted to occur at the solid-liquid interface and inferred from measurements of the heat capacity and superfluid fraction. [S0031-9007(98)08080-6] PACS numbers: 61.12.Ex Superfluid helium is a model system whose microscopic excitations explain the macroscopic thermodynamic and transport properties of the liquid [1]. The superfluid excitation spectrum at low momentum transfers ͑Q , 2.4 Å 21 ͒ is well defined, clearly showing the quantum nature of the liquid via the long-lived excited states. The dominant excitations, which determine the thermodynamics of the liquid, are the phonons at low Q ͑Q , 0.9 Å 21 ͒ and the rotons at higher Q ͑Q Ӎ 1.9 Å 21 ͒. At low temperatures ͑T , 0.7 K͒ the liquid can be treated as a noninteracting gas of phonons, and above 1 K the liquid can be treated as an ideal gas of rotons.Confinement can have dramatic effects on the macroscopic properties of superfluid helium, especially near the superfluid transition temperature. For instance, confinement in aerogel glass results in only a slight decrease in the transition temperature T l but dramatically changes the critical exponent [2]. Alternatively, confinement in porous Vycor glass results in a substantial decrease in the transition temperature ͑T l Ӎ 1.95 K͒ but no change in critical exponents. Recently studies of the microscopic dynamics of helium confined in aerogel have reported [3,4] a crossover behavior of the roton energy from a low temperature regime ͑T , 1.9 K͒ with a weak temperature dependence to a high temperature regime, where it exhibits a strong temperature dependence. This anomalous temperature dependence can be understood in terms of a competition between the temperature-dependent roton mean free path and a temperature-independent length scale set by the porous media.In this Letter we report neutron inelastic scattering measurements of the collective excitation spectrum of superfluid helium confined in porous Vycor glass. We observe a strong excitation similar to the roton observed in the bulk liquid which we shall refer to as the three-dimensional (3D) roton. We also observe additional scattering around the bulk roton minimum but lower in energy. This additional intensity is consistent with a two-dimensional (2D) roton which is confined to the higher density liquid layer near the pore wall [5]. The properties of this 2D roton, which have previously been observed directly (via inelastic neutron scattering) on crystalline substrates such as pyrolytic graphite [6][7][8], and indirectly (via third sound measurements) on amorphous substrates [9], are consistent with the thermodynamic data ...

Surface excitations in thin helium films on silica aerogel

“…The line width of the bulk roton (0.16 meV) is in agreement with the resolution of the instrument and the lifetime of the roton at T = 1.55 K (∼ 0.04 meV). This coincides with the line width found for helium confined in Vycor [9] and shows that the line width of the roton is influenced by a restricted geometry. An other important result is that the line width of the layered roton of ∼ 1.1 meV is doubled with respect to the line width at low temperature of T = 0.45 K, which will be discussed in the next section.…”

“…The line width of the layered roton measured in Vycor [9] is about half the one presented here on aerogel. This agrees with the smoother surface of the Vycor pores with respect to the rough surface of the aerogel composed of silica strands.…”

“…The layered roton of our data exhibits an even more increased line width at a temperature of T = 1.55 K (see Fig. 2), which amounts to about 2 meV; however a theoretical background to explain this behaviour as it is done for the bulk roton in a confining geometry [9] is still missing.…”

Phonon-roton modes of liquid4He beyond the roton in the porous medium MCM-41

“…In porous media 19,[21][22][23][24]26,[28][29][30][31]34 and in helium films, 35,38,39 a layer mode is also observed. This is a mode that propagates in the 1-4 liquid layers adjacent to the porous media walls.…”

Localized Bose-Einstein condensation in disordered liquidHe4films

“…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 .…”