Nuclear magnetism of sub-monolayer solid He on graphite

Nuclear magnetization has been measured with NMR down to temperatures around 0.1 mK for sub-monolayer solid ³He formed on graphite preplated with two layers of HD. The data for the commensurate phase show no evidence of a magnetic transition and a rather slow increase of the magnetization even at ultra-low temperature (0.15 mK), much lower than the antiferromagnetic exchange interaction of −6.3 mK.     

Binding of a 3He impurity to a screw dislocation in solid 4He

Using first-principles simulations for the probability density of finding a 3He atom in the vicinity of the screw dislocation in solid 4He, we determine the binding energy to the dislocation nucleus E(B)=0.8+/-0.1 K and the density of localized states at larger distances. The specific heat due to 3He features a peak similar to the one observed in recent experiments, and our model can also account for the observed increase in shear modulus at low temperature. We further discuss the role of 3He in the picture of superfluid defects.     

Selective adsorption of 3He and 4He on the basal plane surface of graphite

Atomic beam scattering was used to measure the binding energies of selectively adsorbed states of two isotopes of helium on the basal plane of graphite. The results are used to determine semi-empirical potential parameters, and the ground state energies are compared with values derived from adsorption studies for both isotopes.     

Relaxation of spin polarized 3He by magnetized ferromagnetic contaminants

In the first in a series of three papers on wall relaxation of spin polarized ³He we have reported on a breakdown of relaxation times which is observed after exposing the ³He containing glass cells to a strong magnetizing field. In this third paper we give a quantitative analysis of this phenomenon, based on magnetic signal detection by means of SQUIDs, on the pressure dependence of relaxation times in magnetized cells, as well as on Monte Carlo simulations of ³He-relaxation in a macroscopic dipole field. Our analysis allows to identify the contaminants as being aggregates of dust-like Fe₃O₄ particles (magnetite) with a radius R ? 10 mR \approx 10~\mu m and a remanent magnetic moment of the order of m ≈O(10 ^-10 ^{-10}~ A m²). The particles are located at or close to the inner glass surface.     

Low-temperature magnetization of submonolayer 3He adsorbed on HD preplated graphite

NMR studies of submonolayer 3He adsorbed on a bilayer of HD preplated graphite have been made down to 100 &mgr;K, which is more than 1 order of magnitude smaller than the exchange energy ( J). In the highly frustrated antiferromagnetic solid region, the magnetization obeys a Curie-Weiss law even at temperatures around J and then increases gradually down to 100 &mgr;K. Nevertheless, it does not show any anomalous behavior corresponding to a spin gap. The normalized magnetization versus the reduced temperature ( T/J) is independent of the density just after solidification. This is consistent with the result in the high-temperature region, that the main multiple-spin exchanges have a similar density dependence.     

Model of quantum criticality in He3 bilayers adsorbed on graphite

Recent experiments on He3 bilayers adsorbed on graphite have shown striking quantum critical properties at the point where the first layer localizes. We model this system with the Anderson lattice plus interlayer Coulomb repulsion in two dimensions. Assuming that quantum critical fluctuations come from a vanishing of the effective hybridization, we can reproduce several features of the system, including the apparent occurrence of two quantum critical points, the variation of the effective mass and coherence temperature with coverage.     

Equation of state of solid 3He

We present results of diffusion Monte Carlo calculations for the bcc and hcp phases of solid 3He, using a recent ab initio interatomic potential, including two- and three-body terms. This potential is found to yield an equation of state for condensed 4He in excellent agreement with experiment, in a wide density range. For 3He, we find a systematic discrepancy, worth 0.7 K, between our computed equation of state and a commonly accepted experimental one. We attribute such a discrepancy to an improper choice of reference energy in the determination of the experimental equation of state.     

Josephson effect through a ferromagnetic layer

We report transport measurements on Superconductor/Ferromagnet/Superconductor (S/F/S) junctions: Nb/Al/Gd/Al/Nb where gadolinium (Gd) is a weakly polarized ferromagnet. A sizeable critical current I _c is observed in the I(V) characteristics. This current can be modulated by a weak magnetic field, as expected for a Josephson current. With these experiments, we establish that superconducting coherent transport survives across a small ferromagnetic layer. The penetration depth of Cooper pairs in Gd has been measured. An extensive study of the Josephson critical current in temperature for different thicknesses of magnetic compounds is presented. A comparison of transport measurements with S/N/S junction is given through measurements made on Nb/Al/Y/Al/Nb, where yttrium (Y) is used as non magnetic rare earth metal. Received 20 September 2000 and Received in final form 22 February 2001     

Texture of superfluid 3He probed by a Wigner solid

The resistivity of the Wigner solid floating on the free surface of superfluid 3He has been measured in both the A and B phases down to 200 microK in the magnetic field. The resistivity in the A phase shows the asymptotic behavior to the temperature-square dependence at low temperatures. The temperature dependence is successfully explained by the uniform l[over] texture oriented normal to the surface and by the specular scattering of quasiparticles excited along l[over] vector. In the B phase, the resistivity exhibits the exponential decrease at low temperatures. The steep increase of the resistivity observed at high magnetic field is attributable to the nonuniform texture of the field-distorted order parameter induced near the surface.     

Muon spin relaxation in solid3He

Muon spin relaxation in solid³He depends non-monotonously on the temperature. It is shown that this is entirely due to the magnetic dipole-dipole interactions. The observed line narrowing cannot be explained by the mechanism of positive charge hopping described by an Arrhenius-like law. The relaxation rate at low temperatures was found to increase under the influence of an external electric field.