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1 UBT - Ultra-basses températures NEEL - Institut Néel 2 NEEL - Institut Néel 3 Aalto - Aalto university low temperature physics 4 ILL - Institut Laue-Langevin 5 ORNL - Oak Ridge National Laboratory Oak Ridge 6 Kepler - Institute for Theoretical Physics 7 SUNY - University at Buffalo, physics department

Abstract : Understanding the dynamics of correlated many-body quantum systems has been a challenge for modern physics. Due to the simplicity of their Hamiltonian, 4He bosons and 3He fermions have served as paradigm for strongly interacting quantum fluids. For this reason, substantial efforts have been devoted to their understanding. An important milestone was the direct observation of the collective -phonon-roton- mode in liquid 4He by neutron scattering, verifying Landau-s prediction and his fruitful concept of elementary excitations. In a Fermi system, collective density fluctuations -zero-sound- in 3He -plasmons- in charged systems as well as incoherent particle-hole PH excitations are observed. At small wave-vectors and energies, both types of excitations are described by Landau-s theory of Fermi liquids. At higher wavevectors, the collective mode enters the PH band, where it is strongly damped. The dynamics of Fermi liquids at high wave-vectors was thus believed to be essentially incoherent. We report here the first observation of a roton-like excitation in a Fermi liquid, obtained in a monolayer of liquid 3He, studied by inelastic neutron scattering. We find that the collective density mode reappears as a well-defined excitation at momentum transfers larger than twice the Fermi momentum. We thus observe unexpected collective behaviour of a Fermi many-body system in the region outside the scope of Landau-s theory. A satisfactory interpretation of the measured spectra is obtained within a novel dynamic many-body theory.

Autor: Henri Godfrin - Matthias Meschke - Hans-Jochen Lauter - Ahmad Sultan - Helga M. Böhm - Eckhard Krotscheck - Martin Panholzer -



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