Investigations of Ultrasound Propagation in Porous Impurity-Helium Solids

The velocity and attenuation of ultrasound passing through porous impurity-helium solids immersed in liquid ⁴He have been measured in the temperature range 1.1–2.3 K. These solids were formed by injecting a mixture of impurity (e.g. D₂, Ne, N₂ or Kr) and helium gases into superfluid ⁴He. The sound signal seemed to propagate mainly in the helium contained in the pores, rather than through the solid sample itself. We found that the speed of sound at low temperatures is close to and decreases more rapidly with temperature than first sound in bulk helium, similar to behavior observed in aerogel. The attenuation of sound in helium in the compressed impurity-helium solids is bigger than in bulk helium and increases rapidly with temperature up to 1.65 K, after which a crossover to a much weaker temperature dependence was observed.     

Magnetic Resonance Studies of Impurity-Helium Solids Containing Hydrogen and Deuterium Impurities

Impurity-Helium Solids (Im-He) produced by injecting a mixed beam of helium and impurity gases into superfluid ⁴He have been investigated by electron spin resonance (ESR) and nuclear magnetic resonance (NMR) techniques. NMR signals from deuterium molecules in a D₂-He solid have been studied. Only samples prepared from gaseous mixtures containing high concentrations of D₂ molecules gave observable signals. The ESR experiments were performed on H and/or D atomic impurities in Im-He solids containing H, D, H₂, and D₂ in various combinations. The exchange tunneling reactions D+H₂HD+H and D+HDD₂+H were used to generate high concentrations of H atoms (10¹⁷/cm³) in Im-He samples.     

Paramagnetic Attraction of Impurity-Helium Solids

A small permanent magnet was used to attract impurity-helium solid samples composed of hydrogen, deuterium, and nitrogen radicals. The magnetic field gradient was sufficiently strong to lift each of the impurity-helium solids while submerged in superfluid helium, but only strong enough to lift one of four samples through the liquid surface. This suggests ranges of local atomic radical concentrations that partially agree with previous ESR measurements. The attractive paramagnetic force is strong enough to be useful as a trap for the formation of a pure hydrogen impurity-helium solid, for use in radical concentration measurements and for sorting and moving impurity-helium solids.     

Measurements of Thermal Diffusivity of Impurity-Helium Solids

Thermal diffusivity of the Kr- and N₂-impurity-helium solids (IHS) has been measured for the samples, immersed into liquid helium in the temperature range of 2.2–4.2 K. It has been found that the thermal diffusivity for both solids does not differ from that of liquid helium when the convection is absent even for the case when the samples were compact 10 times as compared to their initial volume. It should be emphasized that the presence of less than 0.5% mole fraction of a rare gas in the form of IHS allows us to suppress completely any convection in liquid helium (this effect was not observed when deuterium was used as an impurity). IHS can be used as a specific porous medium for investigations of critical phenomena of the He-I–He-II phase transition, as the He-aerogel systems.     

X-Ray Studies of Structural Changes of Impurity-Helium Solids

We have performed studies of the formation and evolution of samples of Im-Helium (Im=Ne, D₂, N₂) solids obtained by allowing a mixed beam of helium and impurity molecules to fall onto the surface of superfluid helium. Clusters consisting of single impurity molecules and/or small aggregates of the molecules surrounded by layers of solid helium agglomerate to form porous solids. These solids have been studied by x-ray diffraction techniques. As the solids were heated above their formation temperature of T=1.5 K, the small aggregates or clusters of impurities tended to form larger clusters, which x-ray diffraction revealed to be nanocrystallites ranging in size from 3 to 6 nm.     

Hyperfine Resonance of Deuterium Atoms Stabilized in Impurity-Helium Solids

We describe a method of combining the stabilization of impurities in superfluid helium and magnetic resonance techniques to study hyperfine resonance in atomic deuterium obtained by passing a D ₂ -He mixture through a radiofrequency discharge. The resulting D-D ₂ -He gas mixture penetrates a superfluid helium surface and forms a Van der Waals solid mixture containing atoms of D and He as well as D ₂ molecules. In preliminary experiments we have applied magnetic resonance (309 MHz) to the – transition to study spin-spin and spin-lattice relaxation in atomic deuterium and to provide information about the surroundings of atomic deuterium isolated in the D-D ₂ -He solid.     

Bubbles in Liquid Helium Containing Electrons in Excited States

When an electron enters liquid helium, it forces open a cavity within the liquid. We calculate the size and shape of these electron bubbles for different quantum states of the electron, and determine the negative pressure at which the different bubbles explode.     

Stabilization of High Concentrations of Nitrogen Atoms in Impurity-Helium Solids

Impurity-helium (Im-He) solids created by injecting gaseous helium with an admixture of nitrogen atoms and molecules into superfluid ⁴He have been studied via electron spin resonance (ESR). We have studied the efficiency of stabilization of N atoms in Im-He samples prepared from nitrogen-helium gas mixtures with different fractions of nitrogen varying from 0.25% to 4%. Some of the observed ESR spectra of N atoms in the Im-He samples are very broad. The highest local concentration of N atoms determined from dipole-dipole broadening of the ESR line is ～8×10²⁰ cm^?3. The highest average concentrations of N atoms in N-N2-He solids were much lower (of order 10¹⁹ cm^?3). The samples with high concentrations of N atoms were stable in liquid helium, and remained stable even after draining liquid helium from the sample at T≤3.5 K.     

ESR and X-ray Investigations of Deuterium Atoms and Molecules in Impurity-Helium Solids

Impurity-helium solids created by injecting deuterium atoms and molecules into superfluid ⁴He have been studied via electron spin resonance (ESR) and x-ray diffraction methods. We measured the g-factor, the hyperfine constant and the spin-lattice relaxation time of D atoms in D-D₂-He solids. These measurements show that D atoms are mainly stabilized in D₂ clusters. Using an x-ray method we found the size of D₂ clusters to be ～90Å in diameter and the densities of D₂ molecules in the samples to be of order 2.5?10²¹ cm^?3 . The highest average concentration of D atoms achieved in D-D₂-He solids was ～1.5?10¹⁸ cm^?3 . The local concentrations of D atoms within D₂ clusters is found to be large (～2?10¹⁹ cm^?3).     

Pulse Electron Spin Resonance Studies of H and D Atoms in Impurity-Helium Solids

We have performed measurements of the two-pulse ESEEM (electron spin echo envelope modulation) spectra of H and D atoms within impurity-helium solids. The local environments of the atoms were determined from the modulation of the ESEEM signal by neighboring D₂ and HD molecules. We have measured changes in the atom environments due to coalescence of the nanoclusters within the impurity-helium solids and due to the tunneling exchange chemical reaction D+HD→H+D₂.      

Watergel in Liquid Helium

Semitransparent soft water clouds forming in superfluid He-II by condensation of a gaseous mixture of ⁴He with water impurities transform with time to more rigid, highly porous icebergs. The icebergs suspended in the bulk of He-II are stable at a constant temperature T1.6 K, and they can beak down to small ice pieces on heating the liquid above T. The temperature of decomposition of the icebergs in He-I depends strongly on the vapor pressure above the surface of the liquid: at P0.2 atm they start to decompose at T_d2.5 K, but increasing the pressure to 1 atm causes T_d to rise to 4 K. When withdrawn from He-II the dry icebergs segregate to an ice powder and He on heating them above 1.8 K in He gas atmosphere. The total content of the water in the bulk of the icebergs, estimated from the ratio by volume of icebergs and powder, is 10²⁰ molecules/cm³. From observations of acoustic oscillations in the cell filled with He-1 (the ratio of amplitudes of vibrations of the iceberg and He-I level is about 0.2–0.3) it can be estimated that the density of the iceberg is only a few percent higher than the density of the surrounding liquid. We suggest that the highly porous water condensate (watergel) is composed of water nanoclusters, coated with 1-2 layers of solidified helium, which form a dispersed system of gel, and of liquid Helium, filling the pores between these van-der-Waals complexes, which serves as the dispersion medium of the gel.     

Laser-Induced Breakdown in Liquid Helium

We report on experiments in which focused laser light is used to induce optical breakdown in liquid helium-4. The threshold intensity has been measured over the temperature range from 1.1 to 2.8 K with light of wavelength 1064 nm. In addition to the measurement of the threshold, we have performed experiments to study how the breakdown from one pulse modifies the probability that a subsequent pulse will result in breakdown.     

Recent Progress in Studies of Nanostructured Impurity–Helium Solids

Impurity–helium (Im–He) solids are porous materials formed inside superfluid ⁴He by nanoclusters of impurities injected from the gas phase. The results of studies of these materials have relevance to soft condensed matter physics, matrix isolation of free radicals and low temperature chemistry. Recent studies by a variety of experimental techniques, including CW and pulse ESR, X-ray diffraction, ultrasound and Raman spectroscopy allow a better characterization of the properties of Im–He solids. The structure of Im–He solids, the trapping sites of stabilized atoms and the possible energy content of the samples are analyzed on the basis of experimental data. The kinetics of exchange tunneling reactions of hydrogen isotopes in nanoclusters and the changes of environment of the atoms during the course of these reactions are reviewed. Analysis of the ESR data shows that very large fraction of the stabilized atoms in Im–He solids reside on the surfaces of impurity nanoclusters. The future directions for studying Im–He solids are described. Among the most attractive are the studies of Im–He solids with high concentrations of stabilized atoms at ultralow (10–20 mK) temperature for the observation of new collective quantum phenomena, the studies of practical application of Im–He solids as a medium in neutron moderator for efficient production of ultracold (∼1 mK) neutrons, and the possibilities of obtaining high concentration of atomic nitrogen embedded in N₂ clusters for energy storage.     

Imbibition of Liquid Helium in Aerogels

We report optical measurements of the imbibition of liquid helium in a sample of silica aerogel with 90 % porosity. Both direct imaging and light scattering experiments were performed to determine the dynamics and the properties of the liquid-gas interface in both the normal and superfluid phases of liquid helium. In the normal phase, a classical Lucas Washburn behavior is observed for the rise of the imbibition front while the behavior in the superfluid phase is markedly different, as the fluid invades the sample from all sides with a constant speed. In both phases, the interface is rough, leading to light scattering. In addition, condensation ahead of the imbibition front is observed at low temperature in the superfluid phase.     

Thermal Decomposition Mechanism of N2-Impurity Helium Solids

The mechanism for the thermal decomposition of N₂ containing impurity helium solids (IHS) in superfluid ⁴He is reported. We show that the solid undergoes rapid thermal annealing leading to mobilization of the nitrogen atoms. Subsequently, the nitrogen atoms form molecular N₂ excimers, which are mobile in the solid and are responsible for various emission bands observed over the thermal decomposition of IHS. The present interpretation of the spectroscopic results indicates that IHS contains only ground state atomic species.PACS numbers: 33.20 Kf, 33.70 Jg     

Cavitation in Normal Liquid Helium 3

We have studied cavitation, i.e. bubble nucleation, by focusing ultrasound hurts in normal liquid helium 3 at temperatures down to 40 mK. As in helium 4, cavitation is found to be stochastic, with a cavitation probability 0.5 at a given value of the sound amplitude, which we define as a cavitation threshold. This threshold is found significantly lower in helium 3 than in helium 4, a result which agrees with theoretical predictions of a spinodal limit at - 3.1 bar in helium 3 instead of- 9.5 bar in helium 4- We also measured the temperature variation of this cavitation threshold; it decreases with temperature as expected for a thermally activated nucleation process. We have not yet found any evidence for a crossover toward cavitation by quantum tunneling below 120 mK as predicted by several authors; if confirmed, it might indicate that the superfluid coherence plays a role in quantum cavitation.     

Fractional Electrons in Liquid Helium?

We argue that electrons in liquid helium bubbles are not fractional, they are in a superposed state.     

Collapse of Vapor-Filled Bubbles in Liquid Helium

Multielectron bubbles (MEBs) are charged cavities in liquid helium which provide an interesting platform for the study of electrons on curved surfaces. Very recently, we have reported an experiment to trap these objects in a two-dimensional Paul trap, where they could be observed from ten to hundreds of milliseconds. During this time, the vapor inside the bubble condensed which resulted in a steady reduction in their size such that beyond a certain time the MEBs could no longer be detected. In this paper, we present experimental data on the lifetime of the bubbles as a function of their initial radius and compare the results with a theoretical model.     

Properties of Electron Bubbles in Liquid Helium

We present calculations of a number of properties of electron bubbles in liquid helium. The size and shape of bubbles containing electrons in different quantum states is determined based on a simplified model. We then find how the geometry of these bubbles changes with the applied pressure. The radiative lifetime of bubbles with electrons in excited states is calculated. Finally, we use a quantum Monte Carlo method to determine the properties of a bubble containing two electrons. We show that this object is unstable against fission.     

Electrons on Liquid Helium in a Resonator

An investigation of the microwave absorption for 2-dimensional electron layers in a resonator cavity are presented. The difference in the eigenmodes of the resonator in case of an empty cavity and in presence of a 2-dimensional electron layer on a helium film within the cavity are calculated. When introducing electrons into the cavity a pronounced frequency dependence is found. The expected shift in the resonance frequency is compared to previous and new data of resonance response measurements.