Dynamics of helium films

We present quantitative calculations for the static structure and the dynamics of quantum liquid films on a translationally invariant substrate. The excitation spectrum is calculated by solving the equations of motion for time-dependent one- and two-body densities. We find significant corrections to the Feynman spectrum for the phonon-like collective excitations.     

Some observations with helium films

This appendix contains several comments relevant to the lectures of Roger Bowley.¹ We begin with some introductory remarks on third sound and proceed to utilize third sound to study the stability of flow in helium films. We conclude with a remark about the effective thermal conductivity of a helium film.This is an appendix to the series of lectures delivered by Roger Bowley at the Elba Summer School; much of this material was presented during one of Roger Bowley's lectures.     

Inhomogeneities in thin helium films

We report a study of the inhomogeneities in thin helium films due to the van der Waals potential by a resonant acoustical technique. The van der Waals constant between helium and quartz is estimated to be =65±5 K (layer)³; the model leads to a solid helium layer on the substrate of the order ofH=2.2a ₀ . A comparison is made with data extracted from interferometric acoustical methods and third-sound type experiments.Associated with the Centre National de la Recherche Scientifique.     

Stability of helium and hydrogen films

We use the variational theorem to evaluate the domain of parameter space such thatsome film exists on a given surface. This is accomplished by determining the binding energy variationally for a monolayer film at T=0. The He results are for the most part consistent with those of Cheng, et al., for the domain of wetting films. New results for H₂ are compared with calculated parameters of noble gas adsorption potentials.     

Structure in thin superfluid helium films

Precision measurements of third sound in atomically thin⁴He films on Ne, Ar, and CO₂ substrates reveal a periodic structure not corresponding to whole layers. Treating the He film as an incompressible, continuous fluid, the data indicate that the chemical potential has a contribution proportional to l^–6, where l is the He coverage in layers, which is modulated at intervals of 0.62±0.04 layers and which weakens the van der Waals potential. Alternatively the data indicates the existence in the film of a damped smectic density wave with a 0.54±0.40 layer periodicity.     

X-ray induced electrostatic charging of helium films

It is well known that free electrons can be held onto the free surface of liquid helium through either their own image charges or through the effect of an externally applied electric field. The resultant electrostatic pressure causes films to thin. We have recently measured x-ray reflectivity from static films of isotopic mixtures of helium with an intense x-ray beam in the temperature range between 0.37 K and 1.3 K. Normally, no significant thickness variation with x-ray intensity is expected over a wide range of temperatures when the film is superfluid. We have found that even modest x-ray intensities affect the thickness of films containing only trace amounts of³He. We believe that the effect is due to x-ray produced photoelectrons, which thermalize in the vapor and then reside on the surface, attracted by both the film and a charged substrate. The temperature and concentration dependence is then due to the transport properties of the electrons at the surface. It may be possible to study the 2-D electron gas produced in this way by diffraction techniques.     

Magnetopolarons on the surface of helium films

The energy spectrum of the surface electrons on liquid helium films in the presence of a perpendicular applied magnetic field is determined in different approaches within second order perturbation theory. The influence of the electron-ripplon interaction on the Landau levels are studied for arbitrary strength of the magnetic field and several values of the coupling constant. Current theories of the polaron ground-state and cyclotron resonance are compared to the exact weak-coupling results obtained. We found a shift down in the cyclotron resonance frequency due to polaron effects.     

Theory of quantum nucleation of bubbles in liquid helium

We calculate the rate at which bubbles form by quantum tunneling in liquid helium-3 and helium-4 at negative pressure. We find that quantum tunneling should be observable at temperatures below about 0.1 K in helium-3 and 0.2 K in helium-4, and at pressures close to the critical negative pressure at which the liquid becomes unstable against long wavelength density fluctuations.     

Third-sound resonance and thermal resistance in unsaturated helium films

We have obtained a thermodynamic state of thin helium films which permits precision measurements of their properties. Third-sound resonances indicate a transition in the superfluid density of these films. This transition is accompanied by an exponential rise in the thermal resistance of the helium film which indicates the formation of high-energy excitations of 0.08 eV. The dissipation at resonance indicates that thermodynamic fluctuations are the principal mode of third-sound damping.A revised approach to the data in this paper appeared inPhysical Review Letters, Vol. 25, No. 11, page 711ff, September 14, 1970.This research was supported in part by the Advanced Research Projects Agency under Contract SD-131 and in part by the Alfred P. Sloan Foundation.Alfred P. Sloan Fellow.     

Low-temperature heat capacity of helium films adsorbed on copper

Heat capacity measurements are described for films of pure⁴ He and a 12%He ³ mixture ofHe ³ andHe ⁴He adsorbed on copper, for 0.1<T<1.2 K.He ⁴ heat capacity isotherms show a step-type behavior as a function of coverage for the first two layers, while the mixture isotherms only show the first step. A comparison is made with recent multilayer data for helium films on Vycor published by other authors.Work supported by the National Science Foundation.     

Theory ofT 2 heat capacities of monolayer helium films

Heat capacities of adsorbed monolayers of He³and He⁴indicate that the dominant thermal excitations at low temperatures are phonons rather than single-particle modes when the films are near maximum density. A recent theoretical study of noninteracting particles adsorbed on a crystalline substrate yielded a two-dimensional band structure, and explored the thermodynamic properties of the single-particle modes. The theory is now extended to include the influence of hard-core interactions between the particles moving in the lowest band. Resembling the high-density approximations for liquid and solid helium, the hard-core interactions are shown to give rise to a collective zero-point kinetic energy from which a two-dimensional pressure and a compressibility are derived. The corresponding velocity of sound and phonon heat capacity are calculated in the weak-binding and tight-binding approximations. Comparison with experiment indicates that the weak-binding approximation is more appropriate and that it yields effective mass ratiosm*/m1.7 for the two isotopes at the experimental film densities. A novel feature of the tight-binding calculation that may be applicable to other experimental systems is the prediction of phase condensation for adsorbed gases of relatively small molecular diameter.Research supported by the National Science Foundation.     

Experimental approaches to the wetting transition with helium films

Alkali metal surfaces have been suggested not to be wetted by helium at zero temperature. Several experiments have been realized to check if these surfaces, as well as other weak binding substrates, indeed exhibits this particularly interesting property at any finite temperature.Although works are still in progress, essential results are (i) substrates non wetted by helium films do exist (ii) prewetting jumps from low to high coverage have been observed for weakly wetted ones (iii) the consequences of substrate roughness and chemical impurities are suspected to be important and remain to be assessed. These results are reviewed and discussed. Applications of those substrates for both³He and⁴He isotopes are considered.Unité de recherche de l'Ecole Normale Supérieure et de l'Université Pierre et Mavie Curie, associé au CNRS (URA 18).     

Equilibrium helium films under the influence of surface roughness

No Heading Equilibrium helium films adsorbed on solid substrates are investigated. Due to their thickness these films are mainly in the retardation regime where the influence of the roughness of the substrates, (x), can be strong enough to be observed. For the definition of (x) we use a simple corrugation model. This model is supported by experimental results using the surface plasmon resonance technique to determine the thickness of helium films grown on different Ag surfaces.PACS numbers: 67.70.+n, 68.15.+e, 68.43.–h, 68.55.–a     

Self-organized CdSSe quantum dots thin films

Self-organized and strained CdSSe quantum dot (Q-dot) thin films have been grown on ITO-coated glass substrates by novel wet chemical route. The Q-dots are (002) oriented faceted pyramids with average particle size of 7.5 nm. The X-ray diffraction results reveal the formation of a solid solution of CdSSe. Atomic force microscopy is used to investigate the morphology of the nanocrystalline thin films. The energy dispersive X-ray analysis spectrum confirms the presence of Cd, S, and Se in the films. Optical absorption and photoluminescence spectra show the blue shift for quantum dot thin films.     

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.     

The influence of electrostatic fields on films of liquid helium

Prompted by the recent striking experimental results reported by Babkin and Hakonen that appeared to show that liquid helium-II does not wet magnesium fluoride, we have examined the effects that an inhomogeneous electrostatic field has on thin films of liquid helium at temperatures below 0.5K. Our model includes the influence of gravity, surface tension, the electric field and the van der Waals interaction between the helium and its supporting substrate. We show that, an inhomogeneous charge on the substrate can produce effects that mimic the surface profiles between wetted and non-wetted areas. The calculations also indicate that some special precautions may be necessary when studying films of liquid or solid helium on insulators.