Thin Helium Films Research Articles

Measurement of High Density Electrons Above a Helium Film on an Amorphous Metal Substrate

We have measured two-dimensional electron systems bound to a thin helium film supported by a metallic substrate. We report on our measurement of electron density obtained via a Kelvin probe technique. The underlying metallic substrate is an amorphous metallic alloy (TaWSi), which can support large uniform densities due to its low surface roughness and homogeneous work function. We find that this substrate is able to support high enough densities that the electrons are expected to be Fermi-degenerate.

Vortex Motion Quantifies Strong Dissipation in a Holographic Superfluid.

Holographic duality provides a description of strongly coupled quantum systems in terms of weakly coupled gravitational theories in a higher-dimensional space. It is a challenge, however, to quantitatively determine the physical parameters of the quantum systems corresponding to generic holographic theories. Here, we address this problem for the two-dimensional holographic superfluid, known to exhibit strong dissipation. We numerically simulate the motion of a vortex dipole and perform a high-precision matching of the corresponding dynamics resulting from the dissipative Gross-Pitaevskii equation. Excellent agreement is found for the vortex core shape and the spatiotemporal trajectories. A further comparison to the Hall-Vinen-Iordanskii equations for point vortices interacting with the superfluid allows us to determine the friction parameters of the holographic superfluid. Our results suggest that holographic vortex dynamics can be applied to experimentally accessible superfluids like strongly coupled ultracold Bose gases or thin helium films with temperatures in the Kelvin range. This would make holographic far-from-equilibrium dynamics and turbulence amenable to experimental tests.

Transport Measurements of Surface Electrons in 200-nm-Deep Helium-Filled Microchannels Above Amorphous Metallic Electrodes

We report transport measurements of electrons on helium in a microchannel device where the channels are 200 nm deep and 3 $\mu$m wide. The channels are fabricated above amorphous metallic Ta$_{40}$W$_{40}$Si$_{20}$, which has surface roughness below 1 nm and minimal variations in work function across the surface due to the absence of polycrystalline grains. We are able to set the electron density in the channels using a ground plane. We estimate a mobility of 300 cm$^2$/V$\cdot$s and electron densities as high as 2.56$\times$10$^9$ cm$^{-2}$. We demonstrate control of the transport using a barrier which enables pinchoff at a central microchannel connecting two reservoirs. The conductance through the central microchannel is measured to be 10 nS for an electron density of 1.58$\times$10$^9$ cm$^{-2}$. Our work extends transport measurements of surface electrons to thin helium films in microchannel devices above metallic substrates.

Quasi-One-Dimensional Electronic States Inside and Outside Helium-Plated Carbon Nanotubes

About one-half a century ago, it was realized that electrons experience a repulsive barrier when approaching the surface of condensed phases of helium, hydrogen, and neon. This led to the proposal and subsequent observation of image-potential surface-bound electronic states, which exhibit intriguing quasi-two-dimensional behavior. In the present work, we report similar quasi-one-dimensional electronic states by exploring single-wall carbon nanotubes coated both inside and outside by thin helium films. Electrons near such structures are localized in the radial direction, but free to move along the nanotube axis. The many-body aspects of the system are discussed qualitatively.

Stability of Surface State Electrons on Helium Films

Electrons on helium substrates form a model Coulomb system in which the transition from classical electron liquid to Wigner crystal is readily observed. However, attempts to increase the electron density in order to observe the ‘quantum melting’ of the system to a Fermi degenerate gas are hindered by an instability of the helium surface. Here we describe experimental efforts to reach the degenerate regime on thin helium films and microstructured substrates, for which the surface instability is suppressed. We demonstrate that, although the electron densities obtained exceed those for bulk helium substrates, observation of quantum melting remains challenging. We discuss possible solutions to the technical challenges involved.

A Study of the Superfluid Transition in $$^4\hbox {He}$$ 4 He Films Adsorbed to Rough $$\hbox {CaF}_2$$ CaF 2 Over a Large Temperature Range

Rough two-dimensional substrates, such as thermally deposited \(\hbox {CaF}_2\), have been shown to modify the experimental signatures of the superfluid transition in adsorbed thin helium films. Previous experiments have investigated a series of increasingly rough surfaces over a limited temperature range and found that the features at the superfluid transition become less defined as substrate roughness is increased. In this work we study the superfluid transition in adsorbed helium films over a wide range of temperatures for a series of \(\hbox {CaF}_2\) substrates. Our results show that as the transition temperature increases the abrupt jump in superfluid density at the transition becomes less distinct. The changing characteristics of the transition on a single \(\hbox {CaF}_2\) substrate with temperature suggest that the reduced observability of the transition on rough substrates cannot be explained entirely by superfluid drag. We discuss several other possible scenarios which may be relevant to the helium films on \(\hbox {CaF}_2\).

Boundary Tension at the Interface of Nanoscopic Helium Films on an Heterogeneous Substrate

This paper reports the first calculation of the two-dimensional interfacial profile and energetics of nanoscopically thin films of helium, on an heterogeneous planar substrate consisting of two adjoining metals. The calculations are performed in the frame of density functional theory at zero temperature, with the purpose of identifying the formation process of the interface at the boundary between the two substrates when few atomic layers are involved, to elucidate the possible relationship of the magnitude of the boundary tension with the displacement of layers between the half films, and to extract keys to organize future calculations of film coexistence at finite temperatures.

Dynamic phase transitions in a two-dimensional electronic crystal and in a two-dimensional helium film

The phase transitions induced by an electric field in the plane of the electron layer (dynamic transitions) in a two-dimensional (2D) electronic crystal over liquid helium are investigated experimentally. They are compared with the superfluid transition in thin helium films at nonlinear conditions (at high velocities of a substrate). A qualitative correspondence between the transitions is found. Because the melting of the two-dimensional crystal and the superfluid transition in the two-dimensional film belong to one type of phase transitions, the correspondence found indicates that the dynamic phase transition in the 2D electronic crystal can be considered as a nonlinear or dynamic melting.

A Low Power Photoemission Source for Electrons on Liquid Helium

Electrons on the surface of liquid helium are a widely studied system that may also provide a promising method to implement a quantum computer. One experimental challenge in these studies is to generate electrons on the helium surface in a reliable manner without heating the cryo-system. An electron source relying on photoemission from a zinc film has been previously described using a high power continuous light source that heated the low temperature system. This work has been reproduced more compactly by using a low power pulsed lamp that avoids any heating. About 5×103 electrons are collected on 1 cm2 of helium surface for every pulse of light. A time-resolved experiment suggests that electrons are either emitted over or tunnel through the 1 eV barrier formed by the thin superfluid helium film on the zinc surface. No evidence of trapping or bubble formation is seen.

Coupling and proximity effects in the superfluid transition in 4He dots

Some effects that have long been observed in superconductors are difficult to see in superfluid helium-4, despite the similar physics governing these systems, owing to the small correlation length in helium-4. Proximity effects have now been observed in that system, using an array of micrometre-size boxes linked by a thin helium film.

Electron tunneling through barriers of adjustable width: Role of the image potential and the wetting behavior of Cs by He

Photocurrents from cesium, flowing through gaseous 3 He or 4 He and also through thin liquid helium films, are investigated as a function of the chemical potential of helium at T = 1.33 K. At low pressures, the two isotopes behave similarly as the photocurrent is governed by scattering by the gas. At higher pressures, a film of 3 He grows on the Cs and forms a tunnel barrier; but for 4 He, the film is too thin to form a tunnel barrier below liquid-vapor coexistence. This is because 4 He does not wet Cs at this temperature and the finite thickness needed to form a tunnel barrier is larger than the thickness of the thin-film state. 3 He enables a continuously variable tunnel barrier thickness to be studied. We show that the image potential is important and confirm that an electron in liquid 3 He has a potential energy of 1.0 eV. We find that the thickness d of a helium film is given by C3 /d 3 =� kBT lnp /p0 for films thicker than approximately three monolayers.

Generic electron mobility in surface states on thin helium films

We study the mobility of electrons adsorbed on thin $^{4}\mathrm{He}$ films on flat, uniform, dielectric substrates. Utilizing the time-dependent version of the Euler-Lagrange, hypernetted chain variational theory, we compute the inelastic scattering rate of an electron due to collisions with film excitations (third sound). We obtain an analytic result valid in the long-wavelength limit. In agreement with experiment, the mobility shows oscillations due to the underlying transverse film structure. The oscillations are due to the explicit appearance of the third sound speed in the scattering rate, since the third sound speed itself oscillates in conjunction with the $^{4}\mathrm{He}$ film structure. The calculated mobilities tend to be higher than reported mobilities on thin films. We attribute this difference to the contribution to the mobility from substrate structure and defects that are omitted in this model. We interpret our results as generic mobilities that are valid in the limit of perfectly smooth, structureless substrates.

On the design of capacitive sensors using flexible electrodes for multipurpose measurements

This article evaluates the potential of capacitive measurements using flexible electrodes to access various physical quantities. These electrodes are made of a thin metallic film, typical thickness 0.2 microm, evaporated on a plastic substrate. Their large flexibility enables them to be mounted in complex geometries such as curved surfaces. In the configuration of planar condensers, using a very sensitive commercial capacitive bridge and a three-terminal measurement method, several measurements are presented. A relative resolution of 10(-8) for the thermal expansion of samples is obtained at low temperature in a differential configuration. The same technique adopted for pressure gauge measurements at low temperature led to a typical 0.1 Pa resolution over a dynamic range of 10(4) Pa. In the configuration of interleaved electrodes, condensers have been used to measure wetting by either bulk liquid helium or by thin continuous helium films in a cylindrical pipe. Both experimental and numerical evidence is provided, showing that the close proximity of a reference ground potential significantly increases the relative sensitivity to fluid wetting. Further, interleaved electrodes can be used to access both the area that is covered by a liquid film but also to determine the thickness of this film, provided it is comparable to the periodicity of the electrode pattern.

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.

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.

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

Quartz microbalance study of superfluid helium thin films adsorbed on a hydrogen pre-plated gold surface

We have developed a highly sensitive torsional oscillator technique for probing the effect of the substrate character on the Kosterlitz–Thouless [1] transition in thin helium films. We have measured the vortex diffusivity in thin superfluid helium films adsorbed on a hydrogen pre-plated gold substrate. Helium adsorbed on such a surface exhibits the Kosterlitz–Thouless transition at pressure substantially higher than that for helium adsorbed on a gold substrate. We study finite amplitude and frequency effects for substrate velocities up to 32 cm/ s and in the frequency range from 3.3 to 36 MHz for single quartz crystal microbalance.

Torsion oscillator study of three-dimensional superfluidity of [formula omitted] thin films in [formula omitted] porous glass

The superfluid density of thin helium films (superfluid transition temperature 0.1 K<T C <0.88 K ) condensed on 10 μm porous glasses was studied. In spite of two-dimensional linear behavior of T C-film thickness relation, the three-dimensional coherence length is found to be larger than the pore size over the whole superfluid temperature range. We discuss the three-dimensional nature of the superfluidity of the system.

Surface excitations in thin helium films on silica aerogel

First measurements are reported on pure surface excitations in thin superfluid 4He films on silica aerogel. The ripplon dispersion curve is found to be the same for helium on graphite and silica aerogel substrates. However, the layered roton line width in helium films on aerogel shows a pronounced broadening with respect to that found in helium films on graphite.

Instability and reconstruction of a thin charged helium films

Reconstruction of thin charged liquid films is discussed within of the multielectron dimple formation scenario. It is shown, that two multielectron dimples on bulk helium can form metastabile bound statc with the equilibrium dimple-dimple distance around capillary length. Such a property is a suitable indicator for the possibility of periodical bulk chat-ged liquid surface reconstruction. In the case of thin helium films dimple-dimple bound state don't exists. And therefore the periodical reconstruction becomes impossible.