The elementary excitations in 3He and 4He quantum fluids

Orateur : Ketty BEAUVOIS

Understanding the elementary excitations of quantum many-body systems is a challenge for modern Physics. Experimentally, the problem can be addressed by the investigation of quantum fluids like liquid 4He (bosons) and liquid 3He (fermions) at very low temperatures. Because they can be obtained very pure, liquid 3He and 4He are model systems and serve to study strongly correlated systems of fermionic and bosonic particles. The dynamics of superfluid 4He at and above the Landau quasiparticle regime is investigated by high precision inelastic neutron scattering measurements of the dynamic structure function. The results are analysed with a dynamic many-body calculation of the same physical quantity. Theory and experiment are in quantitative agreement, showing, besides the familiar phonon-maxon-roton spectrum, a highly structured response characterized by sharp thresholds for phonon-phonon, maxon-roton and roton-roton coupling processes [1]. Pioneering investigations have established the existence of Fermi Liquid particle-hole excitations, zero sound and paramagnon collective modes in bulk liquid 3He. Following a recent theoretical breakthrough [2], the dynamical many-body theory now provides accurate predictions for the dynamical structure factor of 3He, calling for neutron data of similar precision and resolution. Experimental studies were concentrated until now on low wavevectors (below 1.2 A-1), mainly because this was the only region of the spectrum where theories were available and reliable. But now, the theory describes new results and notably the damping of the zero-sound mode as it enters the particle-hole band as well as features observed in the spectrum at atomic (larger) wave vectors. In order to test these new theoretical results, an accurate experimental investigation is now possible, on modern time of flight instruments such as IN5B at ILL. In this talk, I will show you firstly why the study of the elementary excitations in quantum fluids is interesting and I will describe the elementary excitations in Fermi liquid 3He and in superfluid 4He. Then, I will present the results of the measurements of the excitations in superfluid 4He by inelastic neutron scattering. And I will show the comparison of the experimental results with those of the dynamical many-body theory. I will present also quickly the pressure dependence of the excitations from the saturated vapor pressure to the solidification pressure. To finish, I will describe the preparation of a forthcoming neutron scattering experiment to investigate the dynamics in bulk liquid 3He.

Télécharger dans mon agenda