Liquid Helium Film Research Articles

On the Possibility of a Significant Increase in the Storage Time of Ultracold Neutrons in Traps Coated with a Liquid Helium Film

It is shown that rough inner walls of a trap of ultracold neutrons can be coated with a superfluid helium film much thicker than the depth of penetration of ultracold neutrons into helium. This coating should reduce the rate of loss of ultracold neutrons caused by absorption in the walls of the trap by orders of magnitude. It is demonstrated that triangular roughness is more efficient than rectangular for the reduction of the rate of loss of ultracold neutrons. Triangular roughness is more easily implemented technically and such diffraction gratings are fabricated industrially. Other methods are proposed to increase the thickness of the protective helium film.

Inelastic Neutron Scattering As an Indication of a New Type of Gapped Surface Excitations in Liquid He

We analyze the experimental data on inelastic neutron scattering by a thin ~5-atomic-layer film of liquid helium at three different temperatures: T=0.4K, 0.98K and 1.3K. These data were partially published previously, but here we present them in a better quality and at various temperatures. The neutron scattering intensity plots, in addition to the previously know dispersion of phonons and ripplons, suggest a branch of gapped surface excitations with activation energy $\sim 4.5$K and the dispersion similar to that expected for surfons - the bound quantum states of helium atoms above liquid helium surface, proposed and investigated theoretically. These data, probably, provide the first direct experimental confirmation of surfons. Before these surface excitations received only indirect experimental substantiation, based on the temperature dependence of surface tension coefficient and on their interaction with surface electrons. The existence of surfons as an additional type of surface excitations, although being debated yet, is very important for various physical properties of He surface. We also analyze previous numerical results on excitations in liquid helium and argue that surface excitations similar to surfons have been previously obtained by numerical calculations and called resonance interface states.

Quantum reflection of antihydrogen from a liquid helium film

We study the quantum reflection of ultracold antihydrogen atoms bouncing on the surface of a liquid helium film. The Casimir-Polder potential and quantum reflection are calculated for different thicknesses of the film supported by different substrates. Antihydrogen can be protected from annihilation for as long as 1.3 s on a bulk of liquid 4He, and 1.7 s for liquid 3He. These large lifetimes open interesting perspectives for spectroscopic measurements of the free fall acceleration of antihydrogen. Variation of the scattering length with the thickness of a film of helium shows interferences which we interpret through a Liouville transformation of the quantum reflection problem.

Quantum transition and decoherence of levitating polaron on helium film thickness under an electromagnetic field

We have studied the transition probability and decoherence time of levitating polaron in helium film thickness. By using a variational method of Pekar type, the ground and the first excited states of polaron are calculated above the liquid-helium film placed on the polar substrate. It is shown that the polaron transits from the ground to the excited state in the presence of an external electromagnetic field in the plane. We have seen that, in the helium film, the effects of the magnetic and electric fields on the polaron are opposite. It is also shown that the energy, transition probability and decoherence time of the polaron depend sensitively on the helium film thickness. We found that decoherence time decreases as a function of increasing electron–phonon coupling strength and the helium film thickness. It is seen that the film thickness can be considered as a new confinement in our system and can be adjusted in order to reduce decoherence.

Mobility of Surface State Electrons on Liquid Helium Films in Micro- Structured Geometry

The present paper discusses the studies of surface state electrons (SSE) on liquid helium films in confined geometry using suitable substrate structures, microfabricated on a silicon wafer which resembles Field Effect Transistors (so called Helium FET). The sample has a Source and Drain regions, separated by a Gate structure, which consists of 2 gold electrodes with a narrow gap (channel) through which the electron transport takes place. After developing a new model of number of saved electrons as a function of gate barrier, it becomes easier to study the mobility of SSE. The experimental results show a reasonable agreement with the literature.

Finite-temperature Wigner solid and other phases of ripplonic polarons on a helium film

Electrons on liquid helium can form different phases depending on density, and temperature. Also the electron-ripplon coupling strength influences the phase diagram, through the formation of so-called "ripplonic polarons", that change how electrons are localized, and that shifts the transition between the Wigner solid and the liquid phase. We use an all-coupling, finite-temperature variational method to study the formation of a ripplopolaron Wigner solid on a liquid helium film for different regimes of the electron-ripplon coupling strength. In addition to the three known phases of the ripplopolaron system (electron Wigner solid, polaron Wigner solid, and electron fluid), we define and identify a fourth distinct phase, the ripplopolaron liquid. We analyse the transitions between these four phases and calculate the corresponding phase diagrams. This reveals a reentrant melting of the electron solid as a function of temperature. The calculated regions of existence of the Wigner solid are in agreement with recent experimental data.

The Helium Field Effect Transistor (II): Gated Transport of Surface-State Electrons Through Micro-constrictions

We present transport measurements of surface-state electrons on liquid helium films in confined geometry. The measurements are taken using split-gate devices similar to a field effect transistor. The number of electrons passing between the source and drain areas of the device can be precisely controlled by changing the length of the voltage pulse applied to the gate electrode. We find evidence that the effective driving potential depends on electron–electron interactions, as well as the electric field applied to the substrate. Our measurements indicate that the mobility of electrons on helium films can be high and that microfabricated transistor devices allow electron manipulation on length scales close to the interelectron separation. Our experiment is an important step toward investigations of surface-state electron properties at much higher densities, for which the quantum melting of the system to a degenerate Fermi gas should be observed.

Localized Bose-Einstein condensation in disordered liquidHe4films

We present neutron scattering measurements of the phonon-roton and layer modes of liquid helium films adsorbed in 25-\AA{} pore diameter gelsil as a function of temperature. The goal is to determine whether the well-defined phonon-roton and layer modes observed at low temperature at partial fillings of the porous media persist as well-defined modes at higher temperatures above the superfluid phase, as is the case of fully filled pores (3D liquid). We find that well defined P-R modes exist in films at temperatures well above the superfluid phase. This is the case at all fillings investigated. Liquid $^{4}\mathrm{He}$ in porous media is an example of a Bose liquid in disorder. The existence of P-R modes above ${T}_{c}$ suggests the existence of patches or islands of Bose condensed liquid that have order [localized Bose-Einstein condensation (LBEC)] above ${T}_{c}$. The interpretation is that the well-defined P-R modes propagate in the patches of BEC. Gelsil partially filled with liquid helium shows the same LBEC behavior as found previously in fully filled gelsil (as in a 3D liquid).

The polaron state of surface electrons on helium covering a structured substrate

In the temperature range 1.5–2.7 K the conductivity of surface electrons over a liquid helium film that covers a structured silicon substrate containing a regular system of micropores is investigated experimentally. It is found that in the range T ≥ 2.5 K the conductivity drops sharply, which can be explained by the formation of the autolocalized polaron state of an electron over the helium film. The hypothesis that the polaron state of a surface electron appears in a dense helium vapor was examined by calculating the free energy of the polaron with a minimum showing up when the temperature rises up to a certain critical value, and the dependence of the pressing field is in a qualitative agreement with experimental data on the temperature of the sharp drop in the conductivity. The calculation predicts a dependence of the critical temperature on parameters of the potential acting in a plane of the helium surface and associated with distortion of the helium surface due to the structure of the substrate, which favors the appearance of the localized charge over the helium film.

Low-Dimensional Confining Structures on the Surface of Helium Films Suspended on Designed Cavities

We investigate the formation of quantum confined structures on the surface of a liquid helium film suspended on a nanostructured substrate. We show theoretically that, by nanostructuring the substrate, it is possible to change the geometry of the liquid helium surface, opening the possibility of designing and controlling the formation of valleys with different shapes. By applying an external electric field perpendicular to the substrate plane, surface electrons can be trapped into these valleys, as in a quantum dot. We investigate how the external parameters, such as the electric field strength and the height of the liquid helium bath, can be tuned to control the energy spectrum of the trapped surface electrons.

A Study on the Roton-limited Thermal Boundary Shock Wave in Liquid Helium Films

A Study on the Roton-limited Thermal Boundary Shock Wave in Liquid Helium Films

Charged macroparticles over liquid helium

An analysis of properties of a plasma with macroparticles (dust plasma) at cryogenic temperatures led us to the need to account for the effects of helium condensation on these particles. Micrometer particles in the saturated helium vapor are coated with a film of liquid helium with the thickness of about 100 Å. This limits their electronic charge, since the helium film is weakly permeable to electrons. The exceptions are clusters of Cs, Rb and K, which are not wetted by helium.

Superfluid behaviour of a two-dimensional Bose gas

Two-dimensional (2D) systems play a special role in many-body physics. Because of thermal fluctuations, they cannot undergo a conventional phase transition associated to the breaking of a continuous symmetry. Nevertheless they may exhibit a phase transition to a state with quasi-long range order via the Berezinskii-Kosterlitz-Thouless (BKT) mechanism. A paradigm example is the 2D Bose fluid, such as a liquid helium film, which cannot Bose-condense at non-zero temperature although it becomes superfluid above a critical phase space density. Ultracold atomic gases constitute versatile systems in which the 2D quasi-long range coherence and the microscopic nature of the BKT transition were recently explored. However, a direct observation of superfluidity in terms of frictionless flow is still missing for these systems. Here we probe the superfluidity of a 2D trapped Bose gas with a moving obstacle formed by a micron-sized laser beam. We find a dramatic variation of the response of the fluid, depending on its degree of degeneracy at the obstacle location. In particular we do not observe any significant heating in the central, highly degenerate region if the velocity of the obstacle is below a critical value.

James Dewar and the vanishing electrical resistance at absolute zero temperature

History of science is full of surprises and there is no end to instances where experimental observations or theoretical predictions come to overthrow intuitive expectations. But it is one thing to be faced with what for the specific period is considered counterintuitive, and it is another to deal with results whose mere description cannot be realized within the descriptive framework that contemporary theories allow. Of these events we do not have too many. There is, however, one area – low temperature physics – that has offered a plenitude of such events. From the observation of zero electrical resistance of mercury at helium temperatures to the phenomena of superfluid helium, low temperature physics has generously provided us with “counterintuitive” instances: the “unreasonable” effectiveness of the Gorter-Casimir thermodynamic calculations, the “unexpected” results of the Meissner-Ochenfeld experiment, the “unique” macroscopic wave function proposed by Fritz London, the “mysterious” case of the measurement of viscosity of liquid helium below 2 ◦K with two different yet perfectly equivalent ways and the difference of 100 000 in the measured values, the “phantasmagoric” fountain effect as well as the creeping film of liquid helium, these and more, became the trademark of low temperature physics. The “birth date” of all these phenomena, was the discovery of suFigure 1 Variation of electrical resistance of mercury at liquid oxygen temperatures. The dotted part represents the extrapolated expected values (from [7]).

The Helium Field Effect Transistor (I): Storing Surface State Electrons on Helium Films

We present investigations of surface state electrons on liquid helium films in confined geometry, using a suitable substrate structure microfabricated on a silicon wafer, similar to a Field Effect Transistor (FET). The sample has a source and drain region, separated by a gate structure, which consists of two gold electrodes with a narrow gap (channel) through which the transport of the surface state electrons takes place. The sample is illuminated to provide a sufficient number of free carriers in the silicon substrate, such that a well-defined potential distribution is achieved. The eventual goal of these experiments is to study the electron transport through a narrow channel in the various states of the phase diagram of the 2D electron system. In the present work we focus on storing the electrons in the source area of the FET, and investigate the spatial distribution of these electrons. It is shown that under the influence of a potential gradient in the silicon substrate the electrons accumulate in front of the potential barrier of the gate. The electron distribution, governed by Coulomb repulsion and by the substrate potential, is determined experimentally. The result is found to be in good agreement with a parallel-plate capacitor model of the system, developed with the aid of a finite element calculation of the surface potential profile of the device.

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.

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.

Electron Conductivity on Helium Films

Electrons on liquid helium films form a two-dimensional (2D) array with a wide range of electron density. This system is also very interesting for applications in restricted geometry. The conductivity σ of the electron arrays, however, strongly depends on the thickness d of the helium films adsorbed above solid substrates. This behaviour of σ is discussed in detail for a randomly rough substrate. It turns out that for the dependence of the conductivity σ(d) there exist three regions of helium thicknesses: d>d min , d∼d min , and d<d min . Here d min is the helium film thickness which corresponds to a relatively deep minimum of the 2D conductivity. In the first interval, d>d min , a two-fraction scenario determines the behaviour of σ(d). In the vicinity of d min percolation phenomena develop and the conductivity exhibits different types of the so-called dip effect. For even thinner helium films, i.e., when d<d min , an activation type of mobility is stimulated. The presented model fits quite well to existing data of ac and dc electron mobility.

Plasma modes in the electron chain over a liquid helium film

We calculate the dispersion relations of longitudinal and transverse plasma oscillations in the quasi-one-dimensional electron system over liquid helium film within the quasicrystalline approximation. The plasma spectrum is obtained by solving the classical equations of motion. We study screening effects of the electron-electron interaction on the dispersion relations due to different solid substrates under the helium film. The influence of a magnetic field on the spectrum is also investigated.

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.