Specific heat of amorphous3He films and confined liquid3He

We have measured the heat capacities of³He films and liquid³He in porous Vycor glass at 10 to 600 mK. With increasing the film thickness from 1 to 3 atomic layers, the specific heat evolves gradually from that typical to solid to that of liquid³He. At about 2 atomic layers, however, its low-temperature part is nearly temperature-independent; we interpret this as a result of gradual freezing of spins in an amorphous solid³He film with decreasing the temperature. The contribution of liquid³He in the center of the Vycor pores can be described as the specific heat of bulk liquid³He at corresponding pressures in the range 0 to 28 bar. The thickness of amorphous solid on the pore walls increases with external pressure roughly linearly. Preplating the walls with⁴He allows to determine the positions of³He atoms contributing to the surface specific heat at 10 to 50 mK. In addition, the contribution from the specific heat of³ He -⁴He mixing at 100 to 600 mK is discussed as a function of pressure and amount of⁴He.0n leave from ISSP Acad. Sci. of Russia, Chernogolovka, Russia     

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.Presented by H. J. Lauter.     

Determination of the thermodynamic phases of4He adsorbed on H2 plated graphite

Thermodynamic measurements have been used to investigate the surface phases of monolayer⁴He adsorbed on graphite plated with 1.45, 2.15, and 3.15 layers of H₂. In each case the⁴He preferentially adsorbed on the lower completed layer. In this paper we report the measurements of⁴He on the 1.45 layer H₂ graphite plating. For this plating the⁴He goes into a condensed phase below 0.87 K. The high temperature-low density film entropy has been calculated.     

Layer by Layer Growth of Solid 4He on Graphite down to 0.1 K

The growth of solid ⁴He on graphite from the superfluid phase is known to occur at pressures well below the bulk solidification point. The number of adsorbed layers increases with pressure and the solid growth undergoes non-continuous layer-by-layer growth at low temperature. We have studied this growth using the torsional oscillator method for isotherms down to 0.1?K. In contrast with simple layer-by-layer growth scenarios, our evidence suggests that the growth of adsorbed solid ⁴He is more complex and less solid is present on the graphite substrate at low temperature.     

High thermal conductivity diamond coated graphite by microwave plasma chemical vapour deposition

Diamond film was grown on high thermal conductivity graphite substrate using microwave plasma chemical vapour deposition method. Nanodiamond particles were uniformly seeded on the substrate to generate high nucleation density by a spray gun. The continuous and high purity diamond film was obtained, and growth rate was up to 2.7 μm h^??1. The thickness, surface morphology, quality and composite phase of the film were analysed by SEM, Raman and X-ray diffraction. It was shown that graphite coated with diamond presented a higher thermal conductivity (520?W?m^??1 k^??1) than copper. Furthermore, this coated material with high thermal conductivity, good strength and non-conductive surface will make it possible to be widely used in thermal management field.     

NMR study of the solidification and melting of3He in porous glasses

TheP-T phase diagrams of the liquid-solid phase transition of³He in three porous glasses with different pore sizes have been determined from spin-lattice relaxation measurements in the temperature range 0.5–4.2 K. The onset of solidification of³He in the pores occurs at excess pressure over the bulk phase transition. The excess pressure depends on the pore size. A model of the phase transition in small pores which takes into account the contribution of the surface energy to the free energy is described and compared with experimental results. TheT ₁ relaxation mechanism of³He in the pores is found to be due to the surface relaxation when³He is in the liquid phase and due to the relaxation of bulk solid³He when it is in the solid phase.     

Compression of3He by refluxing4He: A model for computing HEVAC effects in3He-4He mixtures

This article comprises the first part of a study concerning the effect that a flow of gaseous⁴He has on the concentration distribution of³He atoms in the presence of a super fluid film. We present a simple model in which hydrodynamic effects in the gas phase (diffusion and viscosity), and local thermodynamic equilibrium with a superfluid film are considered. Results are derived and discussed for the simple case in which a heat flow is sustained along a cylindrical tube lined with a helium film. This heat flux drives a superfluid flow in the film, and a corresponding counterflow of⁴He in the vapour. The pressure, temperature,³He concentration, and film thickness profiles along the tube are computed. Over a wide range of conditions, a dramatic reduction of the³He concentration, a large temperature increase, and a spectacular film thinning towards hotter regions are predicted to result from a heat flow. The results of a series of experiments supporting this model will be presented in a forthcoming article.Unité de recherche de l'Ecole Normale Supérieure et de l'Université Pierre et Marie Curie, associée au CNRS (URA 18).     

Vortex-Loops and Phase Nucleation in Superfluid 4He and 3He

We propose a new model for the nature of the nucleation of solid from the superfluid phases of ⁴ He and ³ He. A fast solidification event in the superfluid results in a local release of pressure and a velocity field in the superfluid. This in turn facilitates the nucleation of vortex-loops. The kinetic energy gain of this process balances the surface tension, as the solid surface is quickly covered by many vortex-loops (hairy snow-ball). We show that this scenario gives good agreement with experiments on heterogeneous nucleation.     

Superfluidity of4He films adsorbed on hydrogen plated graphite

The superfluidity of⁴He on graphite with various preplatings of HD is under investigation using a torsional oscillator. Results for a bilayer of HD follow a similar pattern to those obtained earlier on pure⁴He films and are consistent with third sound measurements on graphite plated with a bilayer of hydrogen. In our case the first⁴He layer solidifies, consistent with behaviour observed in NMR studies of³He on a similarly prepared substrate. At a coverage of 11nm^–2 there is the onset of a region in which the transition temperature increases rapidly with coverage and the transition itself becomes sharper. At certain coverages pronounced signatures have been observed in the period shift and dissipation, which may indicate a further phase transition in the film at lower temperatures.     

Nuclear magnetic susceptibility of3He adsorbed on graphite below 4.2 K

The nuclear magnetic susceptibility of³He films adsorbed on graphite substrates was measured by means of pulsed NMR techniques at 10 and 20 MHz. Submonolayer, monolayer, and multilayer coverages were examined, as well as one monolayer of³He mixed with various amounts of⁴He. The temperature range of measurements extended from 0.35 to 4.2 K. For pure³He and for³He mixed with⁴He the nuclear magnetic susceptibility displays departures from Curie's law. The observed behavior in pure³He layers can be fitted to an ideal-Fermi-gas curve with an effective degeneracy temperature of 0.15 K. The nuclear magnetic susceptibility of³He–⁴He films is also found to fall below the values predicted by Curie's law as the temperature is lowered, but the results suggest that in this case the film is not uniform, possibly as a result of a phase separation in the film at low temperatures.Work supported in part by the National Science Foundation and a Navy Equipment Loan contract.     

Heat capacity study of4He desorbing from H2-plated graphite

We report measurements of the surface binding energy of⁴He adsorbed on 1.45, 2.15, or 3.15 layers of hydrogen plating graphite. The heat capacity of partial monolayer coverages of helium was measured using adiabatic heat pulse calorimetry from 0.2 to 9 K. Desorption of low surface density⁴He films was indicated above 2 K by a broad peak in the heat capacity. A model was developed to calculate the specific heat of an ideal gas in two dimensions as it desorbs into an ideal gas in three dimensions. This model was used to obtain the binding energies from our experimental data. The binding energies per atom are –39, –25.8, and –22 ± 0.5 K, respectively, for the three hydrogen platings. The 2-d compressibility of the partial solid second layer of H₂ in the 1.45 layer H₂ coverage was calculated from an increase in its melting temperature with increase of⁴He 2-d pressure.     

Dissipation in adsorbed submonolayer helium films and two dimensional melting

The mechanical dissipation of a submonolayer helium film adsorbed on the surface of graphite has been studied as a function of coverage and temperature through the two dimensional melting transition. The measurements were made with a high Q torsional oscillator over the temperature range 1.5 to 5K on both⁴He and³He films. The largest dissipation was observed in the incommensurate solid phase and increased with decreasing temperature and increasing surface density. Plots of dissipation vs. temperature break away from a common curve at the melting temperature defined by the observed maxima in the film heat capacity. This may provide evidence for continuous melting of the two dimensional solid. It is believed that the dissipation arises not from interfacial friction between the adsorbate and substrate but rather from internal friction in the solid crystallites. The effect of surface morphology has been investigated by using Grafoil and UCAR ZYX as substrates.     

Vortex-Loops and Solid Nucleation in Superfluid 4He and 3He

We propose a new model for the nature of the nucleation of solid from the superfluid phases of ⁴He and ³He. Unique to the superfluid phases the solid nucleation involves an extremely fast solidification front. We depart from the usual quasi-static treatment of solid nucleation by proposing that the nucleation of a solid seed is helped by the simultaneous nucleation of vortex-loops in the superfluid around it. It is the composite entity which is nucleated out of the over-pressurized liquid. This occurs when the local release of pressure creates a velocity field in the superfluid which in turn facilitates the nucleation of vortex-loops. The kinetic energy gain of this process balances the surface tension, as the solid surface is quickly covered by many vortex-loops (hairy snow-ball). We show that this scenario gives good agreement with many experiments on heterogeneous nucleation, where the energy barrier is found to differ with the classical theory of homogeneous nucleation by 8 orders of magnitude. We propose several experiments that could show the involvement of vortices with solid nucleation.     

Solidification of helium in confined geometries

Solidification and melting of helium in confined geometries has been studied in a series of in situ pressure measurements, using both³He and⁴He, in porous glass with pore sizes from 12 to 191 Å in radius. These measurements covered the temperature range from 0.04 to 2 K and pressures up to 6 M Pa. In³He, a pressure minimum is observed in the pores at the same temperature as for the bulk. At large pore sizes the elevation in pressure in³He is inversely proportional to the pore radius as predicted by the model based on homogeneous nucleation of solid as a result of density fluctuations. However, the magnitude of the pressure elevation does not agree with this model. In⁴He, comparison with the model is complicated by the strong temperaturedependence of the pressure elevation.     

Observation of Zero-Sound at Atomic Wave-Vectors in a Monolayer of Liquid 3He

The 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 liquid ³He 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 plasmon excitations in electronic systems).     

Calculation of third-sound velocities for superfluid4He films with3He impurities

The velocities of third sound in superfluid⁴He films containing submonolayer densities of³He are calculated from the variation of film height with the concentration of³He. The present model assumes that the population of the first-excited, transverse, surface state of the³He impurities directly increases the height of the film because these atoms are preferentially found within the film rather than on the surface. The relative populations of the ground-state and the first excited-state are calculated from a many-body, mean-field theory which takes account of the first two transverse branches of the spectra of the states. Intermolecular and one-ripplon exchange interactions are included. Graphs of the variation of third-sound velocity with³He density are given.     

Imaging the edge of a 4He prewetting film on a cesium metal surface

The edge of a metastable ⁴ He prewetting film on a cesium metal surface is investigated using polarization interference microscopy. This technique images the local gradient of the coverage through its optical thickness. The cesium metal is a film evaporated on a polished copper substrate. A liquid helium film is deposited on the surface through raising and lowering the bulk liquid surface. Its edge is clearly observed for temperatures below 1.6 K. The apparent optical thicknesses of the films, larger than what is expected for a normal saturated film, remain to be explained.     

Nuclear magnetic relaxation of3He gas. II.3He-4He mixtures

Longitudinal relaxation timesT ₁ have been measured in³He-⁴He gas mixtures, using pulsed NMR, in the temperature range 0.6–15 K. Helium-3 number densities of the order of 10²⁴ atoms m^–3 were used. Relaxation takes place on or near the walls of the Pyrex sample cells and measurements ofT ₁ give information about the surface phases. A cryogenic precoating of solid molecular hydrogen was used to reduce the helium-substrate binding energy from 100 K on Pyrex to 13 K for³He and 15 K for⁴He. TheT ₁ data at high temperatures were similar to those observed previously in the pure³He-H₂ system. The presence of⁴He generally causedT ₁ to rise on cooling below 2 K due to the preferential adsorption of⁴He over³He at the surface. However,³He atoms that go into quasiparticle states in the superfluid helium film can be an extra source of relaxation. In uncleaned cells, relaxation probably takes place in quasiparticle states at the free surface of the superfluid film, which are bound with an energy of 5.1±0.3 K. Baking the Pyrex cells under vacuum and rf discharge cleaning the walls before sealing in the sample gas were found to increase the bulk gasT ₁ by two or three orders of magnitude. In a cleaned, sealed cell aT ₁ of 8 h was measured at 7.7 MHz and 0.8 K. In this case relaxation is probably occurring two or three helium layers away from the helium-hydrogen interface. It may be possible to observe a predicted minimum in the intrinsic dipolarT ₁ of the bulk gas by using a⁴He wall coating to suppress wall relaxation effects (which usually dominate the nuclear relaxation of the bulk gas).     

2D liquid3He near solidification: a highly correlated Fermi liquid

The magnetic susceptibility of two-dimensional liquid³He adsorbed on graphite has been measured by NMR techniques for submonolayer and second layer coverages in a large temperature range around the Fermi temperature. The susceptibility enhancement factor determined in the vicinity of second layer solidification is larger than that found in the bulk liquid at high pressures. The results are discussed in the framework of the quasi-localized and the paramagnon models of liquid³He.     

Solidification of the third helium layer on graphite

No Heading We report heat capacity and magnetization measurements of ³He adsorbed on graphite pre-plated by a solid bilayer of ⁴He. The first ³He layer adsorbed on this composite substrate remains fluid up to and beyond layer promotion. However it is observed to solidify significantly before the second ³He layer is completed. Remarkably, at fluid coverages on the verge of solidification, we observe a distinctive heat capacity and magnetisation maximum, which occurs at progressively lower temperatures as the coverage is increased. Possible magnetic instabilities in the fluid and analogues with heavy fermion systems are discussed.PACS numbers: 67.70 n+n, 67.80. Jd