Helium Films Research Articles

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.

Electronically excited rubidium atom in helium clusters and films. II. Second excited state and absorption spectrum

Following our work on the study of helium droplets and film doped with one electronically excited rubidium atom Rb(∗) ((2)P) [M. Leino, A. Viel, and R. E. Zillich, J. Chem. Phys. 129, 184308 (2008)], we focus in this paper on the second excited state. We present theoretical studies of such droplets and films using quantum Monte Carlo approaches. Diffusion and path integral Monte Carlo algorithms combined with a diatomics-in-molecule scheme to model the nonpair additive potential energy surface are used to investigate the energetics and the structure of Rb(∗)He(n) clusters. Helium films as a model for the limit of large clusters are also considered. As in our work on the first electronic excited state, our present calculations find stable Rb(∗)He(n) clusters. The structures obtained are however different with a He-Rb(∗)-He exciplex core to which more helium atoms are weakly attached, preferentially on one end of the core exciplex. The electronic absorption spectrum is also presented for increasing cluster sizes as well as for the film.

Characterization of MEMS Devices for the Study of Superfluid Helium Films

Measurements on the mechanical properties of MEMS resonators were performed to characterize such devices at room temperature and low temperatures. Using state-of-the-art silicon integrated circuit technology, we have designed, fabricated, and manufactured resonators consisting of a pair of parallel plates with a well-defined gap whose size can be controlled with a high accuracy down to the sub-micron range. A full study of resonance properties at various pressures was performed at room temperature. We will discuss the details of design, fabrication, and operation. These studies open up a window of opportunities to look for novel phenomena in quantum fluids such as in superfluid 3He films.

RESISTANCE IN SUPERCONDUCTORS

In this pedagogical review, we discuss how electrical resistance can arise in superconductors. Starting with the idea of the superconducting order parameter as a condensate wave function, we introduce vortices as topological excitations with quantized phase winding, and we show how phase slips occur when vortices cross the sample. Superconductors exhibit non-zero electrical resistance under circumstances where phase slips occur at a finite rate. For one-dimensional superconductors or Josephson junctions, phase slips can occur at isolated points in space-time. Phase slip rates may be controlled by thermal activation over a free-energy barrier, or in some circumstances, at low temperatures, by quantum tunneling through a barrier. We present an overview of several phenomena involving vortices that have direct implications for the electrical resistance of superconductors, including the Berezinskii–Kosterlitz–Thouless transition for vortex-proliferation in thin films, and the effects of vortex pinning in bulk type II superconductors on the nonlinear resistivity of these materials in an applied magnetic field. We discuss how quantum fluctuations can cause phase slips and review the non-trivial role of dissipation on such fluctuations. We present a basic picture of the superconductor-to-insulator quantum phase transitions in films, wires, and Josephson junctions. We point out related problems in superfluid helium films and systems of ultra-cold trapped atoms. While our emphasis is on theoretical concepts, we also briefly describe experimental results, and we underline some of the open questions.

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.

Magnetic trapping of NH molecules with 20 s lifetimes

Buffer gas cooling is used to trap NH molecules with 1/e lifetimes exceeding 20 s. Helium vapor generated by laser desorption of a helium film is employed to thermalize 105 molecules at a temperature of 500 mK in a 3.9 T magnetic trap. Long molecule trapping times are attained through rapid pumpout of residual buffer gas. Molecules experience a helium background gas density below 1×1012 cm−3.

Influence of Discharge Atmosphere on the Ageing Behaviour of Plasma-Treated Polylactic Acid

The effect of a plasma treatment on polymers is not permanent, since the treated surfaces tend to recover to the untreated state (ageing process). This paper investigates the influence of discharge atmosphere on the ageing behaviour of plasma-treated PLA foils: these foils are plasma-treated with a DBD in 4 different atmospheres (air, nitrogen, argon and helium) and are subsequently stored in air. Results of contact angle and XPS measurements show that the discharge gas has a significant influence on the ageing behaviour of the PLA foils. This influence can be explained by the different cross-linking degree of the plasma-treated surfaces: helium and argon plasma-treated PLA films have a high cross-linking degree, which limits polymer chain mobility and as a result reduces the ageing process. In contrast, the ageing behaviour of air and nitrogen plasma-treated films is more pronounced due to their low cross-linking degree.

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.

Temperature and Pinning Effects on Driving a 2D Electron System on a Helium Film: A Numerical Study

Using numerical simulations we investigated the dynamic response to an externally driven force of a classical two-dimensional (2D) electron system on a helium film at finite temperatures. A potential barrier located at the center of the system behaves as a pinning center that results in an insulator state below a threshold driving force. We have found that the current-voltage characteristic obeys the scaling relation I=f ξ, with ξ ranging from ∼(1.0–1.7) for different pinning strengths and temperatures. Our results may be used to understand the spread range of ξ in experiments with typical characteristic of plastic depinning.

Porous plug and superfluid helium film flow suppressor for the soft X-ray spectrometer onboard Astro-H

Suppression of superfluid helium flow is critical for the Soft X-ray Spectrometer (SXS) onboard Astro-H, to achieve a life time of the liquid helium over 5 years. The superfluid film flow must be sufficiently small, compared to a nominal helium gas flow rate of the SXS ( 25 μ g / s ) . For this purpose, four devices composed of a porous plug, an orifice, a heat exchanger, and knife edge devices will be employed based on the experience of the X-ray microcalorimeter (XRS for X-Ray Spectrometer) onboard Suzaku. The porous plug is a phase separator of the liquid and gas helium. A potential film flow leaking from the porous plug is suppressed by the orifice. Almost all the remaining film flow evaporates at the heat exchanger. The knife edge devices stop the remaining film flow by using atomically sharp edges. In this paper, we describe the principle and design of these four devices.

Electrostatic Manipulation of Level of Bulk Liquid for Studies of Saturated Superfluid 3He Films

In studies of saturated superfluid helium films, the level of bulk liquid is an important parameter in determining the thickness of the film. To change the level at low temperatures, a complicated system with a bellows and an extra fill line is usually needed. In this paper, we report a simple device that can be used to change the level using an electrostatic force. This level controller consists of a stack of 60 silver plates of 0.1 mm thickness, separated from each other by a 0.1 mm gap. The application of a dc voltage V to every other plate pulls the dielectric liquid 3He into the gaps, resulting in the drop of the bulk level. The change in level is in agreement with that expected by the balance between the electrostatic force and gravity, and the decrease is as large as 0.27 mm at V=200 V. The level controller is also used to detect the superfluid transition of bulk liquid 3He through the sudden reduction of the response time after V is varied.

Transport in Mesoscopic 3He Films on Rough Surfaces

Measurements of the flow of thick 3He films over a highly polished silver surface, using a high precision torsional oscillator, have found unexpectedly long momentum relaxation times (Casey et al., Phys. Rev. Lett. 92, 255301, 2004). This results in a decoupling of the normal state helium film from the oscillator motion at low temperatures. In the ballistic regime the relaxation rate varies linearly with temperature. This result is explained by a theory in which the effect of surface roughness is incorporated into the transport equation as a disorder potential (Meyerovich and Stepaniants, J. Phys., Condens. Matter 12, 5575, 2000). This potential is fully characterised experimentally by AFM measurements of the surface roughness power spectrum, which we find to have a stretched exponential form. Incorporating the measured spectrum into the theory allows a direct comparison with the observed relaxation rate, with good agreement. The anomalous temperature dependence of the relaxation rate is accounted for by interference between bulk inelastic scattering and this weak surface elastic scattering. Decorating the substrate with scattering centres restores film-substrate coupling at the measurement frequency of 2.5 kHz. These results on the nature of quasiparticle scattering at rough surfaces may have implications in the understanding of the effects of confinement on superfluid 3He.

Helium Film Formed in 1.2 nm Pore in Zeolite Templated Carbon

We have measured the vapor pressure of 4He adsorbed in zeolite templated carbon (ZTC) at 4.2 K, in order to examine 4He film growth in the nanopore. ZTC has a framework made of non-stacked graphene, and has a relatively uniform 1.2 nm pore with three-dimensional (3D) periodicity of 1.4 nm. The nanoporous substrate is a candidate to realize superfluid 4He film with closer 3D connectivity than those studied so far. From the vapor pressure, coverage dependences of the thickness and the two-dimensional (2D) isothermal compressibility of 4He film were derived using thermodynamic equations. The 2D compressibility indicates the onset coverage n1 of the second adsorption layer around 20 µmol/m2. It is confirmed that the 4He film in the 1.2 nm pores grows uniformly at least up to 1.3–1.5 n1.

Erratum: Influence of Film-Mediated Interactions on the Microwave and Radio Frequency Spectrum of Spin-Polarized Hydrogen on Helium Films [Phys. Rev. Lett.101, 165301 (2008)

Received 6 July 2009DOI:https://doi.org/10.1103/PhysRevLett.103.049902©2009 American Physical Society

Thermionic emission and a novel electron collector in a liquid helium environment

We study two techniques to create electrons in a liquid helium environment. One is thermionic emission of tungsten filaments in a low temperature cell in the vapor phase with a superfluid helium film covering all surfaces; the other is operating a glowing filament immersed in bulk liquid helium. We present both the steady state and rapid sweep I-V curves and the electron current yield. These curves, having a negative dynamic resistance region, differ remarkably from those of a vacuum tube filament. A novel low temperature vapor-phase electron collector for which the insulating helium film on the collector surface can be removed is used to measure emission current. We also discuss our achievement of producing multielectron bubbles in liquid helium by a new method.

Numerical analysis of two-dimensional quantum turbulence

This is the first numerical study of two-dimensional quantum turbulence by the Gross-Pitaevskii equation. We start from a uniform state and obtain quasi-steady quantum turbulence. Then the energy spectrum of the incompressible kinetic energy shows an indication of inverse cascade which is a remarkable phenomenon of two-dimensional classical turbulence. It is possible to make experiments of two-dimensional quantum turbulence in the film of superfluid helium and atomic Bose-Einstein condensates.

How the thickness of a liquid helium film influences the spectrum of electrons localized above its surface

We consider the problem of finding the energy spectrum of electrons localized above the surface of a liquid helium film under the electrostatic attraction of induced charges. We show that the forces of attraction to the charges induced on the surface of a conductor located beneath the film begin to additionally influence the forces of attraction of the electrons to the free liquid surface as the film thickness decreases. When the film thickness becomes less than 10μm, this influence causes a significant increase in the energy difference between the lower levels and a decrease in their depth. We suggest a numerical method for solving the problem and present the results of our calculations of the energy spectrum of electrons localized above the surfaces of liquid 3He, 4He, and neon films. The influence of the pressing electric field on the energy spectrum is considered.

Fluidization of granular media wetted by liquidHe4

We explore experimentally the fluidization of vertically agitated polymethylmethacrylate spheres wetted by liquid 4He . By controlling the temperature around the lambda point, we change the properties of the wetting liquid from a normal fluid (helium I) to a superfluid (helium II). For wetting by helium I, the critical acceleration for fluidization (Gamma_{c}) shows a steep increase close to the saturation of the vapor pressure in the sample cell. For helium II wetting, Gamma_{c} starts to increase at about 75% saturation, indicating that capillary bridges are enhanced by the superflow of the unsaturated helium film. Above saturation, Gamma_{c} enters a plateau regime where the capillary force between particles is independent of the bridge volume. The plateau value is found to vary with temperature and shows a peak at 2.1K , which we attribute to the influence of the specific heat of liquid helium.

Addition spectra of Wigner islands of electrons on superfluid helium

We present here the first experimental study of Wigner islands formed by electrons floating over helium. Electrons are trapped electrostatically in a mesoscopic structure covered with a helium film, behaving as a quantum dot. By removing electrons one by one, we are able to find the addition spectrum, i.e. the energy required to add (or extract) one electron from the trap with occupation number $N$. Experimental addition spectra are compared with Monte Carlo simulations for the actual trap geometry, confirming the ordered state of electrons over helium in the island.

连续波DF/HF化学激光器收缩段氦气膜注入式新型喷管的理论研究

连续波DF/HF化学激光器收缩段氦气膜注入式新型喷管的理论研究