Liquid Helium Film Research Articles

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.

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.

Variational approach to the problem of the energy spectrum of surface electrons over a liquid helium film

The energies of the first two subbands are calculated, within a variational approach, for electrons localized over the surface of a liquid helium film covering a solid substrate. The results are obtained for arbitrary value of the dielectric constant of the solid substrate, covering both the limit of a substrate with a dielectric constant close to unity (such as a rare gas solid) and a metal. The results for the subband energies in the case of a metallic substrate are compared with those obtained previously by a different method by Gabovich, Ilchenko, and Pashitskii. The agreement is rather good, supporting the applicability of the variational method for calculating the energy spectrum of surface electrons in a wide range of substrate parameters.

Scattering of atoms from liquid helium films and slabs

We report new results in describing the scattering of 4 He atoms off 4 He films and slabs. The description incorporates elastic and inelastic scattering off the host liquid. We have computed the probabilities of elastic and inelastic reflection, sticking and, in the case of slabs, transmission for normal incidence.

Quasi-one-dimensional electron transport over the surface of a liquid-helium film

Quasi-one-dimensional mobility of surface electrons over a liquid-helium suspended film is studied for a conducting channel. The electron mobility is calculated taking into account the electron scattering by helium atoms in the vapor phase, ripplons, and surface defects of the film substrate both in one-electron regime and in the so-called complete-control limit where the influence of inter-electron collisions on the electron distribution function is taken into account. It is shown that the mobility for low temperatures is dominated by the surface-defect scattering and its temperature dependence is essentially different from that of the electron–ripplon scattering.

Mobility of electrons on a liquid-helium film with a rough substrate

The origin of the specific activation-type behavior of the mobility of electrons on liquid-helium films with different kinds of substrates is discussed. The characteristic feature of the activation energy Ea observed in the experiment is its dependence on the effective film thickness d in the form Ea∝d−2. A scenario of this effect is proposed with consideration for the roughness of the substrate underlying the liquid-helium film.

Detrapping aspects of ripplonic polarons on a liquid helium film

The problem of two-dimensional electron detrapping from the polaron to the quasifree state above a liquid helium film is studied employing the actual dispersion form of media vibrations. In contrast with the frequently used cutoff approximation, the actual form is shown to result in continuous and quite smooth changes of the polaron energy and mass near the detrapping point. At the same time, the electron localization radius (L) is found to increase rapidly, when the coupling constant $({\ensuremath{\alpha}}_{\mathrm{CC}})$ approaches unity: $L\ensuremath{\propto}({\ensuremath{\alpha}}_{\mathrm{CC}}\ensuremath{-}{1)}^{\ensuremath{-}1/2}\ensuremath{\rightarrow}\ensuremath{\infty}.$ In the framework of the general stability analysis, this electron detrapping is explained to be a result of an interesting relationship between the stability index and ${\ensuremath{\alpha}}_{\mathrm{CC}}$ for the media excitations involved in the polaron cloud.

Adsorption of Thin Liquid Helium Films on Cs Measured via Photoelectron Tunneling

The alkali metals Cs and Rb are the only surfaces which are not wetted by superfluid 4He below a certain temperature. In our experiments, using the photoelectron tunneling method, we can highly resolve the growth of the non-wetting thin-film state of 4He on a quench-condensed Cs surface. It turns out that far from coexistence there is little adsorption of helium. In contrast, close to coexistence a rapid growth up to two monolayers of helium is observed, but the surface is still non-wet under the usual convention.

Fabrication of the structure for qubits using electrons on liquid helium

In the previous papers, the system of qubits using electrons on a liquid helium film was described. In this paper we describe the physical realization of the system that we have begun to fabricate. We will not in this brief discussion describe how we intend to operate the system. We will show that we are nano- and micro-fabricating a new type of electronic device that differs from other microelectronic devices in that the final step of the fabrication deposits a layer of helium rather than some other dielectric. The operation of the device will differ in that it manipulates single electrons and it must operate at low temperatures.

Polaron effects in electron channels on a helium film

Using the Feynman path-integral formalism we study the polaron effects in quantum wires above a liquid helium film. The electron interacts with two-dimensional (2D) surface phonons, i.e. ripplons, and is confined in one dimension (1D) by an harmonic potential. The obtained results are valid for arbitrary temperature ($T$), electron-phonon coupling strength ($\alpha $), and lateral confinement ($\omega_{0}$). Analytical and numerical results are obtained for limiting cases of $T$, $\alpha $, and $\omega_{0}$. We found the surprising result that reducing the electron motion from 2D to quasi-1D makes the self-trapping transition more continuous.

4Heliquid-vapor interface below 1 K studied using x-ray reflectivity

The free surface of thin films of liquid helium adsorbed on a solid substrate has been studied using x-ray reflectivity. The film thickness and interfacial profile are extracted from the angular dependence of measured interference between signals reflected from the liquid-vapor and liquid-substrate interfaces. Polished silicon wafers, chemically cleaned and passivated, were used as substrates. Results are reported for measurements for ${}^{4}\mathrm{He}$ films 35 to 130 \AA{} thick in the temperature range 0.44 to 1.3 K. The 10%/90% interfacial width for temperature $T=0.45$ K varies from $5.3\ifmmode\pm\else\textpm\fi{}0.5$ \AA{} for $36\ifmmode\pm\else\textpm\fi{}1.5$ \AA{} thick films to $6.5\ifmmode\pm\else\textpm\fi{}0.5$ \AA{} for $125\ifmmode\pm\else\textpm\fi{}1.5$ \AA{} thick films. The profile width at zero temperature should not differ significantly from that measured at $T=0.45$ K. For $T=1.22$ K, the width is $7.8\ifmmode\pm\else\textpm\fi{}1.0$ \AA{}.

Equilibrium thickness of saturated superfluid 3He films on a copper substrate

Much theoretical interest focuses on the equilibrium thickness of liquid helium films adsorbed onto a substrate. To address this issue in liquid 3He, an experiment to measure the thickness d of a saturated superfluid 3He film, on a horizontal copper substrate which is a height h above the free surface of the liquid bath, was carried out. The free surface of the bulk superfluid 3He is slowly lowered, thus increasing h, by expanding a bellows (actuated with superfluid 4He) at the base of the cell. While doing this, continuous measurements of the thickness of the superfluid 3He film, from which we can obtain d( h), are made. These measurements then allow us to extract information on the attractive (van der Waals) potential between the superfluid 3He film and the copper substrate.

Diplons in the degenerate regime: exchange and correlation effects

We study the exchange and correlation effects on the ground-state properties of dipolar complexes (diplons) formed by the surface electrons on a liquid helium film and positive ions in the substrate supporting the helium film. We use the self-consistent-field approximation including the short-range correlations. The local-field correction, static structure function, exchange and correlation energies, and collective mode dispersions are calculated for systems with different substrates.

Quantum computing with electrons floating on liquid helium

A quasi-two-dimensional set of electrons (1 < N < 10(9)) in vacuum, trapped in one-dimensional hydrogenic levels above a micrometer-thick film of liquid helium, is proposed as an easily manipulated strongly interacting set of quantum bits. Individual electrons are laterally confined by micrometer-sized metal pads below the helium. Information is stored in the lowest hydrogenic levels. With electric fields, at temperatures of 10(-2) kelvin, changes in the wave function can be made in nanoseconds. Wave function coherence times are 0.1 millisecond. The wave function is read out with an inverted dc voltage, which releases excited electrons from the surface.

Critical point explosions in two-dimensional wave fields

We discuss degenerate critical points in two-dimensional fields described by complex wave functions. Typical examples include the familiar optical field in a plane normal to the direction of propagation, electrons in two-dimensional heterojunctions, the Abrikosov lattice in superconductors, thin films of liquid helium at low temperatures, etc. Using simple topological arguments, we show that under the influence of small perturbations a degenerate critical point can decay explosively into a large number of irreducible (nondegenerate) components, and that this explosion may trigger a chain reaction that fragments other aspects of the wave field. We also discuss controlled explosions designed to produce particular decay products. We show that vortices (phase singularities) may be divided into two classes, generic and nongeneric, and that each class has its own special properties and modes of decay. The fallout from a critical point explosion can be extensive. The mound of debris resulting from decay of a single 5th-order optical vortex can easily contain over 150 new critical points. In superconductors vortex orders can approach 10 3, and the number of new critical points generated during explosion of vortices of this order can exceed 5×10 6.

Phase diagram of the screened two-dimensional electron crystal

The melting line of the two-dimensional crystal formed by surface electrons above liquid helium films adsorbed on PMMA has been mapped out by means of a microwave cavity technique at temperatures above 1 K. The influence of the dielectric substrate on such a transition has been measured at different temperatures and at various helium film thicknesses. The results are found to be in good agreement with existing theories. In general, screening reduces the stability range of the Wigner crystal in the phase diagram of the surface electrons.

Aharonov-Bohm effect for electrons on a liquid-helium surface

A method for obtaining finite electronic systems on a liquid-helium surface is proposed. If a thin film of liquid helium lies above a bottom capacitor plate made in the form of metal rings connected with one another, then electrons will accumulate in potential troughs near these rings. The purity of the helium surface, i.e., the absence of impurities and pinning centers on it, affords an excellent opportunity for investigating the Aharonov-Bohm effect in an ideal ring of a Wigner crystal and a Luttinger liquid

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.

Stationary soliton on a charged surface of liquid helium and hydrogen films

A change in the shape of a charged surface of liquid hydrogen and helium — the formation of a solitary wave (a positively charged hump for hydrogen and a negatively charged dimple for helium)-is observed in an electric field exceeding a critical value under conditions of total compensation of the applied field by the surface charge.

Polaron impurity states on a liquid helium film

We calculate the ground and the first excited states of an electron trapped within a dimple on a thin liquid helium film. The electron is localized by a positive impurity charge which is located on the substrate supporting the film. The mechanical equilibrium equation for the liquid helium surface is coupled to the Schr\odinger equation which are solved using a variational approach. The energies of the ground and first excited states are obtained as a function of the helium film thickness and external electrical, and magnetic fields which are directly perpendicular to the two-dimensional system.