Surface Of Helium Research Articles

A mixed scenario for the reconstruction of a charged helium surface

A mixed scenario for the periodic reconstruction of a charged surface of a liquid when the liquid is close to occupancy saturation by 2D charges is discussed. It is shown that the unit cell of the periodic structure arising is a modified multicharged dimple.

Thermodynamic model of helium and hydrogen co-implanted silicon surface layer splitting

A thermodynamic model of the evolution of microcracks in silicon caused by helium and hydrogen co-implantation during annealing was studied. The crack growth rate relies on the amount of helium atoms and hydrogen molecules present. Here, the crack radius was studied as a function of annealing time and temperature, and compared with experimental results. The mean crack radius was found to be proportional to the annealing temperature and the helium and hydrogen implanted fluence. The gas desorption should be considered during annealing process.

10.1007/s11447-008-2010-3

The temperature dependence of electron mobility is examined. We calculate the contribution to the electron scattering rate from the surface level atoms (SLAs), proposed in [10]. This contribution is substantial at low temperatures T < 0.5, when the He vapor concentration is exponentially small. We also study the effect of depopulation of the lowest energy subband, which leads to an increase in the electron mobility at high temperature. The results explain certain long-standing discrepancies between the existing theory and experiment on electron mobility on the surface of liquid helium.

Characteristics of the Helium Circulation System on the 440 CH Vector type MEG

Event Abstract Back to Event Characteristics of the Helium Circulation System on the 440 CH Vector type MEG Tsunehiro Takeda1*, Masayoshi Okamoto1 and Keishi Katagiri1 1 University of Tokyo , Japan We report the characteristics of the Helium Circulation System (HCS) mounted on the 440 CH vector type MEG of the University of Tokyo Japan, that re-liquefies all the evaporating helium gas, consumes far less electric power and has low magnetic noises. It collects warm helium gas about 300 K, cools it to about 40K and returns it to the neck tube of the Dewar of the MEG to keep it cold. It also collects helium gas just above the liquid helium surface while it is still cold, re-liquefies and returns it to the Dewar. A special transfer tube (TT) about 2 m length with 7 multi-concentric pipes was developed to allow the dual helium streams. It separates the HCS with a MEG to reduce magnetic noise. A refiner to collect the contaminating gases such as oxygen and nitrogen effectively by freezing the gases is developed. It has an electric heater to remove the frozen contamination in the form of gases into the air. A gas flow controller is also developed, which automatically control the heater to cleanup the contamination. The developed TT has very low heat inflow less than 0.1W/m to the liquid helium ensuring the efficient operation. The HCS can re-liquefy up to 35.5 l/D of liquid helium from the evaporated helium gas using two 1.5W@4.2K GM cryocoolers (SRDK-415D, Sumitomo Heavy Industries, Ltd.). It has been confirmed that the HCS could be used with the real MEG system without any noise problem for about two year. The noise level is below 10 fT/Hz1/2 for 2-40 Hz, below 30 fT/Hz1/2 at 1 Hz, and 200 fT/Hz1/2 at 50 Hz, which is the power supply frequency. As the amount of liquid helium used decreases less than one percent, the maintenance cost of the MEG becomes less than one-tenth of the previous cost. Conference: Biomag 2010 - 17th International Conference on Biomagnetism , Dubrovnik, Croatia, 28 Mar - 1 Apr, 2010. Presentation Type: Poster Presentation Topic: Instrumentation and Multi-modal Integrations: MEG, Low-field MRI,EEG, fMRI,TMS,NIRS Citation: Takeda T, Okamoto M and Katagiri K (2010). Characteristics of the Helium Circulation System on the 440 CH Vector type MEG. Front. Neurosci. Conference Abstract: Biomag 2010 - 17th International Conference on Biomagnetism . doi: 10.3389/conf.fnins.2010.06.00026 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 19 Mar 2010; Published Online: 19 Mar 2010. * Correspondence: Tsunehiro Takeda, University of Tokyo, Kashiwa, Japan, takeda@brain.k.u-tokyo.ac.jp Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Tsunehiro Takeda Masayoshi Okamoto Keishi Katagiri Google Tsunehiro Takeda Masayoshi Okamoto Keishi Katagiri Google Scholar Tsunehiro Takeda Masayoshi Okamoto Keishi Katagiri PubMed Tsunehiro Takeda Masayoshi Okamoto Keishi Katagiri Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

INFLUENCE OF HELIUM SURFACE ON PHOTODETACHMENT OF NEGATIVE IONS

The photodetachment of H - on the surface of helium is investigated by quantum and semiclassical Green function approaches. It is found that a new sinusoidal oscillation is introduced in the photodetachment cross-section because of the existence of helium surface. Both quantum and semiclassical approaches give similar results but with minor difference when the photon energy is higher than the surface potential. The new oscillation is formed due to transmissions and reflections of the photodetached electronic waves on helium surface. It provides a simple system to test quantum reflection.

Transport of Electrons on Liquid Helium Across a Tunable Potential Barrier in a Point Contact-like Geometry

We present transport measurements of electrons bound to the surface of superfluid 4He in a microchannel of width 10 μm. A set of electrodes 2 μm beneath the helium surface, fabricated in a split-gate configuration using electron beam lithography, are used to control the current along the microchannel as in a point contact device. As the split-gate bias V SG is swept negative the current decreases to zero. The value of V SG at which the current is suppressed is dependent on the AC driving voltage applied to the electron system. We explain our results using a simple model in which a potential barrier created by the split-gate electrodes must be overcome in order to allow current to flow in the microchannel. The control of electron transport in such confined geometries may offer new possibilities for mesoscopic experiments with electrons on the surface of liquid helium.

Ground-plane screening of Coulomb interactions by a nearby two-dimensional system

The use of a nearby metallic ground-plane to limit the range of the Coulomb interactions between carriers is a useful approach in studying the physics of two-dimensional (2D) systems. This approach has been used to study Wigner crystallization of electrons on the surface of liquid helium, and insulating behaviour of semiconductor two-dimensional systems. We recently reported a variation of this technique where an adjacent 2D system was used as the ground plane instead of a metal gate. In this paper, we perform calculations of the ground plane screening effect of a 2D system and show that it is as effective as a metal in screening Coulomb interactions.

Ground-plane screening of Coulomb interactions in two-dimensional systems: How effectively can one two-dimensional system screen interactions in another

The use of a nearby metallic ground-plane to limit the range of the Coulomb interactions between carriers is a useful approach in studying the physics of two-dimensional (2D) systems. This approach has been used to study Wigner crystallization of electrons on the surface of liquid helium, and most recently, the insulating and metallic states of semiconductor-based two-dimensional systems. In this paper, we perform calculations of the screening effect of one 2D system on another and show that a 2D system is at least as effective as a metal in screening Coulomb interactions. We also show that the recent observation of the reduced effect of the ground-plane when the 2D system is in the metallic regime is due to intralayer screening.

Thermo-mechanical analysis of a DEMO divertor cooling finger under the EFREMOV test conditions

A divertor component of the forthcoming DEMO fusion reactor should be able to withstand heat flux loads larger than 10 MW/m 2. Successful design should withstand high flux loads for a number of load cycles since initially the DEMO reactor is expected to operate in a non-steady-state mode. Computations for evaluating the structural response of the divertor published so far have, however, been based on the stationary approach. A combined computational fluid dynamics and structural model for evaluating the structural response of a divertor under the non-stationary load conditions is therefore developed in this work. Heat transfer coefficients between the helium and inner surface of the thimble are calculated first by solving the helium steady-state flow equations. Spatially distributed heat transfer coefficients are then used as a boundary condition in a non-stationary thermo-mechanical analysis of the divertor. This analysis is performed for a number of load cycles under different surface heat flux levels. The model is validated against the EFREMOV test experimental conditions, designed to be close to reactor operation conditions. Good agreement of the highest temperatures on the tile’s top surface with the experimental data is obtained. The results suggest that there are three critical regions in the design where damage could initialize: (a) the thimble’s inner surface with the highest thermal gradients, (b) the tile’s outer surface and (c) the filler layer of the brazed tile-thimble joint where the temperature is higher than permissible. Post-examination data of experimental specimen confirm these conclusions as cracks were observed in the above mentioned areas (a) and (b), while melting of the layer (c) was also observed.

Self-Sustained Microwave Absorption Induced by Extremely High Radiation Intensities in Surface Electrons on Liquid Helium

AbstractA new nonlinear-optical absorption effect is observed in electrons bound to the liquid helium surface. We study absorption of mm-wave radiation due to resonant excitation of electron bound states. Below 1 K, almost all electrons occupy the ground state. Therefore, the system should be transparent for resonant radiation connecting any two excited states. On the contrary, we observe strong absorption peaks associated with transitions between the first excited and the higher excited states. We show that this anomaly results from the bistability of the electron system induced by extremely high radiation intensities and the long electron relaxation time.

Nature of a nonzero frequency shift of a hyperfine transition in two-dimensional atomic hydrogen

We propose the explanation of a surprisingly small hyperfine frequency shift in the two-dimensional (2D) atomic hydrogen bound to the surface of superfluid helium below 0.1 K. Owing to the symmetry considerations, the microwave-induced triplet-singlet transitions of atomic pairs in the fully spin-polarized sample are forbidden. The apparent nonzero shift is associated with the density-dependent wall shift of the hyperfine constant and the pressure shift due to the presence of H atoms in the hyperfine state $a$ not involved in the observed $b\to c$ transition. The interaction of adsorbed atoms with one another effectively decreases the binding energy and, consequently, the wall shift by the amount proportional to their density. The pressure shift of the $b\to c$ resonance comes from the fact that the impurity $a$-state atoms interact differently with the initial $b$-state and final $c$-state atoms and is also linear in density. The net effect of the two contributions, both specific for 2D hydrogen, is comparable with the experimental observation. To our knowledge, this is the first mentioning of the density-dependent wall shift. We also show that the difference between the triplet and singlet scattering lengths of H atoms, $a_t-a_s=30(5)$ pm, is exactly twice smaller than the value reported by Ahokas {\it et al.}, Phys. Rev. Lett. {\bf101}, 263003 (2008).

Dynamic spin–electric and spin–thermal effects in a system of electrons on the surface of liquid helium

A study is made of the natural spin-electric and thermoelectric oscillations in an electron ring and of forced oscillations of the spin density and temperature driven with the help of external electrodes. Effects that may be used to investigate the spin characteristics of the system, in particular, spin-flip processes, by means of electrical measurements are proposed.

Conductivity Measurement of Helium Surface Electrons in the Coexistence of Adsorbed 2D Atomic Hydrogen Gas

Cold hydrogen atoms (H) are adsorbed on liquid helium surface to form two-dimensional (2D) quantum gas. Because of the boson nature of H atoms, when 2D H gas is cold and dense enough, it is expected to exhibit a transition to superfluid phase by Kosterlitz-Thouless mechanism. In order to sense the state of 2D H gas, we used helium surface electrons as a probe. Since the conductivity of surface electrons is sensitively influenced by collisions with its environment, such as He vapor atoms and ripplons, it is expected to detect the existence of H atoms on the same surface. We prepared a 2D mixture gas of H and electrons on a helium surface and measured the conductivity of the electrons. Strong magnetic field was applied to avoid chemical processes: electron attachment on H atom and H-H recombination. We employed pulse excited edge-magnetoplasmon (EMP) technique for the conductivity measurement. We show EMP spectrum under the influence of H atoms and discuss the potentiality of surface electrons for probing states of adsorbed 2D H gas.

Capillary turbulence on the surface of normal and superfluid He4

The capillary turbulence on the surface of liquid helium with excitation of surface waves by a harmonic force at low frequency ωp is investigated. It is shown for the first time that at the transition of helium out of the superfluid into the normal state the high-frequency edge ωb of the inertial interval shifts in the direction of low frequencies. The magnitude of the shift is described well on the basis of the theory of weak wave turbulence. In the superfluid state the relative width ωb∕ωp of the inertial interval, where a developed turbulent cascade is established, can reach two decades.

Performance of an efficient Helium Circulation System on a MEG

We report a Helium Circulation System (HCS) that re-liquefies all the evaporating helium gas, consumes far less power and has extremely lower magnetic noise compared with conventional systems. It collects warm helium gas about 300 K, cools it to about 40K and returns it to the neck tube of the Dewar to keep it cold. It also collects helium gas just above the liquid helium surface while it is still cold, re-liquefies and returns it to the Dewar. A special transfer tube (TT) about 2 m length with 7 multi-concentric pipes was developed to allow the dual helium streams. It separates the HCS with a MEG to reduce magnetic noise. A refiner to collect the contaminating gases such as oxygen and nitrogen effectively by freezing the gases is developed. It has an electric heater to remove the frozen contamination in the form of gases into the air. A gas flow controller is also developed, which automatically control the heater to cleanup the contamination. The developed TT has very low heat inflow less than 0.1W/m to the liquid helium ensuring the efficient operation. The HCS can re-liquefy up to 35.5 1/D of liquid helium from the evaporated helium gas using two 1.5W@4.2K GM cryocoolers (SRDK-415D, Sumitomo Heavy Industries, Ltd.). It has been confirmed that the HCS could be used with the real MEG system without any noise problem for over one year. The maintenance cost (electricity charges and cryocoolers maintenance fee) of the MEG has reduced to be less than 1/10 of the previous cost.

Nonlinear resonant microwave absorption in strongly correlated hot 2D electrons on a liquid helium surface

The microwave (MW) absorption resonance in 2D electrons bound to the surface of liquid 3He is studied at high MW powers. In our experiment, we induce quantum transitions between two lowest surface states of electrons by letting them interact with the resonant MW radiation. Under the conditions of the experiment, the resonant absorption induces strong heating of the electron system. The electron temperature can be accurately determined by measuring the variation of the electron magneto-resistivity at the resonance. Electron heating causes a shift in the resonance frequency, which is measured as a function of the electron temperature. The shift is compared with a theory that takes into account strong electron-electron correlation, and a reasonable agreement is found. When the shift exceeds the width of the absorption line, their emerges bistability of the nonlinear response and hysteresis. Our result demonstrate the occurrence of a new nonlinear optical effect due to strong correlations in the many-electron system on liquid helium.

Neutron reflection from the surfaces of liquid 4He and a Dilute 3He—4He solution

We report and discuss the first neutron reflection measurements from the free surfaces of normal and superfluid 4He and of a liquid 3He-4He mixture. In case of liquid 4He the surface roughness is different above and below the lambda transition, being smoother in the superfluid state. For the superfluid, we also observe the formation of a surface layer ∼ 200 Å thick which has a subtly different neutron scattering cross-section. The results can be interpreted as an enhancement of the Bose-Einstein condensate fraction close to the helium surface. We find that the addition of 3He isotopic impurities leads to the formation of Andreev levels at low temperatures.

A description of bubble growth and gas release of helium implanted tungsten

Bubble growth and gas release during annealing of helium implanted tungsten is described using a Kinetic Monte Carlo approach. Implantation of tungsten with low energy He ions results in the unusual formation of large numbers of oversized surface pores, which eventually lead to drastic changes of the tungsten surface morphology. Traditional rate-theory models are not suitable to simulate surface pore formation. Results of the KMC simulations illustrate the rapid formation of very large helium bubbles and surface pores. The simulation results agree well with observed results of low energy He implanted tungsten experiments, and thus provide an explanation for the observed surface morphology changes caused by low energy He implantation of tungsten.

Electronically excited rubidium atom in a helium cluster or film

We present theoretical studies of helium droplets and films doped with one electronically excited rubidium atom Rb( *) ((2)P). Diffusion and path integral Monte Carlo approaches are used to investigate the energetics and the structure of clusters containing up to 14 helium atoms. The surface of large clusters is approximated by a helium film. The nonpair additive potential energy surface is modeled using a diatomic in molecule scheme. Calculations show that the stable structure of Rb( *)He(n) consists of a seven helium atom ring centered at the rubidium, surrounded by a tirelike second solvation shell. A very different structure is obtained when performing a "vertical Monte Carlo transition." In this approach, a path integral Monte Carlo equilibration starts from the stable configuration of a rubidium atom in the electronic ground state adsorbed to the helium surface after switching to the electronically excited surface. In this case, Rb( *)He(n) relaxes to a weakly bound metastable state in which Rb( *) sits in a shallow dimple. The interpretation of the results is consistent with the recent experimental observations [G. Aubock et al., Phys. Rev. Lett. 101, 035301 (2008)].

Features of the conductivity of a 2D electron crystal over liquid helium at high driving fields

An electron crystal with a surface electron density ns=6.2×108cm−2 over the surface of superfluid liquid helium is investigated as a function of the amplitude of an ac electric field (1–30mV∕cm) in the plane of the layer at a temperature T=80mK. The frequency dependence of the response of the experimental cell to an ac voltage is measured at frequencies where coupled electron-ripplon resonances are excited (1–20MHz), and the dependence of the conductivity of an electron layer on the driving field at two fixed frequencies, 4 and 5MHz, is investigated. A complex nonmonotonic dependence of the conductivity of the crystal on the driving field is observed, which apparently reflects a dynamic phase transition in the system.