High Q-Fermions

This project proposes a coordinated study of elementary excitations of quasi-two-dimensional 3He up to atomic wave lengths of approx. 10 -15nm. The investigations are to be carried out simultaneously by an experimental team at CNRS/ILL in Grenoble, and by the many-particle theory group in Linz. In this proposal, we address the problem of the dynamics of fermionic systems at elevated wave-vectors, i.e., beyond the "grey zone" dominated by Landau damping. This is not done as an end in itself, but rather because we see, in this momentum regime, evidence for new excitation mechanisms that require a revision of some of the common terms in which the dynamics of strongly interacting Fermi systems is described. The proposed research at elevated wave numbers has the promise of pro- viding new information on the nature of dynamic interparticle correlations. Our investigations have been initiated by pilot neutron scattering experi- ments that show evidence of the appearance of a collective mode beyond the particle-hole continuum. While the observation per se is new and unantic- ipated, it has serious consequences for the paradigms in terms of which we have learned to describe interacting Fermi fluids. It is customary to discuss low-energy excitations in Fermi fluids in terms of "quasi-particles", and many-body effects are primarily seen in a modification of quasi-particle properties (such as an effective mass). We shall present evi- dence that, in order to have a reasonably consistent picture, it is unavoidable to introduce the concept of pair fluctuations which are intermediate states that can not be characterized by the quantum numbers of a single (quasi- )particle. We are proposing a new view of some of the elementary excitations mech- anisms in Fermi fluids, it is therefore important to proceed carefully and systematically. The theoretical investigations will combine diagrammatic ex- pansions with correlated, non-orthogonal wave functions, which will provide a consistent picture of collective, single-particle, and multi-particle excitations.

The goal of microscopic quantum many-body theory is the quantitative prediction of properties of strongly interacting many-particle systems from no other information than the underlying microscopic input: particle number, temperature, statistics, and an empirical manybody Hamiltonian that incorporates the state-of-the-art knowledge of the particle interactions. The past decades have seen enormous progress in that area in the sense that the ground state properties of the most strongly interacting quantum fluids, e. g. the helium liquids, can be predicted with high accuracy. The situation has for decades been much less satisfactory for dynamic properties.The work of this project has closed this gap to a large extent once and for all and has brought the theoretical understanding of the dynamics of highly correlated many-body systems to the same quantitative level that has been reached some time ago for the static ground state.Our work has contained both analytic and numerical components. The method of choice was the generalization of the theory of correlated wave functions which has been most successful in describing ground state properties. To describe dynamic situations, all many-body correlations had to be generalized to be time dependent, and the theory has be executed with no more approximations than those that were made for the ground state theory. Executing the same theory for fermions has been significantly more difficult, but, just because of the greater difficulty, significantly more rewarding.We have been in the fortunate situation that a new facility, the time-of- flight spectrometer IN5 at the Institut Laue Langevin high flux reactor on the has become available. As a consequence we were able to compare theoretical predictions with very precise neutron scattering data. We found that our theory faithfully reproduced even the finer structures of the data.We have also been able to make several methodological statements: Among others, we could make point that neutron scattering data on mono-layers of 3He are manifestly contradictory to common wisdom theoretical descriptions. We have demonstrated that our approach provided an understanding of those experiments.