Prediction of reentrant wetting of3He-4He mixtures on cesium

We examine the effect of ³ He impurities on the wetting behavior of⁴He on cesium, predicting a phase diagram which includes reentrant wetting transitions. This phase diagram is shown to be very sensitive to effects such as a theoretically predicted bound state of³He at the liquid-cesium interface, and the contact angle may be sensitive to interesting temperature dependences of the helium-cesium surface tension resulting from surface rotons or Rayleigh waves.     

The solid-liquid surface tension at a helium/cesium interface

The existence of a wetting transition for helium adsorbed on cesium provides an experimental opportunity to investigate the solid-liquid surface tension. We have extracted the temperature dependence of the solid-liquid surface tension at the⁴He/Cs interface from our earlier measurements of the wetting temperature of pure⁴He on thin layers of cesium overlaying gold. More recently we have measured the wetting temperature as a function of the³He concentration for³He-⁴He mixtures. An analysis of these experiments paralleling Andreev's investigation of the free surface reveals³He bound states at the helium/cesium interface.This work was supported by NSF grant DMR-9223775.     

Contact Angle of Liquid 4He on Cs: Evidence for Ripplons at the He-Cs Interface

Two independent methods to measure the contact angle, , of isotopically pure ⁴ He on Cs are described, and applied to the same Cs sample. The two methods agree well. The contact angle as a function of temperature is measured and clearly shows a first order surface phase transition with a wetting temperature T _w = 2 K. From (T) the difference in surface free energy of the Cs-vapour interface and the Cs-liquid interface is calculated. Its large temperature dependence suggests the existence of ripplons at the liquid He-Cs interface. This surprising possibility is discussed.     

The'thin-film' state of 4He on Cs and Rb

At temperatures less than the wetting temperature, T _w, ⁴ He exists in equilibrium, either as a thin film on the surface of Cs or Rb, or as a macroscopic drop with a finite contact angle. The thin-film state is at most around a few monolayers thick and can exist as a two-dimensional (2D) gas or a 2D liquid. We present evidence of a gas /liquid transition at T _gl 1.5K on Cs. For T < T _gl the gas density falls rapidly with decreasing temperature to 10 ^–6 monolayers at T = 0.1K. On Rb there can be a superfluid thin liquid film for T T _w but no 2D gas state has been seen. We discuss the free energy of the solid/vapour interface and its temperature dependence.     

3He Transverse Excited State Occupation in Thin 3He-4He Films

The discovery that ³ He was occupying transverse excited states at submonolayer coverages in ³ He-⁴ He mixture films on a Nuclepore substrate, was a surprise. In this note we discuss the relationship between theory and experiment in attempting to understand the physics of this behavior. We first discuss various single-atom-limit calculations of the level spacing between the ground-state and first excited state. We then introduce a free, quasi-particle picture for analyzing experimental magnetization step data and compare those results with the single-atom-limit calculations. The experiments clearly show excited state occupation at submonolayer coverages in contradistinction with the calculations. We then briefly discuss a microscopic, semi-phenomenological theory which, in agreement with experiment, yields ³ He occupation of the first excited state at submonolayer coverages. The mechanism is a model ³ He-³ He effective interaction due to one ripplon exchange. This effective interaction is density dependent and very long ranged. It strongly modifies the small-k properties of the ³ He self-energy and, in particular, causes the ground-state to first excited state level spacing to decrease with increasing ³ He areal density.     

Optical Measurement of Wetting by 3He-4He Mixtures Near Their Tri-Critical Point

We have measured the contact angle of the interface of phase-separated ³ He- ⁴ He mixtures against a sapphire window using an interferometric method in an optical cryostat. Our measurement has been performed at saturated vapor pressure, along the phase separation line and near T_t, the temperature of the tri-critical point. We have found that the contact angle is finite, contrary to what was generally accepted up to now, i.e., complete wetting by the ⁴ He-rich phase. Furthermore, this angle does not tend to zero when the temperature approaches T_t. This is a remarkable exception to critical point wetting, a phenomenon which is usually observed near ordinary critical points.     

Nucleation and Growth of bcc 4He and of 3He-4He Crystals on Cubic Substrates

We describe experiments on the nucleation and growth of bcc ⁴He and ³He-⁴He mixture crystals on several cubic substrates. In the case of pure ⁴He, the solid does not preferentially nucleate nor does it wet the substrates even when the lattice match is very close. In our view, the absence of preferential nucleation and wetting is a consequence of the interaction between the substrate and the He solid being excessively strong. In the case of mixture crystals, preferential nucleation is observed on a CsCl substrate. Even without preferential nucleation, complete wetting of all the substrates is observed. We suggest that the addition of ³He lowers the crystal-substrate interface energy due to a lower shear modulus of the mixture solid. The microscopic origin of this effect remains a puzzle.     

The influence of3He on the wetting of4He to a cesiated substrate

We report preliminary measurements of the influence of ³ He in the adsorption of ⁴ He to a cesiated substrate by the use of quartz crystal resonance techniques. When compared with the case of pure ⁴ He, the presence of ³ He in the apparatus induces the wetting of the mixture film to Cesium at low temperatures. The wetting is hysteretic, beginning at a concentration-dependent T_c on cooling with continued wetting for T>T_c on warming. The temperatures T_c are given as a function of the total concentration of ³ He in the apparatus.     

Development of Magnetic Resonance Imaging Technique for Ultra Low Temperature Physics

We have developed a Magnetic Resonance Imaging (MRI) technique applicable for ultra low temperature physics. In contrast to conventional MRI for general use where a pulsed-field gradient method is commonly used, we used a steady-field gradient method to avoid an eddy current heating due to metallic parts around the sample cell. We applied the MRI for ³ He-⁴ He mixture liquid with a critical concentration below 1 K and visualized the shape of the phase-separated boundary. We obtained two-dimensional space resolution of a few 10 m. We extracted the interfacial tension of the boundary which was in good agreement with reported values. The contact angle of the boundary to the cell wall was small at low temperature and increased toward 90° near the critical temperature.     

Investigation of quantum systems with surface plasmons and surface state electrons

We report on investigations of thin films of the quantum systems hydrogen and helium both in thermodynamic equilibrium and in metastable states. Information about the film thickness and surface roughness is obtained from the excitation of surface plasmons and mobility measurements of surface state electrons. The equilibrium studies show triple-point wetting for H ₂ on Au substrates, in agreement with earlier results for similar systems. Unexpectedly a distinct hysteretic behavior of the saturated film thickness is found near the triple-point. Superfluid ⁴ He films on as prepared Ag display complete wetting. Investigations of H ₂ films in metastable states prepared by quench-condensation at 1.5K reveal pronounced changes in the film structure upon annealing. These changes set in far below the desorption temperature and are ascribed to surface diffusion.     

3He Impurities in 4He Systems Adsorbed on Curved Substrates

It is shown that ³He impurities in sufficiently large ⁴He systems adsorbed onto substrates with curved geometries form surface bound states, analogous to the Andreev state on a planar liquid--vapor interface. We report the analysis performed for superfluid ⁴He adsorbed on the external surface of the nano-fullerene C₆₀ and on cylindrical nano-wires of Au. It is found that a single ³He impurity diluted into such adsorbed structures behaves as on films on planar substrates and as on pure ⁴He clusters.     

Contribution to the study of reactive wetting in the CuTi/Al2O3 system

The wetting (kinetics of spreading and stationary contact angles) of CuTi alloys on monocrystalline alumina under high vacuum, at a temperature of 1373 K, by the sessile drop technique was investigated. The morphological and chemical characteristics of the metal-ceramic interface were determined by scanning electron microscopy and microprobe analysis. When the results are analysed, three distinct effects of the Ti solute on wetting can be identified and evaluated semi-quantitatively: (a) a reduction in the solid-liquid interfacial tension by adsorption into the liquid side of the interface; (b) a reduction in this tension by formation of a TiO metallic-like oxide layer in the solid side of the interface; (c) a contribution to the wetting driving force due to the free energy released at the interface by the reaction between Ti and Al₂O₃.     

Modification of the gold/graphite interfacial energy by interfacial adsorption of nickel

The effects of nickel additions on the energy of gold/graphite interfaces was investigated. Nickel was found to segregate to the interface forming an adsorbed layer. Solid-state wetting measurements showed that the contact angle of gold on graphite was decreased by 7.8° by the addition of 15 at % Ni. The corresponding decrease in interfacial energy, determined from the Young-Dupré equation was found to be 112 mJm^–2. The interfacial excess of nickel corresponding to 15 at % Ni in the bulk was calculated by means of the Gibbs adsorption isotherm to be about 0.45 equivalent monolayers of gold. The presence of nickel at the interface was further confirmed by crater-edge profiling studies performed in a scanning Auger microprobe.     

Dynamics of3He impurities in4He films

Using a microscopic theory we calculate the binding energy of³ He impurities in films of⁴ He absorbed to a graphite substrate. Without adjustable parameters, we obtain excellent agreement with the experimental binding energies for the ground state of the He impurity. By introducing a timedependent variational wave function, the impurity atom acquires a hydrodynamic effective mass for its motion parallel to the surface due to hydrodynamic backflow, and the excited states have a finite lifetime. When these effects are included, both the energy of the first excited state of the impurity, and the effective mass of the ground state, agree well with experimental data.A comparison with recent density functional results is carried out. It is shown that the substrate bound states on strong potentials are spurious and due to the inappropriate treatment of the effective mass within that theory.     

Surfactants for Exploring the Liquid–Solid Surface Free Energy

Helium-3 and helium-4 can be used as surfactants to modify the observed wetting transitions of hydrogen and deuterium on rubidium. The effect of these surfactants on the wetting temperature is calculated. Prewetting induced by ⁴ He in H ₂ on Rb is studied, and found to be re-entrant. Experiments can use the effect to measure the difference in temperature and concentration dependence of a free solid surface and a liquid/solid interface. This may help to resolve some unresolved questions pertaining to the wetting behavior of helium isotopic mixtures on cesium.     

Ion beam and thermally-induced interface reaction between highT c superconductor thin film and metal overlayer

Ion beam and thermally-induced interface reactions between highT _c superconducting thin film of Y₁Ba₂Cu₃O_7−x and metal overlayer of Ag are studied with a view to control the interfacial property of contact resistance. The interface reaction is induced by 100 keV Ar⁺ ion beam with different ion dose values ranging from 5 × 10¹³ to 3×10¹⁴ ions/cm². The YBaCuO film-metal interface is characterized by using the small angle XRD to study the structural properties of the interfacial phases. The electrical property of the interface, specifically contact resistance, has been investigated for different dose values and thermal treatments. Three-probe vs four-probe configuration has been adopted to measure the contact resistance.     

Frequency Thresholds in the Energy Transmission Across Kapitza Interfaces

When heat is conducted from a solid into liquid helium, a thermal boundary resistance occurs across the interface. This effect, known as Kapitza resistance, has been described with limited success by the acoustic impedance theory. An energy exchange mechanism is proposed in this investigation which is distinct from that of the acoustic impedance theory. The resulting model is successful in predicting the existence of frequency thresholds in the energy transmission across Kapitza interfaces. These frequency thresholds were derived from an analysis of the interaction between helium atoms from the coupling fluid and adsorbed particles which are bound to the solid surface by the Van Der Waals potential of the interface. The predicted ratio of the onset frequencies for a solid surface coupled to ³ He and ⁴ He gas was found to be approximately equal to the square root of the mass ratio. This result is in good agreement with experimental observations.     

3He impurity states on liquid4He: From thin films to the bulk surface

The structure of the states accessible to³He impurities in films of liquid⁴He on Nuclepore is investigated using a density functional approach with a finite-range effective interaction. In thick films, one finds that the two lowest states are localized in the surface region. For thinner films, the variation with film thickness of the first three states results from a delicate balance between the attractive tail of the substrate potential and the quantum finite-size effect. The existence of states localized in the second layer of the films is discussed. The energy difference between the ground state and the first excited state agrees with the recent determination of Higley, Sprague, and Hallock from magnetization measurements. The effective mass of the ground state has a structure similar to that obtained by Krotscheck and coworkers and exhibits a maximum for a⁴He coverage of 0.15 Å^–2, in agreement with the data of Gasparini and coworkers. A similar behavior is predicted for the effective mass of the first, second, and third excited states. The structure of the energy spectrum may also explain former results on third-sound measurements in thin mixture films by Laheurte et al. and by Hallock.Unité de Recherche des Universités Paris 11 et Paris 6 associée au CNRS.