Understanding the dynamics of correlated many-body quantum systems is a challenge for modern physics. Owing to the simplicity of their Hamiltonians, (4)He (bosons) and (3)He (fermions) have served as model systems for strongly interacting quantum fluids, with substantial efforts devoted to their understanding. An important milestone was the direct observation of the collective phonon-roton mode in liquid (4)He by neutron scattering, verifying Landau's prediction and his fruitful concept of elementary excitations. In a Fermi system, collective density fluctuations (known as 'zero-sound' in (3)He, and 'plasmons' in charged systems) and incoherent particle-hole excitations are observed. At small wavevectors and energies, both types of excitation are described by Landau's theory of Fermi liquids. At higher wavevectors, the collective mode enters the particle-hole band, where it is strongly damped. The dynamics of Fermi liquids at high wavevectors was thus believed to be essentially incoherent. Here we report inelastic neutron scattering measurements of a monolayer of liquid (3)He, observing a roton-like excitation. 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 regime beyond the scope of Landau's theory. A satisfactory interpretation of the measured spectra is obtained using a dynamic many-body theory.

The dynamics of superfluid 4 He at and above the Landau quasiparticle regime is investigated by high precision inelastic neutron scattering measurements of the dynamic structure factor. A highly structured response is observed above the familiar phonon-maxon-roton spectrum, characterized by sharp thresholds for phonon-phonon, maxon-roton and roton-roton coupling processes. The experimental dynamic structure factor is compared to the calculation of the same physical quantity by a Dynamic Many-body theory including three-phonon processes self-consistently. The theory is found to provide a quantitative description of the dynamics of the correlated bosons for energies up to about three times that of the Landau quasiparticles. Liquid4 He is the prime example of a strongly correlated quantum many-body system. It has been studied for decades and still offers surprises that lead to new insights. Understanding the helium fluids, due to their generic nature, lies at the core of understanding other strongly correlated many-particle systems, and is therefore of interest not only for the quantum fluids community. The description of the elementary excitations of superfluid 4 He in terms of phonon-roton quasiparticles is a cornerstone of modern physics, with profound implications in condensed matter physics, particle physics and cosmology. The empirical dispersion relation of these excitations proposed by Landau [1, 2] to explain thermodynamic data found support in the microscopic theory of Feynman and Cohen [3], initiating a fruitful development of the field theoretical description of correlated quantum particles.From an experimental point of view, neutron scattering techniques allowed the direct observation of very sharp excitations in superfluid 4 He at low temperatures. The density fluctuations displayed, as predicted, a continuous phonon-maxon-roton dispersion curve: a linear phonon part at low wave vectors followed by a maximum ("maxon") and then a pronounced "roton" minimum at a finite wave vector of atomic dimensions. Phonons naturally arise as the Goldstone mode associated with the continuous symmetry of the interacting system, whereas rotons are a direct consequence of strong correlations. Roton-like excitations have been proposed in cold atomic gases [4], in one-dimensional 4 He [5] and in two-dimensional fermionic systems [6]. Superfluidity emerges phenomenologically as a natural consequence of the dynamics, while the knowledge of the dispersion relation allows the calculation of low temperature thermodynamic properties of superfluid 4 He [7]. The relation between theory and experiment, however, is not straightforward [8,9]. The excitations considered by Landau, Feynman and others correspond to the singleparticle response function associated with the description of an effective vacuum -the superfluid ground stateand non-interacting quasiparticle excitations. Neutron scattering, in turn, gives access to the dynamic structure factor S(Q, ω), a quantity related to the dynamic susceptibility, i.e., the linear response ...

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