Helium Films Research Articles

Study of Thermal Desorption of Helium from HydrogenatedZirconium

A new method prepared for helium and hydrogen co-containing Zr films ispresented to simulate aging metal tritides, in which direct currentmagnetron sputtering with a He/H/Ar mixture is used. The retainedamount and depth profiles of helium and hydrogen are determined byelastic recoil detection analysis. Thermal desorption spectrometry isapplied to investigate He thermal release and the effect of hydrogen.It is found that the high-temperature peaks with a large mount ofhelium release obviously shifted toward lower temperature at highhydrogen concentration, especially at the hydride transformation region,and that the shapes of the release peaks also changed due to theadditional hydrogen. However, at the low-temperature releasing region thepeak intense decreases when phase transformation takes place. Themechanism of helium thermal release and the effect of hydrogen arealso discussed.

Two-dimensional Wigner crystal on helium films: an indication of quantum melting

Using molecular dynamics simulation (MD) we have investigated the melting of the two-dimensional Wigner crystal on 240A-500A liquid helium films supported by substrates of dielectric constants es = 2.2-7.3. Our results show good agreement with available theoretical and experimental results for densities below 1.0×1010cm-2. For higher densities, we notice some disagreements suggesting that quantum effects are important in this regime of densities.

Surface electron mobility over a helium film from Boltzmann and force-balance equations

The mobility of surface electrons localized over helium films underlying solid substrates has been evaluated by solving the Boltzmann equation in the time relaxation approximation and the force balance equation in which an effective mobility is obtained in terms of the dynamical structure factor of the nondegenerate electron liquid. The essential processes of electron scattering by gas atoms, ripplons, and film-solid interface roughness are taken into account. The electron mobility dependence on the film thickness and temperature is determined and compared with experimental data available. We find that the interface-roughness scattering is the dominant process for explaining the experimental results. We estimate the extended defect sizes of the underlying substrate within the Gaussian correlated model for interface roughness.

One-dimensional and quasi-one-dimensional electron systems in nanochannels

A study is made of the energy spectrum of electrons localized in cylindrical nanochannels coated by a thick layer of solid hydrogen or neon. The interaction potential of an electron with the surrounding matter is determined. An algorithm, which includes both analytic and numerical calculations, is devised for solving the wave equation. The dependence of the electron energy on the radius of the channel is investigated, and the electron wave functions are determined. The influence of a helium film on the energy state of the electron is considered. It is shown that at channel radii of the order of 50Å the localization of the electron inside the channel is energetically favorable.

Influence of the superfluid transition on the adsorption of thin helium films

An approach to the calculation of the chemical potential of a thin helium film in equilibrium with the vapor is proposed. In the calculation of the chemical potential the surface layer of helium atoms in contact with the vapor is treated as a two-dimensional system. The chemical potential of superfluid helium is approximated by the chemical potential of an ideal two-dimensional Bose gas, while the chemical potential of normal helium contains an additional, temperature-dependent term. This approach leads to the prediction that the thickness of the film will increase at the transition to the superfluid state. It is also predicted that a coexistence of normal and superfluid films of different thickness is possible under certain conditions.

Phonon Mediated Helium Atom Transmission through Superfluid Helium Four

We report results of experiments in which pulses of helium vapor with translational energies of 3 K are directed at a thin film of superfluid helium at a temperature of about 0.2 K suspended over a cesium covered orifice in a platinum film. The response of the superfluid film was detected by a super-conducting titanium bolometer placed on the side of the film opposite to that of the source. For films of approximately 1 mm in thickness we find no response of the bolometer within the limits of our detector. However, for films of less than 100μ thickness, we find a response which is of the same temporal shape, but smaller in amplitude, than that of the orifice when it is not covered with superfluid helium. We interpret these results to mean that we are seeing phonon mediated transmission in the thin films. Roton and condensate mediated transmission amplitudes for these conditions are apparently too small for us to detect in any of the films. This result is consistent with the theoretical results of Sobnack and Inkson [M. B. Sobnack, J. C. Inkson, and J. C. H. Fung, Phys. Rev. B 60, 3465 (1999)] concerning the amplitude of roton to atom and photon to atom conversion as a function of the atomic energy.

A new path to ultracold hydrogen

Ultracold hydrogen offers unique possibilities for precision spectroscopy, studies of atomic interactions, and the creation of quantum fluids containing mixtures of hydrogen and deuterium. Current techniques for trapping and cooling hydrogen have produced large condensates with N ∼ 109 atoms, but suffer from a variety of experimental limitations. Among these are the slow evaporative cooling rate due to the small H–H elastic-scattering cross section, the need for a superfluid helium film in the initial thermalization process, a geometry that severely limits detection efficiency, and the inability to trap deuterium. We are constructing a new apparatus based on buffer-gas cooling that will overcome these problems. To accelerate evaporative cooling, the thermalization rate is increased by simultaneously loading lithium and hydrogen into a 4.2 T anti-Helmholtz trap. Lithium accelerates evaporative cooling because the Li–H elastic-scattering cross section is ∼1200 times larger than that of H–H. Hydrogen and lithium will be produced by laser ablation of solid LiH in a 3He buffer gas held at temperatures of ∼350 mK. Because no wall collisions are needed for initial thermalization, ablating a solid sample of LiD should enable studies of deuterium. The apparatus is in the final stages of construction. We report on results of initial tests and discuss the new scientific opportunities made possible by this approach.PACS No.: 32.80.Pj

Ellipsometry of Liquid Helium Films on Gold, Cesium, and Graphite

We have developed a modulated null ellipsometer with sub-monolayer resolution to measure adsorbed liquid helium thin films at temperatures below 4 K. Adsorption isotherms of 4 He on gold, cesium, and graphite are presented. For Au and Cs substrates, the reflecting surface for our ellipsometric measurements is the metallic electrode of a quartz crystal microbalance(QCM). Performing both types of measurements simultaneously on the same substrate provides a direct method of converting the ellipsometric signal into an absolute film thickness without constructing a detailed model of the refractive index of the substrate. Isotherms on gold above and below T λ ¸ show that the ellipsometric signal is unaffected by the superfluid transition; the ellipsometer measures the total film thickness independent of the superfluid fraction. Isotherms on cesium above the wetting temperature show a prewetting step. Isotherms on clean graphite show steps due to layering.

Melting temperature of screened Wigner crystal on helium films by molecular dynamics

Using molecular dynamics (MD) simulation, we have calculated the melting temperature of two-dimensional electron systems on $ 240$\AA-$ 500$\AA helium films supported by substrates of dielectric constants $ \epsilon_{s}=2.2-11.9$ at areal densities $n$ varying from $ 3\times 10^{9}$ cm$^{-2}$ to $ 1.3\times 10^{10}$ cm$^{-2}$. Our results are in good agreement with the available theoretical and experimental results.

Equilibrium helium films under the influence of surface roughness

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, δ(x), can be strong enough to be observed. For the definition of δ(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.

Surface Electron Transport over a Liquid-Helium Film Covering a Smooth Solid Substrate

The mobility of surface electrons localized over a superfluid helium film covering a solid substrate is calculated by taking into account the essential scattering processes of electrons by helium atoms, by ripplons and by the helium-substrate interface roughness. The last scattering process dominates for thin helium films with thickness with d ≲ 10−6 cm. A new scattering Hamiltonian, recently derived by the method of image forces, is employed. The calculated results for the mobility are compared with experimental data by fitting the two characteristic sizes of the Gaussian correlation model for surface defects. The agreement is rather good in the case of a glass substrate.

Surface Electron Channels on Suspended Helium Films

The profile of a suspended helium film over a narrow channel is calculated self-consistently for several values of the bulk liquid helium level. We also determine the electron distribution in the channel when surface electrons are deposited on the film. It is shown that a thin van der Waals film with spatially dependent thickness is formed in a wide region at the channel center where a large fraction of electrons are confined. We obtain that the effective lateral confining potential can be described by a parabolic one is experimental conditions reported in the literature.

The role of a surface flow in experiments with atomic hydrogen adsorbed on liquid helium

We apply a quantum hydrodynamics of the surface of 3He-4He solutions to account for the in-plane transport of the two-dimensional (2D) spin-aligned atomic hydrogen (H↓) adsorbed on superfluid helium film. We discuss how the surface flow of 2D H↓ may be traced in experiment thus allowing to study the interaction of the 2D hydrogen with ripplons and surface-bound 3He as well as to observe the superfluidity of the 2D Bose gas of H↓. As an example we consider the formation of the ESR spectrum of the 2D H↓ and find that in spatially non-uniform case the surface flow contributes significantly to the conditions of the ESR spectrum instability observable at high microwave power. We also analyze the conditions at which the surface flow of the 2D hydrogen plays an important role in thermal compression experiments.

Electrons Above Helium Films on Metal Substrates

We report about our investigations on two-dimensional electron ensembles floating above thin helium films supported by metallic substrates. We found a surprisingly high stability of these electrons, although the substrates were conducting which allows breakthrough at weak spots of the helium film due to roughness peaks. We have measured the electron densities and corresponding relaxation times. By pulling the electrons from the area above the loading electrode to the area above a separate electrode one can enhance the lifetime of these surface state electrons.PACS numbers: 67.70.+n, 73.90.+f, 73.50.-h

Adsorption of Helium on Au Nanowires

The growth of helium films on Au nanowires is investigated within finite range density functional theory. It is shown that as in previous studies of helium adsorption on spherical surfaces, submonolayer condensation and stable layering transitions take place, and that a growth instability appears above a finite film thickness. The competition between unstable film growth on a single tube and stable capillary condensation in an array of nanowires is examined in a simplified mean field-like model. Results are displayed to illustrate adsorption and its stable and unstable patterns outside solid nanowires, cavities in bulk gold and in the gap between a cavity and an inner concentric cylinder.

Plasma modes in low-dimensional electron system over surface of liquid helium

The collective plasma modes in a quasi-two-dimensional (Q2D) electron system located over the free surface of liquid helium are studied theoretically within many-body dielectric formalism. The dispersion of modes is considered both over bulk liquid and over helium film where the essential modification of interelectron interaction occurs due to screening effects in the substrate with a large value of dielectric constant. It is shown that the plasma spectrum consists of longitudinal and transverse branches which dispersion laws depend on the values of the dielectric constant of helium and the film thickness. For the helium film covering metal, the longitudinal mode is acoustic differing of that for the surface electron (SE) system over bulk helium.

Melting of ap−H2monolayer on a lithium substrate

Adsorption of $p\text{\ensuremath{-}}{\mathrm{H}}_{2}$ films on alkali metals substrates at low temperature is studied theoretically by means of path integral Monte Carlo simulations. Realistic potentials are utilized to model the interaction between two $p\text{\ensuremath{-}}{\mathrm{H}}_{2}$ molecules, as well as between a $p\text{\ensuremath{-}}{\mathrm{H}}_{2}$ molecule and the substrate, assumed smooth. Results show that adsorption of $p\text{\ensuremath{-}}{\mathrm{H}}_{2}$ on a lithium substrate, the most attractive among the alkali, occurs through completion of successive solid adlayers. Each layer has a two-dimensional density ${\ensuremath{\theta}}_{e}\ensuremath{\approx}0.070\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{\ensuremath{-}2}$. A solid $p\text{\ensuremath{-}}{\mathrm{H}}_{2}$ monolayer displays a higher degree of confinement, in the direction perpendicular to the substrate, than a monolayer helium film, and has a melting temperature of about $6.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The other alkali substrates are not attractive enough to be wetted by ${\mathrm{H}}_{2}$ at low temperature. No evidence of a possible superfluid phase of $p\text{\ensuremath{-}}{\mathrm{H}}_{2}$ is seen in these systems.

Publisher's Note: Structure and energetics of helium films on alkali substrates [Phys. Rev. B70, 024512 (2004)

Publisher's Note: Structure and energetics of helium films on alkali substrates [Phys. Rev. B<b>70</b>, 024512 (2004)

Magnetoplasmons in nondegenerate quantum wires on suspended helium films

Magnetoplasmon spectra for the nondegenerate surface electrons confined in a quasi-one-dimensional channel over a liquid helium film are determined within a microscopic approach in the limit of strong magnetic fields when the electrons occupy the lowest spin-split Landau level. It is shown that the dispersion relation and the spatial structure, transverse to the channel, of the magnetoplasmons have a length scale ${\ensuremath{\ell}}_{T}=\sqrt{2T∕m{\ensuremath{\Omega}}^{2}}⪢{\ensuremath{\ell}}_{0}$, where $\ensuremath{\Omega}$ is the confining frequency and ${\ensuremath{\ell}}_{0}$ is the magnetic length. We find acoustic modes, whose velocities as a function of the gate distance exhibit a highly nonlinear behavior under certain conditions, in particular, the appearance of anticrossings.

Structure and energetics of helium films on alkali substrates

Low-temperature adsorption of $^{4}\mathrm{He}$ films on Alkali metal substrates is investigated theoretically by means of ground-state quantum Monte Carlo simulations. The most accurate potentials currently available are utilized to model the interaction of $^{4}\mathrm{He}$ atoms with the substrate. Continuous growth of film thickness as a function of chemical potential is observed on Li, Na, and K substrates. A superfluid monolayer forms on a Li substrate; on Na and K, thermodynamically stable films are a few layers thick. The uncertainties of the calculation and in the potentials, preclude a definitive conclusion on the existence of a stable $^{4}\mathrm{He}$ film on Rb. A comparison of the results of this calculation with those obtained using the Orsay-Trento density functional shows broad quantitative agreement.