The collective excitations of normal and superfluid : the dependence on pressure and temperature

Abstract

Systematic high-resolution measurements of the collective excitations in normal and superfluid 4 He have been made as a function of both pressure (density) and temperature using neutron inelastic scattering. The IN6 time-of-flight spectrometer at the Institut Laue-Langevin has been used to obtain accurate and consistent data over a wavevector range 0.3 < Q < 2.35 Å −1 . We present and discuss the experimental data. An analysis, based on the simple-subtraction model, is made; single-excitation energies, widths, and magnitudes are extracted. Many changes in the excitations are shown to occur at the λ-transition. The single excitations are discussed in the light of current theory. The structure of the multiphonon continuum and its dependence on pressure is also discussed.

Static Structure Factor and Static Response Function of Superfluid Helium 4: a Comparative Analysis

“…However, the methods give identical results below 1.3 K [19,20]. More recent data [10] (using the simple subtraction method) go down to 0.5 K, and show that Z(q) can be taken as constant below 1.5 K (see Fig. 2).…”

“…However, two sets of data [7,10] exhibited a marked discrepancy, especially obvious in Z pk and χ pk /n. We have taken the recent data from Gibbs et al [10] using the following procedure. The one phonon contribution Z pk was taken from the tables in Appendix A of Gibbs's thesis.…”

“…We have selected here those which give information about S(q), Z(q), and/or χ(q) and their pressure dependence, including neutron [1,[7][8][9][10][11] and X-ray [12][13][14] scattering data. Table 1 summarizes the experimental conditions; when absolute units are used, the corresponding calibration method is described.…”

“…The multiphonon contribution to S pk was estimated by extracting the data for S(q, ω) at q = 20 nm −1 from Fig. 19 of Gibbs et al [10], and integrating them from ω = 12 K (to avoid the tail of the one phonon peak convoluted with the experimental resolution) to 40 K; we expect this to be an underestimate, because the multiphonon peak extends to higher energies. Fig.…”

Roton-Phonon Interactions in SuperfluidHe4

“…Early estimates indicated a negative value −0.1 [16], while more recent neutron scattering measurements give values in the range −0.38 to +1.35 [17,18]. The uncertainty comes mostly from the roton mass and from the density dependence of the roton energy ∆(ρ).…”