Investigation of Ultrasound Attenuation in Impurity-Helium Solids Containing Liquid Helium

The attenuation of ultrasound in Impurity-Helium (Im-He) solids created upon introduction of impurity atoms or molecules (D ₂ , N ₂ , Kr) into a volume of superfluid helium has been investigated. The observed features of attenuation show that a porous substance consisting of a loosely interconnected continuous network is created in superfluid helium. This network is formed by impurity particles encapsulated in solidified helium. Analysis of attenuation allows us to conclude that Im-He solid samples have a wide distribution of pores from 8 nm to 800 nm. It was established that the character of attenuation in D ₂ -He samples is considerably different from that in heavier Im-He solids, for which two maxima of attenuation were sometimes observed. A sharp peak was observed at T_c0 very close to the bulk helium lambda transition temperature and a second broad peak occurred at T_cc0 . This behavior is similar to that predicted theoretically for liquid helium in restricted fractal porous media.     

Theory of the Small Amplitude Shape Oscillations of a Helium-II Drop

We present an analysis of the small amplitude shape oscillations of a superfluid helium drop surrounded by saturated helium vapor. The equations of the two-fluid model are used to describe the liquid motion within the drop. The calculations are performed for two different sets of boundary conditions at the surface of the drop. The first set corresponds to the physical situation in which no evaporation or condensation of helium takes place during the oscillation (no evaporation model), whereas the second set apply when the liquid at the surface of the drop is always in phase equilibrium with the vapor (equilibrium model). The theoretical results for frequency and damping rate are then compared with experimental data.     

Inhomogeneities in thin helium films

We report a study of the inhomogeneities in thin helium films due to the van der Waals potential by a resonant acoustical technique. The van der Waals constant between helium and quartz is estimated to be =65±5 K (layer)³; the model leads to a solid helium layer on the substrate of the order ofH=2.2a ₀ . A comparison is made with data extracted from interferometric acoustical methods and third-sound type experiments.Associated with the Centre National de la Recherche Scientifique.     

New calculation of the acoustical mismatch in the Kapitza conductance of bulk helium

Elastic wave transmission at the interface between a substrate and liquid helium has been calculated using a model that takes into account the inhomogeneous character of the acoustical impedance in helium and in particular the existence of a solid helium layer intermediate between the substrate and bulk liquid helium. The properties of this layer (width, velocity, and attenuation of elastic waves) have a great influence on the calculated transmission. A width of about 10 Å seems to fit many quoted experimental results. The Kapitza conductanceh _k derived from this model and the temperature variation ofh _k agree with experimental results obtained from clean copper samples.Associated with the Centre National de la Recherche Scientifique.     

Simulations of the Behavior of Helium in a Thermal Conductivity Cell

We have performed numerical simulations of the behavior in a one-dimensional thermal conductivity cell of helium as it is ramped through the superfluid transition from below. The goal of the simulations is to be able to at least qualitatively, if not quantitatively, predict the behavior of helium in a reduced gravitational environment. These numerical simulations can model helium cells with effective gravitational fields of |a/g| > 0 by changing the distribution of local transition temperatures to match the desired gravitational environment. The numerical results for the simulated behavior show excellent qualitative agreement with the observed experimental data under a variety of effective gravity values.     

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.      

Damping of a Vibrating Wire in Spin Polarized Liquid 3He-4He Mixtures

In this paper we report measurements of the damping of a vibrating wire in spin polarized liquid helium mixtures. The dilute helium solutions have been cooled by nuclear demagnetisation in high magnetic fields such that the spin polarization for a 0.04% mixture is as high 75%. A PtW alloy vibrating wire has been used to probe the viscosity of the liquid. The increase in viscosity on changing the magnetic field from 3 tesla to 11.5 tesla is a factor 3.5 at the lowest temperatures. The results are compared to theory allowing for both s-wave and p-wave scattering.     

Conditional analysis in a turbulent boundary layer with strong density differences

Summary A study of the fine structure of wall turbulence in the presence of strong density variations through a conditional analysis of experimental data obtained in a wind tunnel is presented in this paper. Density variations within the boundary layer are produced by injection of air or helium into a mixture of these two components. The conditional analysis carried out in this study shows that the injection of helium into the boundary layer generates more violent ejections than in the case of an injection of air. This result is confirmed by the significant contribution of the ejections to the turbulent mass flux. The results obtained show that in the fully turbulent zone the ejections are associated with strong velocity fluctuations for the two injected gases. This is even more so for the fluctuations of density when an injection of helium is considered. At two locations of the flow (x/e = 8.3 and x/e = 33.3, where e = 3 mm is the slot width) we show that the contributions of a large excursion in the two quadrants of the u' – ρ' plane, corresponding to the two events of the fluid movement, the ejection and sweeping, are very intermittent owing to the fact that they are associated only to a very low number of measurement points. The ejections at y⁺ > 50 are more significant and much more energetic in the case of helium.     

Emerging issues in helium turbulence

It has been appreciated recently that because helium has the lowest viscosity of any known material, it can be used in reaching the very highest Reynolds numbers and Rayleigh numbers. Critical helium gas, helium I and helium II are all candidates for such uses. Helium gas and helium I are classical fluids and the advantage stems solely from their low kinematic viscosity. Helium II obeys two-fluid equations and their use in turbulence investigations is under study. This article provides a brief introduction and review of this topic, outlining some of the progress already made and questions which need to be resolved as this relatively new field of investigation evolves. A summary of instrumentation available is included.     

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 ⁴ He 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 ⁴ He 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.     

Imaging of a Suspended Helium Film with 2-D Electrons by Interferometry

A helium film suspended between elevations on a substrate forms an excellent surface for a two-dimensional electron gas. With the structure on the substrate a quasi one-dimensional electron system can be created. Experiments probing the electrical transport properties of this system have raised questions about the suspended helium film itself. We report measurements of the suspended film using interferometry. The profile is mapped out as a function of the bulk helium level beneath the substrate. By measuring the electron signal simultaneously the relation between the signal and the film profile can be obtained.     

Optical Observation of Atomic Hydrogen on the Surface of Liquid Helium

We have optically detected hydrogen atoms adsorbed on the surface of liquid helium, a system relevant for the study of Base degeneracy in two dimensions. The atoms are excited by 121.6 nm light and detected both in fluorescence and in absorption. The optical spectrum of the adsorbed hydrogen atoms was not known a priori. It shows a resonance that is much broader than that of a hydrogen atom in vacuo, and it is shifted to lower frequencies. From the fluorescence intensity we determine that we have reached a surface density corresponding to one atom per square De Broglie wavelength. This means that our experiments take place at the edge of quantum degeneracy. In the regime where the adsorption isotherm is known we can use the measured hydrogen densities to infer the temperature of the helium surface. We use this information to determine the thermal conductance between the surface and the bulk of liquid helium. We find quantitative agreement between the measured temperature drops and the prediction of ripplon-phonon coupling theory.     

Temperature Independent Nucleation of Solid Helium 4 Below 1K

Nucleation of solid helium in superfluid helium was studied in a temperature range between 50mK and 1K. Pulsed electrostatical compression was employed to initiate nucleation in a slightly overpressurized superfluid helium. The nucleation rate w was estimated from a distribution of the overpressure P _eff required for nucleation. The temperature dependence of P _eff at constant was essentially flat between 50mK and 1K. The ln was almost linear in P _eff within the resolution of our data.     

Heat transfer from metallic filaments (whiskers) into helium under overcritical pressure for temperatures between 3.7 and 7.2 K

The heat transfer coefficient α, near the critical temperature, T_cO, was determined for several whiskers from the In-Pb alloy system. For this purpose the hysteresis of the voltage-temperature (V-T_B) transition curves at fixed currents, I, and of the V-I characteristics at fixed helium bath temperature, T_B, was determined. The advantage of using measurements made with whiskers is that there is no heat transfer to a substrate and negligible heat transfer to the contracts. The only heat transfer is that to the surrounding helium.     

Explosion of Electron Bubbles Trapped on Vortices in He-II

Electrons injected into liquid helium become trapped in a spherical cavity from which the liquid is almost completely excluded. When a negative pressure is applied to the liquid the size of the electron bubble increases, and at a critical negative pressure P _c the bubble explodes. We have observed these explosions and have measured P _c as a function of temperature. At low temperatures an electron bubble can become bound to a quantized vortex. The flow of the helium around the quantized vortex leads to a reduction in the magnitude of P _c. We will report measurements of this reduction in P _c and will make a comparison of the results with theoretical predictions.     

Visualisation of Convective Flow Patterns in Liquid Helium

The advantages of using liquid helium as the investigative fluid in Rayleigh-Bénard experiments are reviewed. A low temperature shadowgraphy apparatus is described that permits convective flow pattern visualisation in liquid helium, thus overcoming the main disadvantage until now of using quantum fluids. The factors involved in maximising the optical resolution for both the shadowgraph and schlieren methods are examined for several fluids and this discussion is applied to the low temperature apparatus in assessing its performance. Some preliminary results on pattern formation in liquid helium are presented.     

Using Electrons on Liquid Helium for Quantum Computing

We describe a quantum computer based on electrons supported by a helium film and localized laterally by small electrodes just under the helium surface. Each qubit is made of combinations of the ground and first excited state of an electron trapped in the image potential well at the surface. Mechanisms for preparing the initial state of the qubit, operations with the qubits, and a proposed readout are described. This system is, in principle, capable of 10 ⁵ operations in a decoherence time.     

Helium Droplets: A Nanoscale Cryostat for High Resolution Spectroscopy and Studies of Quantized Vorticity

In this paper we discuss helium droplets as a nanoscale cryostat for a variety of fundamental experiments in condensed matter physics. Specifically, we describe our recent work on the spectroscopy of silver atoms, europium atoms and C ₆₀ in helium droplets and compare the helium droplet, as a matrix for low temperature studies of complex systems, with traditional matrix and gas phase techniques. Further, we discuss our recent work on the production of ultra-cold metal clusters of silver, indium and europium embedded in helium droplets at a temperature (T=0.37K) two orders of magnitude lower than previously achieved in beams of free metal clusters. This work opens the door to high resolution spectroscopic studies of metal clusters and, possibly, high resolution studies of the size dependence of their superconducting properties. We further speculate on a series of experiments where we plan to use standard spectroscopic methods, developed in recent years, to exploit the helium droplet for studies of the existence, stability and dynamics of quantized vortices. Helium droplets may be the ideal system for such studies due to the complete absence of pinning sites that plague many similar experiments performed in bulk helium.     

Optical Measurement of the Non-linear Focusing of Sound in Liquid Helium 4

We have measured the amplitude of 1MHz acoustic waves focused in liquid helium 4. Our resolution is 10 nanoseconds in time and 15 micrometers in space. The waves are focused onto a flat glass plate. We measure the reflection of light at the glass/helium interface, which depends on the refractive index of the liquid, consequently on its density which we could measure with an accuracy of ±10 ^–4 g/cm ³ . At large amplitude, strong non-linear effects are observed, in good agreement with a numerical calculation.     

2D Electron Bubbles in Gaseous Helium

The structure of 2D electron bubbles in gaseous helium is discussed. These cavities appear under the condition n_g > n_c, where n_c ≈ 10²⁰ cm⁻³ is much less than the critical helium gas density for the appearance of 3D electron bubbles. Far from the critical density, the 2D bubble trapping energy is practically a linear function of n_g. Experiment confirms the existence of 2D electron bubbles.