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Etude de la dynamique de l'3He liquide et de l'4He superfluide par diffusion inélastique de neutrons

Abstract : The effect of correlations in quantum fluids is investigated in the present work by inelastic neutron scattering measurements of their elementary excitations.Recent theories provide us with a detailed description of the dynamics up to atomic wave vectors. In addition, the recent improvement of the time-of-flight IN5 spectrometer at the ILL opens new experimental possibilities in terms of neutron flux and resolution, as well as accessible energy and wave vector ranges. In the case of 4He, the bosonic isotope, the neutron measurements have been performed at very low temperatures, from the saturated vapor pressure up to the melting curve. The dispersion relations of the corresponding elementary excitations have been accurately determined. At higher energies, we observe in the dynamic structure factor a highly structured multi-excitation response, characterized by sharp thresholds due to the coupling of elementary excitations. In particular, we observe a ghost phonon related to phonon-phonon coupling. Our measurements on superfluid 4He confirm the predictions of the dynamic many body theory (2015), the agreement being quantitative up to an energy of 2 meV. The investigations on normal liquid 3He were carried out at temperatures well below 100 mK in a cell specially designed for this purpose. Ourmeasurements on this fermionic isotope yield a high precision determination of the collective modes, zero-sound and paramagnon, as well as the particle-hole band. Since they also cover an unexplored region, we were able to confirm the theoretical prediction of a roton-like excitation in bulk liquid 3He. This mode remains within the particle-hole band, contrarily to the case of two-dimensional 3He. A broad vision of the dynamics of interacting Bose and Fermi systems, going from the Landau quasi-particles and multi-excitations regimes up to the high-energy limit, where the independent particle dynamics is recovered, emerges from our work.
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https://tel.archives-ouvertes.fr/tel-01694041
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  • HAL Id : tel-01694041, version 1

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Ketty Beauvois. Etude de la dynamique de l'3He liquide et de l'4He superfluide par diffusion inélastique de neutrons. Physique Quantique [quant-ph]. Université Grenoble Alpes, 2016. Français. ⟨NNT : 2016GREAY094⟩. ⟨tel-01694041⟩

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