H. Godfrin

Two-dimensional nuclear magnets

Abstract We review in this paper the experimental results on two-dimensional nuclear magnetic systems obtained by physical adsorption of 3He atoms on solid substrates. Two-dimensional nuclear antiferromagnets and ferromagnets have been observed and studied in recent years, providing useful model systems for spin 1/2 magnetic Hamiltonians and leading to results of general validity on the magnetic ordering of two-dimensional spin systems. Heisenberg, multiple spin and Rudermann-Kittel exchange as well as vacancy-induced magnetism are discussed in the context of the experimental investigations of adsorbed 3He films.

Strong orientational effect of stretched aerogel on the 3 He order parameter

Deformation of aerogel strongly modifies the orientation of the order parameter of superfluid (3)He confined in aerogel. We used a radial squeezing of aerogel to keep the orbital angular momentum of the (3)He Cooper pairs in the plane perpendicular to the magnetic field. We did not find strong evidence for a polar phase, with a nodal line along the equator of the Fermi surface, predicted to occur at large radial squeezing. Instead we observed (3)He-A with a clear experimental evidence of the destruction of the long-range order by random anisotropy-the Larkin-Imry-Ma effect. In (3)He-B we observed and identified new modes of NMR, which are impossible to obtain in bulk (3)He-B. One of these modes is characterized by a repulsive interaction between magnons, which is suitable for the magnon Bose-Einstein condensation.

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.

New magnetic properties of a dilute spin‐glass

The insulating Sr1−xEuxS system exhibits spin‐glass properties for x≳0.05 due to competing ferro and antiferromagnetic interactions. A scaling law x/Tf=f (ln ν) represents the frequency (ν) dependence of Tf, the temperature of the maximum of χa.c. (or spin‐glass freezing temperature). This frequency dependence of Tf is at least an order of magnitude bigger than in a canonical spin‐glass such as AuFe. The dipolar interaction is responsible for the anisotropy inside magnetic clusters. For x<0.05 the a.c. susceptibility, χa.c., doesn’t exhibit a single maximum, but after an initial maximum or plateau around Tp it increases to a second rounded maximum, at much lower temperatures. Below Tp, which is concentration independent, a remanent magnetization appears proportional to x2. Furthermore, at this temperature deviations from a Curie law of χa.c. increase more rapidly than x. Moreover, the frequency dependence of Tp permits a determination of a blocking energy. This energy is in agreement with the anisotropy b...

Properties of sintered silver powders and their application in heat exchangers at millikelvin temperatures

Abstract We present a systematic study of the properties of sintered silver powders used in heat exchangers at millikelvin temperatures. Scanning electron microscopy and surface area (BET) measurements have been performed on various powders with different degrees of sintering. Values of the Kapitza resistance with pure He3 and He3—He4 mixtures are given and compared with previously reported results.

Ultra-Low Temperature Magnetic Properties of Liquid 3He Films

We report measurements of the nuclear magnetization of submonolayer liquid3He films adsorbed on a graphite substrate (Papyex) preplated by a monolayer of4He. In the submilliKelvin temperature range we observe a substantial enhancement of the nuclear magnetization with respect to the degenerate Fermi Liquid value. The unusual temperature dependence of this new contribution to the liquid3He film magnetization agrees well with that expected from the theory of weak disorder in two-dimensional (2D) correlated Fermion systems. The effects of disorder and reduced dimensionality suppress the superfluid transition at least to below 180 μK.

Search for supersymmetric Dark Matter with superfluid 3He (MACHe3)

Abstract MACHe3 (MAtrix of Cells of superfluid 3 He) is a project of a new detector for direct Dark Matter search, using superfluid 3 He as a sensitive medium. This Letter presents a phenomenological study done with the DarkSUSY code, in order to investigate the discovery potential of this project of detector, as well as its complementarity with existing and planned devices.

Dielectric relaxation of diluted phenols by internal rotation

Abstract Phenol, twenty-one phenol derivatives, two phenol-OD derivatives and three thiophenols all exhibit dielectric relaxations at low temperatures in dilution in hydrocarbon media. The relative relaxation rates throughout the series correlate well with published microwave and far-infrared spectroscopic data. The rates vary slowly with temperature up to about 40 K and thereafter rapidly. From the variation of the relaxation strength of 4-methyl-2,6-di-tert-butyl phenol between 3.5 mK and 750 mK the tunnel-splitting of the ground torsional state is found to take values ranging from 100 MHz to 800 MHz. At the lowest temperatures the mean relaxation rate (but not the relaxation strength) has an unexplained field dependence in remarkably small ac measuring fields.

Observation of a roton collective mode in a two-dimensional Fermi liquid

Understanding the dynamics of correlated many-body quantum systems is a challenge for modern physics. Owing to the simplicity of their Hamiltonians, 4He (bosons) and 3He (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 4He 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 3He, 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 3He, 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.

Some Structural Properties of Solid He Films: Consequences on 3He Film Ferromagnetism

Presently available experimental data on neutron diffraction, elastic properties, specific heat, high temperature susceptibility for solid helium films adsorbed on Grafoil and theoretical calculations of the energy of the fluid are used to give an estimate of first and second solid layer densities as a function of total coverage. Important information on the second layer phase diagram is deduced from this analysis. In3He films, the knowledge of the second-layer-solid density ρ2(x) as a function of total coverage x is essential to elucidate the origin of ferromagnetism for two solid films in the presence of a fluid third layer.