Microscopic dynamics of superfluid He4 : A comprehensive study by inelastic neutron scattering

Abstract

The dynamic structure factor of superfluid 4 He has been investigated at very low temperatures by inelastic neutron scattering. The measurements combine different incoming energies resulting in an unprecedentedly large dynamic range with excellent energy resolution, covering wave vectors Q up to 5Å −1 and energies ω up to 15 meV. A detailed description of the dynamics of superfluid 4 He is obtained from saturated vapor pressure up to solidification. The single-excitation spectrum is substantially modified at high pressures, as the maxon energy exceeds the roton-roton decay threshold. A highly structured multi-excitation spectrum is observed at low energies, where clear thresholds and branches have been identified. Strong phonon emission branches are observed when the phonon or roton group velocities exceed the sound velocity. The spectrum is found to display strong multi-excitations whenever the single-excitations face disintegration following Pitaevskii's type a or b criteria. At intermediate energies, an interesting pattern in the dynamic structure factor is observed in the vicinity of the recoil energy. All these features, which evolve significantly with pressure, are in very good agreement with the Dynamic Many-body calculations, even at the highest densities, where the correlations are strongest. arXiv:1802.08120v3 [cond-mat.other]

Re-examining the quadratic approximation in theory of a weakly interacting Bose gas with condensate: the role of nonlocal interaction potentials

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Roton-like acoustical dispersion relations in 3D metamaterials

“…Rotons can be seen as highly unusual acoustical waves with a parabolic minimum of the energy versus momentum at finite momentum and finite energy 2 . Based on a prediction by Landau 2 and following a suggestion by Feynman 3 , 4 , the roton dispersion relation for longitudinal acoustical waves was observed in liquid Helium-4 (a Bose-liquid) at low temperatures by means of inelastic neutron scattering 5 – 7 . A bulk of later theoretical work interpreted rotons in terms of strong spatial correlations in this quantum system 8 – 10 .…”

Superfluid Optomechanics With Phononic Nanostructures

“…Hence, we observe that at low pressure and at the typical base temperature of a dilution refrigerator (T ∼ 10 mK), the quality factor is independent of frequency up to a cut-off frequency of the order of 1 GHz, above which the quality factor decreases following a 1/Ω 4 q frequency dependence. Recent high-precision neutron measurements from Beauvois et al [54] compared to theory [74] confirm that the dispersion coefficient, negative at low pressure (γ < 0), changes sign and becomes positive (γ > 0) at a pressure P 20 bar (ρ 0 = 169 kg/m 3 ). This indicates a concave dispersion relation at high pressure and low wave numbers.…”

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