Pair Excitations and Vertex Corrections in Fermi Fluids

Based on an equations--of--motion approach for time--dependent pair correlations in strongly interacting Fermi liquids, we have developed a theory for describing the excitation spectrum of these systems. Compared to the known ``correlated'' random--phase approximation (CRPA), our approach has the following properties: i) The CRPA is reproduced when pair fluctuations are neglected. ii) The first two energy--weighted sumrules are fulfilled implying a correct static structure. iii) No ad--hoc assumptions for the effective mass are needed to reproduce the experimental dispersion of the roton in 3He. iv) The density response function displays a novel form, arising from vertex corrections in the proper polarisation. Our theory is presented here with special emphasis on this latter point. We have also extended the approach to the single particle self-energy and included pair fluctuations in the same way. The theory provides a diagrammatic superset of the familiar GW approximation. It aims at a consistent calculation of single particle excitations with an accuracy that has previously only been achieved for impurities in Bose liquids.Comment: to be published in: JLTP (2007) Proc. Int. Symp. QFS2006, 1-6 Aug. 2006, Kyoto, Japa

Observation of zero-sound at atomic wave-vectors in a monolayer of liquid 3He

International audienceThe elementary excitations of a strongly interacting two-dimensional Fermi liquid have been investigated by inelastic neutron scattering in an experimental model system: a monolayer of liquid3He adsorbed on graphite preplated by a monolayer of solid 4He. We observed for the first time the particle-hole excitations characterizing the Fermi liquid state of two-dimensional liquid 3He, and we were also able to identify the highly interesting zero-sound collective mode above a particle-hole band. Contrarily to bulk 3He, at low wave-vectors this mode lies very close to the particle-hole band. At intermediate wave-vectors, the collective mode enters the particle-hole band, where it is strongly broadened by Landau damping. At high wave-vectors, where the Landau theory is not applicable, the zero-sound collective mode reappears beyond the particle hole band as a well defined excitation, with a dispersion relation quite similar to that of superfluid 4He. This spectacular effect is observed for the first time in a Fermi liquid (including plasmons excitations in electronic systems)

Two-Dimensional 3He: A Crucial System for Understanding Fermion Dynamics

International audienceNeutron scattering measurements at the Institut Laue-Langevin off quasi-twodimensional 3He have shown, for the first time, a situation where the collective mode crosses the particle-hole continuum and reappears, at a momentum transfer of q≈ 1.55 ˚A−1 as a well-defined collective excitation. The effect is well described by the Fermion generalization of multi-particle fluctuation theory of Jackson, Feenberg, and Campbell that has been so successful for bosonic quantum fluids. We describe the theory briefly and state that it can be mapped onto the form of time dépendent Hartree-Fock theory (TDHF)containing energy dépendent effective interactions; these are obtained from microscopic ground state theory. Our theoretical result has far-reaching consequences: a popular paradigm in discussing the density-density response function of Fermi systems is the "random phase approximation" (RPA), most frequently applied with some static interaction and, perhaps, some effective mass. Such a "phenomenologically modified" RPA can be justified only under severe simplifying approximations and is unable to describe the experimental situation consistently. As soon as one goes beyond the RPA, intermediate states which cannot be described in terms of the quantum numbers of a single (quasi-)particle become essential for capturing the correct physics. In oder to understand the above mentioned experiment, their appropriate inclusion, as presented in this work, is essential

Reemergence of the collective mode in 3He and electron layers

International audienceNeutron scattering experiments on a3Helayer ongraphite show an unexpected behavior of the collective mode. After having been broadened by Landau damping at intermediate wave vectors, the phonon-roton mode resharpens at large wave vectors and even emerges from the particle-hole continuum at low energies. The measured spectra cannot be explained by a random phase approximation with any static interaction. We show here that the data are well described if dynamic two-pair fluctuations are accounted for. We predict similar effects for electron layers

Roton collective mode observed in a two-dimensional Fermi liquid

International audienceUnderstanding 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

Two-dimensional Fermi liquids sustain surprising roton-like plasmons beyond the particle-hole band

International audienceUsing inelastic neutron scattering, we have investigated the elementary excitations of an isotropic two-dimensional Fermi liquid, 3He adsorbed on graphite. We provide in this article a detailed account of the principles and methods which allowed measuring for the first time inelastic spectra on a liquid monolayer of 3He, a strong neutron absorber. We also summarise the results presented at this Conference, and review our recent experimental and theoretical work on this this interacting many-body system. At low wave-vectors, near the edge of the particle-hole band, a mode identified as the zero-sound excitation by comparison to our theoretical calculations, is found as predicted at energies much lower than in bulk 3He. The mode enters the particle-hole band, where it undergoes Landau damping. Surprisingly, however, intensity is observed in the neutron spectra at wave-vectors larger than twice the Fermi wavevector. This new branch is interpreted as the high wave-vector continuation of the zero-sound mode, in agreement with the theory. The results open new perspectives in the understanding of the dynamics of correlated fermions

RHAPSODY - Internet-based support for caregivers of people with young onset dementia: program design and methods of a pilot study

YesBackground: Young Onset Dementia (YOD), defined by first symptoms of cognitive or behavioral decline occurring before the age of 65 years, is relatively rare compared to dementia of later onset, but it is associated with diagnostic difficulty and heavy burden on affected individuals and their informal carers. Existing health and social care structures rarely meet the needs of YOD patients. Internet-based interventions are a novel format of delivering health-related education, counseling and support to this vulnerable yet underserved group. Methods: The RHAPSODY (Research to Assess Policies and Strategies for Dementia in the Young) project is a European initiative to improve care for people with YOD by providing an internet-based information and skill-building program for family carers. The e-learning program focuses on managing problem behaviors, dealing with role change, obtaining support and looking after oneself. It will be evaluated in a pilot study in three countries using a randomized unblinded design with a wait-list control group. Participants will be informal carers of people with dementia in Alzheimer’s disease or behavioral-variant Frontotemporal degeneration with an onset before the age of 65 years. The primary outcome will be caregiving self-efficacy after 6 weeks of program use. As secondary outcomes caregivers’ stress and burden, carer health-related quality of life, caring-related knowledge, patient problem behaviors and user satisfaction will be assessed. Program utilization will be monitored and a health-economic evaluation will also be performed. Conclusions: The RHAPSODY project will add to the evidence on the potential and limitations of a conveniently accessible, user-friendly and comprehensive internet-based intervention as an alternative for traditional forms of counseling and support in healthcare, aiming to optimize care and support for people with YOD and their informal caregivers.RHAPSODY is an EU Joint Program - Neurodegenerative Disease Research (JPND) project. The project is supported through the following funding organizations under the aegis of JPND (www.jpnd.eu). France: National Research Agency; Germany: Ministry of Education and Research; The Netherlands: The Netherlands Organization for Health Research and Development; Portugal: Foundation for Science and Technology; Sweden: The Swedish Research Council; United Kingdom: Economic and Social Research Council