Static structure factor of two-dimensional liquid 3He adsorbed on graphite

International audienceLiquid 3He is a model system for strongly correlated Fermi liquids. For this reason, many X-ray and neutron scattering experiments have been performed to understand the structure and dynamics of this quantum fluid. We have recently shown that two-dimensional liquid 3He sustains long-lived zero-sound excitations at large wave-vectors (Nature 483, 576, 2012). Here we show that its static structure factor can be obtained with reasonable accuracy by integrating the experimental S(Q,ω) over a suitable energy range. A good agreement is found between the static structure factor deduced from the experiment and theoretical models: Quantum Monte Carlo simulations and Dynamical Many Body Theory (DMBT). At high wave-vectors, the experimental values are underestimated because of the limited accessible phase space; nevertheless, even at atomic wave-vectors a semiquantitative agreement is observed with the theoretical predictions

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)

Pair excitations and vertex corrections in Fermi fluids and the dynamic structure function of two-dimensional 3He

International audienceWe use the equations-of-motion approach for time-dependent pair correlations in strongly interacting Fermiliquidsto develop a theory of the excitation spectrum and the single-particle self energy in such systems. We present here the fully correlated équations and their approximate solutions for3He. Our theory has the following properties: It reduces to both, i) the "correlated" random-phase approximation (RPA) for strongly interacting fermions if the two-particle-two-hole correlations are ignored, and, ii) to the correlated Brillouin-Wigner perturbation theory for boson quantum fluids in the appropriate limit. iii) It preserves the two firstenergy-weighted sum rules,and systematically improves upon higher ones. iv) A familiar problem of the standard RPA is that it predicts a roton energy that lies more than a factor of two higher than what is found in experiments. A popular cure for this is to introduce an effective mass in the Lindhard function. No such ad-hoc assumption is invoked in our work. We demonstrate that the inclusion of correlated pair-excitations improves the dispersion relation significantly. Finally, a novel form of the density response function is derived that arises from vertex corrections in the proper polarization

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

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

Non-Standard Errors

In statistics, samples are drawn from a population in a data-generating process (DGP). Standard errors measure the uncertainty in estimates of population parameters. In science, evidence is generated to test hypotheses in an evidence-generating process (EGP). We claim that EGP variation across researchers adds uncertainty: Non-standard errors (NSEs). We study NSEs by letting 164 teams test the same hypotheses on the same data. NSEs turn out to be sizable, but smaller for better reproducible or higher rated research. Adding peer-review stages reduces NSEs. We further find that this type of uncertainty is underestimated by participants

4He films on graphite studied by neutron scattering

The properties of 4He films adsorbed on graphite have been studied by neutron scattering. In particular excitations of the commensurate phase of the monolayer are discussed. The first two adsorbed layers are solid and the next ones stay liquid. At the boundaries of the superfluid film excitations could be studied. Also the phonons, maxon and rotons of the film are investigated. An explanation of the lower density of the very thin films compared to bulk 4He is given

Magnetic flux distribution inside an YBa2Cu3O7-d superconducting thin film in the mixed state

The study with polarized neutron reflection showed the spatially resolved nuclear and magnetic structure in a high-temperature superconducting (HT)film of YBa2Cu307-d in the mixed state in an external field parallel to its surfam. The obtajned magnetic flux profile is composed of the penetrating Meissner field and the flux-line rows. The flux-line rows show good linear arrangements. A local critical magnetization inside the HT film was determined. The total magnetization exhibits flux-line row transitions and increases in average with increasing external field due to the surface barrier and the flux-line interaction

Phases and Phase Transitions of D2 on Graphite obtained by variable 4He Pressure

Phases and phase transitions of D2 on graphite have been investigated using 4He as 2-dimensional pushing-gas. A quite unusual path in the D2 phase diagram is followed when the 2-dimensional D2-lattice is compressed due to the 4He adsorption as the temperature is lowered: starting in the commensurate phase, passing through the domain wall liquid phase, ending in the striped superheavy domain wall phase