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.     

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.     

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 Nonwetting Behavior of Dilute 3He-4He Mixtures on Cs

We present evidence for ³ He interfacial bound states at the Cs-liquid He interface and show their influence on the interfacial free energies and the nonwetting behaviour of dilute ³ He- ⁴ He mixtures on Cs. The bound state energy and effective mass for the ³ He atoms at this interface are determined and found to be in good agreement with theoretical predictions. From the temperature dependence of the contact angle, direct evidence for a first order wetting transition at the lower wetting temperature is found. A detailed model is presented for the Cs-He mixture interface free energy and the contact angle and this model is shown to be in excellent agreement with the measurements. There is a short discussion on hysteresis and the thin film state. Finally it is emphasised that the large measured contact angles are only consistent with ³ He bound states together with ripplons at the Cs-He mixture interface     

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.     

Observation of Superfluid 4He Adsorbed in One-Dimensional Mesopores

To study one-dimensional (1D) quantum liquid of ⁴ He, we measured the heat capacity and performed a torsional oscillator experiment for ⁴ He adsorbed on a new mesoporous substrate whose adsorption area consists of walls of straight one-dimensional 18Å diameter tunnels. The presence of adsorbed quantum liquid was examined by the isotope effect on the heat capacity for ³ He and ⁴ He adatoms. Above a coverage n _o , the heat capacity isotherms are completely different because of the Fermi and the Bose fluids, respectively. In the torsional oscillator experiment we observed superfluid ⁴ He above n _o . The fraction of the superfluid decoupled from the motion of the substrate is 0.13, which is the same order as 0.18 for packed Pt fine powder and 0.24 for 80 Å-porous glasses. The result indicates that the superfluid state exists in the one-dimensional ⁴ He adatoms formed in the 18 Å diameter pores.     

Investigation of adsorption and wetting of3He on cesium and cesiated glass

Experiments have been carried out to investigate the binding of ³ He on cesium substrates, using optical pumping to spin-polarize the atoms. The behaviour of ³ He on the walls at low temperature can be analyzed through the evolution of the nuclear magnetization of the sample. Preliminary results are presented, including: (i) adsorption studies of gaseous ³ He on cesiated glass; (ii) magnetic relaxation time of polarized liquid ³ He on cesium and cesiated glass; (iii) evidence for wetting of liquid ³ He on cesium.     

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.     

Low Temperature Heat-Capacities of Liquid 3He Thin Films

We have measured the heat capacity of ³ He thin films adsorbed on graphite at an areal density of 15.0 nm ^–2 down to as low as 100 K. The second-layer ³ He behaves as a degenerate 2D Fermi fluid in the whole temperature range we studied. We observed no anomalous behavior in the heat capacity near 3 mK in contradiction to the recent report by other workers. This indicates that possible superfluid transitions would be below 100 K. Instead, a small and temperature-independent contribution to the heat capacity was observed, which we attribute to nuclear-spin degrees of freedom in glassy solid ³ He trapped in substrate heterogeneities.     

Ultra-Low Temperature Magnetic Properties of Liquid 3He Films

We report measurements of the nuclear magnetization of submonolayer liquid ³ He films adsorbed on a graphite substrate (Papyex) preplated by a monolayer of ⁴ He. 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 liquid ³ He 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 K.     

The extraordinary phenomena of weak adsorption

A summary is provided of a variety of intriguing phenomena involving particles weakly bound to surfaces. One kind of problem involves wetting and prewetting transitions (for H₂, He and Ne on alkali metals) and thus far mysterious behavior of He on an H₂ substrate. A second kind involves the adsorption of atoms on the surfaces of liquid⁴He or³He.     

NMR in pure3He films on a Nuclepore substrate

The properties of³He films on a Nuclepore substrate have been measured by pulsed NMR at a Larmor frequency of 10 MHz between 1.3 and 4.2 K. The³He film thickness was varied from 0.14 to 2 layers. The spin-spin relaxation timeT ₂ agrees well with previous measurements of³He films on Mylar and Vycor glass at low temperatures. The spin-lattice relaxation timeT ₁ for submonolayer films shows a strong temperature dependence consistent with a thermally activated process. This behavior has not previously been observed on amorphous substrates. The spin diffusion coefficient was measured for the thickest films at 4.2 and 2.6 K and found to be consistent with free atom motion of the³He in the vapor. In thin films or at low temperatures, the diffusion was too small to be observed. The magnetic coupling between the³He nuclei in a film and the protons in the Nuclepore substrate was determined from the effect of the³He on the proton-lattice relaxation time. It is about 100 times weaker than the interaction between³He and the fluorine nuclei in a Teflon substrate.     

Path integral study of the 4He wetting transition

The ⁴ He wetting transition on a Cs surface is for the first time studied by Path Integral Monte Carlo simulation. Numerical results for a system of 64 ⁴ He atoms are qualitatively consistent with a wetting transition at a temperature T 2 K.     

Experiments to search for a substrate state for3he adjacent to surfaces in3He-4He mixtures

We describe three experiments designed to search for the bound state for³He adjacent to a substrate which has been predicted by Treiner and co-workers. In each case a bulk³He-⁴He solution is in contact with the substrate of interest. For a Nuclepore substrate, NMR techniques are used to establish that there is no 2D behavior for the³He in the cell; thus, no substrate state is present. For a cesium or hydrogen substrate the technique involves the measurement of the velocity of ordinary sound in the bulk solution as a function of temperature. The cesium and hydrogen experiments are incomplete.     

Evaporative Cooling of 3He-4He Mixture Films

Heat transport by saturated ³ He- ⁴ He films has been studied at temperatures 50..350 mK and the bulk concentration of ³ He ranging from 0.1 ppm to 5%. The cooling of the film, when locally heated above 160 mK, is mainly via 2D flow of surface ³ He from colder area followed by evaporation of ³ He. At certain heating power the 2D flow becomes a bottleneck, the heated spot runs out of ³ He and its temperature abruptly increases. The critical power is nearly proportional to the surface density of ³ He. For higher ³ He concentrations another distinct step in temperature has been observed at a lower heating power. It is attributed to the existence of an excited ³ He surface state whose population starts at ³ He surface density of 3.5×10 ¹⁴ cm^–2 . The second state is located about 1.2 K higher in energy than the ground state and provides an additional channel for the 2D flow of ³ He.     

Decoupling of 3He Films Adsorbed on Two-Dimensional Mesoporous Hectorite

The mechanical responses of physisorbed films to the vibration of a substrate were investigated by measuring the sound velocity and attenuation of a two-dimensional mesoporous hectorite. The sound velocity of a pellet with adsorbed ³ He films increased drastically at low temperatures accompanied by the attenuation. The temperature dependence of the sound velocity and attenuation can be described by the thermally activated relaxation process, and the increase in sound velocity was found to be proportional to the amount of adsorbed ³ He. The most plausible explanation for this behavior is that ³ He films decouple from the vibrating substrate at low temperatures.     

Specific heat of adsorbed films of3He,4He,and isotopic helium mixtures near monolayer coverages

Specific heat results are presented for near-monolayer films of³He,⁴He, and isotopic He mixtures adsorbed on Vycor porous glass in the temperature range between 1 and 4°K. In the case of the pure He monolayers and submonolayers, the specific heats depend only onT ² , like a two-dimensional Debye elastic medium. A film of slightly greater than monolayer coverage needs an additional temperature-independent term. Monolayer coverages of mixtures are difficult to define, but all coverages used, some of which are submonolayer, need bothT ² andT-independent terms. The constant term may arise from atoms in the second layer forming a classical noninteracting system, which in the mixture case would indicate selective adsorption of⁴He in preference to³He at the substrate wall, in agreement with previous adsorption experiments on the same substrate.Work supported in part by the U.S. Atomic Energy Commission.     

Wetting of3He-4He mixtures on alkali metal substrates

From available information concerning the wetting behavior of pure³He and pure⁴He on alkali metal substrates, as well as the known properties of bulk³He-⁴He mixtures, the complete wetting phase diagrams for such mixtures, showing prewetting, isotopic separation and lambda transitions for the film phases have been derived. We predict new phenomena such as a triple-point induced dewetting transition, and the absence of a superfluid film wetting Cs, Rb and K walls under concentrated³He solutions.     

The Vortex Lability In a Supersaturated Superfluid 3 He-4 He Mixture and the Quantum Phase-Separation Kinetics

The supersaturated superfluid ³ He-⁴ He mixture in the presence of quantized vortices is considered. A specific type of the lability against the vortex core expansion arises from concentrating ³ He atoms onto the vortex core. This affects significantly the region of the ³ He concentrations at which the rate of nucleating stable ³ He-concentrated phase can be noticeable. The most clear case of the vortex structure is a straight-line vortex with the uniformly distributed 3He atoms along the core. Such lability leads to some features in the thermal activation and quantum tunneling nucleation, resulting, in particular, in a relatively small supersaturation of about 1% which can be achieved. The critical value of the supersaturations corresponding to the vortex lability is dispersed because of the finite size of the ³ He-concentrated phase along the vortex core. We analyze this dispersion and its effect on the quantum separation in supersaturated ³ He-⁴ He mixtures. The value of the supersaturation conserves its order of the magnitude, changing only by a numerical factor. We also emphasize an important difference of the coefficient in the term responsible for the lability from the superfluid density s. The supersaturation observed in experiments lies within the range of the vortex lability and, probably, is connected with this lability.     

Photoelectrochemical investigations on individual ferritic and austenitic grains of a duplex stainless steel oxidized in water vapour

Abstract

This work presents a multiscale photoelectrochemical characterization of oxide layers, and aims mainly at introducing the mesoscopic scale (ca 30 µm). For the first time, photocurrent energy spectra could be recorded on individual ferritic and austenitic grains of a unique 2205 duplex stainless steel sample oxidized for 5 minutes at 650°C under 20% water vapour in nitrogen. These results allowed us to explain contrasting issues in the photoelectrochemical images obtained from the oxidized sample. Moreover, the bandgap energies obtained by fitting these individual mesoscopic photocurrent energy spectra with a novel approach developed in our laboratory, showed that all oxide scales were constituted of Fe₂O₃ and Cr₂O₃ and of an Fe_2-xCr_xO₃ solid solutions, but that the x-value of the latter was different depending on the metallurgic phase of the oxidized substrate. The latter results were shown to be in agreement with those of additional Raman analyses of the oxidized austenitic and ferritic grains.