Layer Of 3He Research Articles

Frustration by multiple spin exchange in 2D solid3He films

The heat capacity and the magnetization of the second solid layer of3He adsorbed on graphite have been measured to temperatures below 1mK. Prior to the formation of a fluid overlayer the temperature dependence of both sets of data is well described by high temperature series expansions for a 2D Heisenberg antiferromagnet on a triangular lattice. Differing exchange constants from the two quantities provide a measure of frustration due to multiple spin exchange. The subsequent evolution to ferromagnetic exchange suggests that this arises from variations in the coverage dependence of the frustration. After promotion it is found that an antiferromagnetic heat capacity coexists with a ferromagnetic susceptibility. At the ferromagnetic anomaly, the heat capacity is well described by series expansions for a 2D Heisenberg ferromagnet with effective exchange constant close to that determined from the magnetisation, indicating weak frustration.

Thin3He films on a disordered substrate: A Metal-Insulator transition

The heat capacity of thin3He films and of liquid3He underpressure up to 28 bar in porous Vycor glass has been measured at temperatures 10–600 mK. An evolution from solid (Insulator) to liquid (Metal) behaviour has been observed with increasing the film thickness from one to three atomic layers. At low temperatures the second layer has a temperature independent specific heat. Using a model of “glassy” second layer of3He on a rough substrate, that has a broad uniform distribution of the logarithm of nuclear exchange interaction, we calculate its specific heat and susceptibility and compare them with the available experimental data.

Weak anomaly in the nuclear magnetic susceptibility of two dimensional fluid 3He films adsorbed on graphite

The nuclear magnetic susceptibility of a single atomic layer of fluid 3He adsorbed on the surface of graphite under various preplating conditions has been measured to below 1mK. A small but clear anomaly has been seen, corresponding to a local maximum in the susceptibility near 3mK. No large frequency shift or change in linewidth suggestive of a p-wave superfluid transition has been observed. The observed susceptibility enhancements over that of an ideal Fermi gas indicate that the quasiparticle interactions do not vary with the preplatings studied thus far.

Fermi liquid parameters of a 2D 3He film

Abstract A temperature independent magnetic susceptibility has been observed for the second layer of 3He on graphite for second layer surface densities less than 0.055 A-2, consistent with 2D Fermi liquid behaviour. The Landau parameter Foa is determined using known values of m∗/m. The relative dependence of these two parameters is in good agreement with almost localised Fermion theory, as is the case in bulk liquid 3He.

Spin heat capacity of the second solid layer of 3He adsorbed on graphite

Precise heat capacity data have been obtained for multilayers of 3 He adsorbed on graphite in the temperature range between 2 and 200 mK. As a function of total coverage, these data show an accurate correspondence with special features in recent magnetization measurements but suggest models very different from those proposed to explain the magnetization data alone. The heat capacity results indicate that the ferromagnetic anomaly observed in the magnetization measurements, occurs in the second layer when this layer exists in a state intermediate between a registered and an incommensurate solid.

Very low field susceptibility of the boundary layers of 3He on grafoil

Very low field susceptibility of the boundary layers of 3He on grafoil

Very low field susceptibility of the boundary layers of 3He on grafoil

Abstract The low-field NMR spectra of 3He boundary layers on graphite show collective modes for T

Spin heat capacity of the second solid layer of 3He adsorbed on graphite

Precise heat capacity data have been obtained for multilayers of 3 He adsorbed on graphite in the temperature range between 2 and 200 mK. As a function of total coverage, these data show an accurate correspondence with special features in recent magnetization measurements but suggest models very different from those proposed to explain the magnetization data alone. The heat capacity results indicate that the ferromagnetic anomaly observed in the magnetization measurements, occurs in the second layer when this layer exists in a state intermediate between a registered and an incommensurate solid.

Ferromagnetism in the Boundary Layer of 3He

The boundary layer of 3He shows a ferromagnetic tendency on many substrates. Recent work at USC and Grenoble has shown both that this layer on graphite undergoes a ferromagnetic ordering and that this magnetism depends sensitively on the structure of the boundary layer. We use a dc SQUID based NMR spectrometer to perform very low field measurements. With T ∼ 1 mK and the applied field under a few gauss, we observe a multiple line spectrum with lines displaced from the Larmor frequency. We interpret these displaced lines as the collective modes of ferromagnetically ordered domains in the 3He surface layers. The data suggests that these domains reorient as the applied field is increased. We conclude that the 3He dipole field can only partially account for the anisotropy energy necessary for the observed line shifts.

Three-Particle Exchange: A Possible Interpretation of Ferromagnetism in Adsorbed 3He

The recent observation of the onset of ferromagnetism for 2.2 3He layers adsorbed on grafoil with a maximum effect for 2.5 layers is interpreted as a consequence of three particle exchange.

Magnetic Properties of Surface Layers of Helium-3 and Methane on Porous Silicon

Porous silicon has been used as a new homogeneous substrate system to study magnetic surface behavior of nuclear spin coupling and dynamic effects at low temperatures. Spin-lattice relaxation time T1 measurements of methane (1H spin) physisorbed on porous silicon have been studied as a function of temperature and coverage. The temperature dependence of T1 in the range of 4-16 K displays a double minima feature similar to previous observations of methane coverages on graphite systems. However, for high coverages of methane on clean porous silicon, the T1 values are in the neighborhood of tens of minutes at 4.0 K, while those of the methane-graphite system are considerably shorter. Preliminary measurements of 3He layers on porous silicon have also been done.

Comments on Surface-Induced Magnetism of Liquid 3He in Restricted Geometries

Mechanisms for the surface-induced magnetizations in liquid 3He confined in restricted geometries are studied. Pressure dependence of the effective Curie constants in the observed Curie-Weiss-law-like susceptibilities is well understood within a simple model including paramagnon effects in liquid 3He. The observed magnetization can be regarded as a giant magnetic moment due to the paramagnon effects in liquid 3He induced by localized spins in the solid 3He layer adsorbed on surfaces of substrates of the sample cell.

Magnetic structure of vacancies and bi-vacancies in 2D 3He layers

Abstract The effect of large elastic interaction between vacancies in 2D 3He layers on their magnetic structure is studied. When the vacancies are on nearest neighbour sites, they induce ferromagnetism and ferromagnetic polarons at finite temperature. In the other cases, they propagate along one-dimensional paths and have no magnetic effect. For smaller interaction, i.e. larger separation, vacancies induce a non ferromagnetic coupling, but weaker than in the case of isolated vacancies, and form non spherical polarons.

Model nuclear relaxation time of inhomogeneous fluids near a substrate: Liquid 3He in confined geometries or physisorbed molecular systems

We extend a previous analysis for the temperature dependence of the longitudinal nuclear spin relaxation time T 1of pure liquid 3He in confined geometries to the case where the various walls are coated with a film of 4He. This leads us to consider relaxation not only of 3He spins on the walls as we did earlier, but also relaxation by coupling between the different layers of 3He of inhomogeneous structure. We characterize the inhomogeneous 3He as a sequence of layers of different densities and thus different static susceptibilities and different fluctuation frequencies. We suggest that the same idea might help to understand the behavior of T 1(T) for adsorbed molecular fluids of BET type III, such as NH3, where one adsorbed monolayer grows into islands of multilayers, for which we propose a model structure, varying with temperature, whose overall T 1 bears much resemblance to that of 3He near a wall. We believe that both cases illustrate the more general behavior of T 1(T) in an inhomogeneous fluid; here the inhomogeneous density has its source in the attraction to the substrate, but any density gradient could in principle yield the same effect. Our phenomenological model is compared with existing data and further experiments are suggested.

SURFACE MAGNETIC RELAXATION - RELATION TO3He↑ EXPERIMENTS

Nuclear relaxation at the walls of experimental cells is an important lifetime limitation for polarized liquid 3He. However, maximum spin-lattice relaxation times T1 (i.e. the intrinsic values T1 bulk) can be obtained by diffusion limited relaxation or boundary limited relaxation. We present NMR measurements on liquid 3He in confined geometries, where the latter process is dominant. Coating of the walls by 2.7 layers of 3He enhances T1 and the thermal boundary resistance by almost two orders of magnitude at low temperatures.

Helium adsorption on exfoliated graphite

High precision adsorption isotherms of3He and4He on bare Grafoil and on Grafoil coated with a monolayer of argon have been measured in the temperature range 4–20 K, using a high resolution, pressure-sensitive capacitance gauge located at 4.2 K close to the sample chamber. This gauge obviated thermomolecular pressure effects and corrections. The isotherms yielded the following resultant data: The monolayer coverage Vm for4He on bare Grafoil was 0.42 cm3 (STP)/m2 and for3He on bare Grafoil was 0.395 cm3 (STP)/m2: the isosteric heat of adsorptionQst/R of the second layer of3He on bare Grafoil was 23.5 K andQst/R for3He on argon-coated Grafoil for the first layers was 47.5 K. Also, the data for3He and4He on bare Grafoil at and just aboveVm have been used by Novaco (see subsequent paper) for determination of the virial coefficients of the gas phase occuring in the early formation of the second layer.