Elementary excitations of highly correlated fermions at atomic wavevectors: Experiments and Theory – HighQ-Fermions

The objective of the experimental part of this proposal is to measure the excitation spectrum of liquid 3He (bulk and films) for several densities, in the wave-vector region around 1 Å-1 (atomic wave-lengths). The corresponding energy is on the order of 1 meV. Technically, this means measuring with inelastic neutron scattering the dynamical structure factor, a magnitude simply related to the dynamical susceptibility of the system. These measurements will show the response of the system: - a) at low wave-vectors (the quantum hydrodynamic region, but in a regime not attainable by low frequency techniques like ultrasound), - b) at intermediate wave-vectors (the zero sound mode is Landau damped by the particle-hole excitations), - c) at high wave-vectors

HIGHLIGHTS

- Observation of a roton collective mode in a two-dimensional Fermi liquid (Nature, 2012).
- Observation of Roton-Phonon repulsive Interactions in Superfluid 4He (PRL, 2012)

We expect to obtain a complete description of the dynamics of bulk 3He and 4He, as well as 2D 3He, in a large density, Energy and k range. This will be done in liaison with the theoretical work of our austrian partner in this ANR (Linz)

• Two-dimensional Fermi liquids sustain surprising roton-like plasmons beyond the particle-hole band.
A Sultan, H Godfrin, M Meschke, H-J Lauter, H Schober, H Böhm, R Holler, E Krotscheck and M Panholzer
Journal of Physics: Conference Series 340, 012078 (2012)

• Observation of a roton collective mode in a two-dimensional Fermi liquid
Henri Godfrin, Matthias Meschke, Hans-Jochen Lauter, Ahmad Sultan, Helga M. Böhm, Eckhard Krotscheck and Martin Panholzer
Nature 483, 576–579 (29 March 2012), doi:10.1038/nature10919

• Static structure factor of two-dimensional liquid 3He adsorbed on graphite
A. Sultan, M. Meschke, H.-J. Lauter and H. Godfrin
J. of Low Temp. Phys. DOI: 10.1007/s10909-012-0649-9 (on-line 3/07/2012)

• Roton-Phonon Interactions in Superfluid 4He
B. Fåk, T. Keller, M. E. Zhitomirsky, and A.L. Chernyshev, Physical Review Letters (Accepted August 17, 2012, in press).

One of the major goals of present physics is understanding the quantum properties of interacting many-body systems. In this proposal, we address for the first time the problem of the dynamics of correlated fermionic systems at elevated wave-vectors, in order to understand the nature and the interplay of incoherent (particle-hole) and coherent (plasmon, zero sound) excitations. The results apply to a large variety of systems: electrons in metals, quantum fluids, neutron stars.

Liquid 3He is an excellent system for such studies, since its Fermi surface is spherical and a large range of densities (interactions) can be studied. In addition, two-dimensional 3He can be studied from the low density limit (Fermi gas) to a highly correlated Fermi liquid state. In this system, the interaction between incoherent particle-hole excitations and the collective zero sound mode can be tuned by changing the density. Under these optimal conditions, we shall investigate the excitations of a Fermi liquid in the high wave-vector sector of the spectrum.

Advanced experimental techniques will be used to measure, by inelastic neutron scattering at very low temperatures, the dynamic structure factor of two- and three-dimensional liquid 3He. This magnitude provides all the necessay information on the elementary excitations of the system.
Very low temperature nuclear magnetic resonance techniques will provide important additional information on other fundamental properties (effective mass, magnetic Landau parameters) as a function of density for bulk and adsorbed liquid 3He.

The proposed experiments will be performed in Grenoble by experts in the field of quantum fluids, low temperature and neutron scattering physics and techniques (Institut Néel -CNRS, and INAC-CEA), using the best neutron and cryogenic instruments for that purpose.

The Austrian partners are developing a microscopic theory that provides a new framework for understanding the dynamics of such strongly correlated Fermions. The theoretical description, based on the variational/equation of motion method, will be developed to a much higher level of accuracy than what has been achieved before, including in particular the important effects of exchanges, spin fluctuations, and a non-trivial single particle spectrum. The theory partner has a distinguished record in producing the most successful theories in liaison with the experiments.

With the experiments and theory proposed here, developed in close collaboration, we expect to answer simple, yet fundamental questions: What is the nature of the dynamics of a Fermi liquid? Do coherent excitations survive at elevated wave-vectors and energies in a Fermi liquid?