Helium In Aerogel Research Articles

Superfluidity of 4He in dense aerogel studied using quartz tuning fork

Superfluid 4He in aerogel is of interest because it has a normal component coupling to gel strand due to viscosity and a superfluid component with zero viscosity. Superfluid helium in aerogel has two sound modes, a slow critical mode and a fast one. In this study, quartz tuning fork was used in order to study acoustic properties of liquid 4He in aerogel with 90% porosity. Two pieces of aerogel were glued on both prongs of quartz tuning fork that had a resonance frequency of 33 kHz. The tuning fork was immersed in liquid 4He from 2 to 20 bar. The resonance frequency increased in the superfluid phase due to decrease in loaded mass. Temperature variation of resonance frequency was explained by that of superfluid density. Superfluid transition in aerogel was 2 mK lower than that without gel. Additional dissipation was observed in the temperature range between 1 K and transition temperature.

Condensation of helium in aerogel and athermal dynamics of the random-field Ising model.

High resolution measurements reveal that condensation isotherms of (4)He in high porosity silica aerogel become discontinuous below a critical temperature. We show that this behavior does not correspond to an equilibrium phase transition modified by the disorder induced by the aerogel structure, but to the disorder-driven critical point predicted for the athermal out-of-equilibrium dynamics of the random-field Ising model. Our results evidence the key role of nonequilibrium effects in the phase transitions of disordered systems.

Observation of a Superfluid Component within Solid Helium

We demonstrate by neutron scattering that a localized superfluid component exists at high pressures within solid helium in aerogel. Its existence is deduced from the observation of two sharp phonon-roton spectra which are clearly distinguishable from modes in bulk superfluid helium. These roton excitations exhibit different roton gap parameters than the roton observed in the bulk fluid at freezing pressure. One of the roton modes disappears after annealing the samples. Comparison with theoretical calculations suggests that the model that reproduces the observed data best is that of superfluid double layers within the solid and at the helium-substrate interface.

Coexistence of superfluid and solid helium in aerogel

The results of recent neutron scattering studies of solid helium in silica aerogel are discussed. Previously I.V. Kalinin et al., Pis’ma Zh. Eksp. Teor. Fiz. 87 (1), 743 (2008) [JETP Lett. 87 (1), 645 (2008)], we detected the existence of a superfluid phase in solid helium at a temperature below 0.6 K and a pressure of 51 bar, although, according to the phase diagram, helium should be in the solid state under these conditions. This work is a continuation of the above studies whose main goal was to examine the detected phenomenon and to establish basic parameters of the existence of a superfluid phase. We have determined the temperature of the superfluid transition from solid to superfluid helium, TC = 1.3 K, by analyzing experimental data. The superfluid phase excitation parameters (lifetime, intensity, and energy) have a temperature dependence similar to that of bulk helium. The superfluid phase coexists with the solid phase in the entire measured temperature range from T = 0.05 K to TC and is a nonequilibrium one and disappears at TC.

Deformation of silica aerogel during fluid adsorption

Aerogels are very compliant materials---even small stresses can lead to large deformations. In this paper we present measurements of the linear deformation of high porosity aerogels during adsorption of low surface tension fluids, performed using a linear variable differential transformer. We show that significant deformation can occur under conditions common in studies of helium in aerogel, and that the degree of deformation of the aerogel during capillary condensation scales with the surface tension. This scaling may then be used to suggest limits on safe temperatures for filling and emptying low density aerogels with helium.

NOVEL SOUND PHENOMENA IN IMPURE SUPERFLUIDS

In the last decade, new techniques for producing impure superfluids with unique properties have been developed. This new class of systems includes superfluid helium confined to aerogel, HeII with different impurities, superfluids in Vycor glasses, and watergel. These systems exhibit very unusual properties including unexpected acoustic features. We discuss the sound properties of these systems and show that sound phenomena in impure superfluids are modified from those in pure superfluids. We calculate the coupling between temperature and pressure oscillations for impure superfluids and show that this coupling increases significantly. This leads to the existence in impure superfluids of such unusual sound phenomena as slow "pressure" waves and fast "temperature" waves. This also decreases the threshold values for nonlinear processes as compared to pure superfluids. Sound conversion, which has been observed in pure superfluids only by high intensity waves should be observed at moderate sound amplitude in impure superfluids. Cerenkov emission of second sound by first sound (which has never been observed in superfluids) could be observed in impure superfluids. Even the nature of the sound modes in impure superfluids turns out to be changed. We have also derived for the first time the nonlinear hydrodynamic equations for superfluid helium in aerogel.

Nonlinear hydrodynamic equations for superfluid helium in aerogel

Aerogel in superfluids is studied very intensively during last decade. The importance of these systems is connected to the fact that this allows to investigate the influence of impurities on superfluidity. We have derived for the first time nonlinear hydrodynamic equations for superfluid helium in aerogel. These equations are generalization of McKenna et al. equations for nonlinear hydrodynamics case and could be used to study sound propagation phenomena in aerogel–superfluid system, in particular—to study sound conversion phenomena. We have obtained two alternative sets of equations, one of which is a generalization of a traditional set of nonlinear hydrodynamics equations for the case of an aerogel–superfluid system and, the other one represents à la Putterman equations (equation for v → s is replaced by equation for A → = ρ n ρσ w → , where w → = v → n− v → s ).

Novel sound phenomena in superfluid helium in aerogel and other impure superfluids

Impure superfluids exhibit very unusual properties including unexpected acoustic features. We calculate the coupling between temperature and pressure oscillations for impure superfluids and for superfluid He in aerogel and show that the coupling between first and second sound modes is governed either by c∂ ρ/∂ c ( c is impurity concentration) or by σρ a ρ s ( ρ a is aerogel density), rather than by the thermal expansion coefficient ∂ ρ/∂ T, which is enormously small in pure superfluids. This replacement plays a fundamental role in sound phenomena. It leads to the existence of such phenomena as the slow “pressure” waves and fast “temperature” waves and modifies significantly all sound conversion phenomena.

Novel sound phenomena in superfluid helium in aerogel and other impure superfluids

During the last decade new techniques for producing impure superfluids with unique properties have been developed. This new class of systems includes superfluid helium confined to aerogel, HeII with different impurities (D 2, N 2, Ne, Kr), superfluids in Vycor glasses, and watergel. These systems exhibit very unusual properties including unexpected acoustic features. We discuss the sound properties of these systems and show that sound phenomena in impure superfluids are modified from those in pure superfluids. We calculate the coupling between temperature and pressure oscillations for impure superfluids and for superfluid He in aerogel. We show that the coupling between these two sound modes is governed either by c∂ρ/ ∂c or σρ a ρ s (for aerogel) rather than thermal expansion coefficient ∂ρ/ ∂T, which is enormously small in pure superfluids. This replacement plays a fundamental role in all sound phenomena in impure superfluids. It enhances the coupling between the two sound modes that leads to the existence of such phenomena as the slow mode and heat pulse propagation with the velocity of first sound observed in superfluids in aerogel. This means that it is possible to observe in impure superfluids such unusual sound phenomena as slow pressure (density) waves and fast temperature (entropy) waves. The enhancement of the coupling between the two sound modes decreases the threshold values for nonlinear processes as compared to pure superfluids. Sound conversion, which has been observed in pure superfluids only by shock waves should be observed at moderate sound amplitude in impure superfluids. Čerenkov emission of second sound by first sound (which never been observed in pure superfluids) could be observed in impure superfluids.

Acoustic study of the liquid–vapor critical point of neon and helium in aerogel

We used low-frequency acoustic resonators to study the liquid–vapor coexistence curve and critical behavior of simple fluids in low-density silica aerogels. Resonators provide direct measurements of sound speed at low frequency. Sound speeds exhibit sharp kinks at the liquid–vapor phase boundary that allow us to map out coexistence curves near the critical point. Our previous measurements showed that neon in 95% aerogel exhibits a narrowed coexistence curve shifted to higher fluid density, but were complicated by a significant quantity of bulk neon present in the cell. We discuss results from our current experiments on two different fluids (neon and helium). Results are compared to our earlier measurements and to studies by other groups.

A very low-frequency measurement of the ζ exponent of helium in aerogel

The inertial mass of a long cylinder of aerogel, moving in He-II, is very sensitive to the helium superfluid density both inside and outside the aerogel, and involves conversion of the superfluid components into one another. The very low resonance frequency of a simple pendulum built with the cylinder gives access to the superfluid density inside the aerogel. An accuracy of 10 −4 is obtained. For three tested samples, the exponent ζ is found larger than the bulk value.

Excitations of Liquid 4He in Disorder

We discuss the effect of disorder and confinement on the excitations in superfluid and normal liquid4He. Neutron scattering measurements of the excitations to date are limited to helium in aerogel. There the phonon-roton energy and width are slightly modified by disorder but there is no evidence for additional excitations at low energy nor of a gap in the phonon energy at long wavelengths. Experimental difficulties are discussed. In a recent path-integral Monte-Carlo study, in which a high density of point impurities are introduced at random positions, a significant broadening and energy shift are found together with additional low-energy excitations.

Helium in Aerogel

An important theme that threads through many areas of current interest in condensed matter physics is the effect of randomness and disorder. Prior to the 1960s, disorder and impurities were often viewed as unavoidable nuisances that masked the true behavior of ideal systems. We have since learned that disorder itself can bring forth fascinating and often unexpected new phenomena in condensed phases of matter. (See the December 1988 special issue of PHYSICS TODAY, dedicated to disordered solids.)

Phase transitions of helium in aerogel

The effect of dilute quenched impurities on the liquid-vapor critical point, superfluid and He3−He4 phase separation transitions were studied by introducing liquid helium into aerogel. Aerogel is a highly porous glass consisting of atomically thin silica strands interconnected at random sites. In spite of the random environment, the transitions were found to be remarkably sharp and well defined. Although the silica network constitutes as little as two percent of the total volume, the nature of the transitions is completely changed. When a He3−He4 mixture is placed inside aerogel of 98 percent porosity, the coexistence region is found to be detached from the superfluid transition, line, giving rise to a new superfluid mixture that is rich in He3.

Critical behavior of superfluid 4He in aerogel.

We report on Monte Carlo studies of the critical behaviour of superfluid $^4$He in the presence of quenched disorder with long-range fractal correlations. According to the heuristic argument by Harris, uncorrelated disorder is irrelevant when the specific heat critical exponent $\alpha$ is negative, which is the case for the pure $^4$He. However, experiments on helium in aerogel have shown that the superfluid density critical exponent $\zeta$ changes. We hypothesize that this is a cross-over effect due to the fractal nature of aerogel. Modelling the aerogel as an incipient percolating cluster in 3D and weakening the bonds at the fractal sites, we perform XY-model simulations, which demonstrate an increase in $\zeta$ from $0.67 \pm 0.005 $ for the pure case to an apparent value of $0.722\pm 0.005$ in the presence of the fractal disorder, provided that the helium correlation length does not exceed the fractal correlation length.

Second and fourth sound modes for superfluid helium in aerogel

Because of the interest in superfluid onset in porous media (high porosity aerogel in particular) one would like to measure the superfluid fraction as well as the heat capacity near T c . However, standard techniques such as torsional oscillators and fourth sound have encountered difficulties because the compliant aerogel allows the viscous normal fluid to move. Using data we obtained for fourth sound propagation below 1.8 K, we develop a theory which fits the data with no adjustible parameters, and show that the theory predicts another second-sound-like mode which may not be damped near T c .

Second and fourth sound modes for superfluid helium in aerogel

Because of the interest in superfluid onset in porous media (high porosity aerogel in particular) one would like to measure the superfluid fraction as well as the heat capacity near T c . However, standard techniques such as torsional oscillators and fourth sound have encountered difficulties because the compliant aerogel allows the viscous normal fluid to move. Using data we obtained for fourth sound propagation below 1.8 K, we develop a theory which fits the data with no adjustible parameters, and show that the theory predicts another second-sound-like mode which may not be damped near T c .