Solutions Of 3He In 4He Research Articles

He 4 nanoclusters around vacancies in solid solutions of He4 in He3

Precise measurements of the pressure in solid solutions of 4He in 3He are performed with temperature cycling in the phase-separation range. It is found that as a result of such cycling the difference ΔP of the pressures between the minimum (≈100 mK) and maximum (≈200 mK) temperatures decreases by a factor of approximately 3 and then remains unchanged for a long time. The initial values of ΔP and the pressure P0 in a uniform solid solution 4He-3He are restored only after the sample is heated to a temperature substantially above the phase-separation point. The data obtained are explained on the basis of the hypothesis that under these conditions clusters of pure 4He form around quasiequilibrium vacancies. A thermodynamic calculation of such clusters is performed. The computational results made it possible to give a quantitative explanation of the distinctive features observed. Comparing the calculations with experiment made it possible to determine the cluster concentration, 8×10−5, and the cluster radius, which is 3–4 interatomic distances at low temperatures. The characteristic equilibration time under these conditions is determined, and it is suggested that processes occurring at cluster boundaries determine its value.

New evidence for zero-temperature relaxation in a spin-polarized fermi liquid.

Spin-echo experiments are reported for 3He-4He solutions under extremely high B/T conditions, B=14.75 T and T>or=1.73 mK. The 3He concentration x(3) was adjusted close to the value x(c) approximately 3.8% at which the spin-rotation parameter muM0 vanishes. In this way the transverse and longitudinal spin-diffusion coefficients D( perpendicular ),D( parallel ) were measured while keeping |muM(0)|<1. It is found that the temperature dependence of D( perpendicular ) deviates strongly from 1/T(2), with anisotropy temperature T(a)=4.26(+0.18)(-0.44) mK. This value is close to the theoretical prediction for dilute solutions and suggests that spin current relaxation remains finite as the temperature tends to zero.

The Wall Potential in the Pores of Vycor Glass Experienced by 3He Quasiparticles in a Dilute 3He-4He Solution

We have studied the Fermi fluid, 3He in 3He-4He solution, confined within the narrow channels of Vycor glass. Continuous wave NMR signals were observed in two almost identical coils, one containing bulk solution and the other the solution in Vycor glass. Three different solutions (0.1, 1.6, and 6%) have been studied over the temperature range 3mK to above 1K. The measurements relate to two exclusion effects of 3He from Vycor glass. One concerns potential energy, and we demonstrate the partial exclusion of 3He independent of concentration as the temperature is lowered from high temperatures down to around 200mK, thus giving an experimental measure of the van der Waals forces between the wall and the helium. The second effect is the additional quantum exclusion which arises from the influence of quantum wire confinement on the kinetic energy, as the temperature is progressively lowered below 50mK in the very dilute (0.1%) solution.

Temperature Increase of Highly-Polarized Fermi Liquids in Spin-Echo Experiments

We show that there are restrictions on the maximum tipping angle that can be used without significantly raising the temperature of the 3He distribution in high B/T spin-echo experiments with pure liquid 3He and 3He-4He solutions. The temperature increase occurs during the diffusion process as quasiparticles in mixed-spin states are scattered and converted into thermal excitations at the spin-up and spin-down Fermi surfaces. This temperature increase can mimic the effects of zero temperature attenuation, leading to a higher values of the measured anisotropy temperature Ta. We analyze the dependence of the increase on polarization, initial temperature, 3He concentration, and tip angle, and estimate the size of the effect in recent experiments.

Measurement of the Solubility of 4He in Liquid 3He in the Fermi-Liquid Region

The phase separation temperatures of dilute solutions of 4He in liquid 3He in the Fermi-liquid region have been measured by detecting the 4He film formation sensitively. From the obtained solubility curve, the effective mass of 4He atom and the difference in binding energy of 4He in liquid 4He and in liquid 3He are proposed.

Study of Bulk Longitudinal Spin Relaxation in 3He-4He Liquid Mixtures

Longitudinal spin relaxation has been investigated in spin polarised liquid 3He-4He mixtures in cesium-coated glass cells. The contribution of wall relaxation is found to be either negligible or easy to separate, and bulk relaxation times can be derived from systematic studies. We report on relaxation measurements at saturated vapour pressure, for temperatures ranging from 1.1 to 3K. 3He densities vary from 0.07 to 11 mmol.cm−3 (3He concentrations between x3≃0.3 and 34%), so that both normal and superfluid 3He-4He solutions are studied. At fixed temperature, the longitudinal relaxation time is inversely proportional to the 3He number density over the whole range of investigated concentrations. In contrast, it is almost independent of temperature (changes do not exceed 10%). Comparison with published data and expected behaviours is discussed.

Acoustic properties of supersaturated superfluid 3He 4He solutions

The first sound velocity in metastable long-lived superfluid 3He 4He mixtures are measured in the temperature range 30–200 mK and the pressure interval 0–20 atm. The metastable state is reached by continuously increasing the 3He concentration in situ due to changing the osmotic and fountain pressure. The experimental data on the relative sound velocity show a linear dependence for all pressures and can be described by scattering of acoustic phonons by 3He quasiparticles in the solution. The experimental data show no essential anomaly under transition from saturated to supersaturated state.

Observation of anomalously fast diffusion in 3He–4He solid solutions near the BCC-HCP transition

The pulsed NMR technique was used to investigate diffusion on the BCC-HCP phase-equilibrium and melting curves of a dilute solution of 3He in 4He. The contributions from all coexisting phases were identified using the spin-echo method. It is established that, along with the contributions from the equilibrium BCC and HCP phases or from bulk liquid (in the melting curve measurements), there is an additional diffusional process that is characterized by an anomalously high diffusion coefficient. It is found that the latter is close to the diffusion coefficient in liquid helium, while the diffusion itself is spatially restricted. The observed effect may be caused by the formation of liquid droplets in the course of the BCC-HCP transition.

Observation of Andreev-Pushkarov vacancy clusters in phase-separated solid solutions of 4He in 3He

New features are observed for the pressure in a phase-separated dilute solid solution of 4He in 3He subjected to multiple temperature cycling within the phase-separation region. The results are explained within the framework of the hypothesis of A.F. Andreev and D.I. Pushkarov that the vacancies in a crystal without ideal periodicity are surrounded by clusters with a periodic structure. The equation for determining the radius of a cluster of pure 4He in a solution of 4He in 3He is refined. This hypothesis is shown to provide quantitative agreement between the calculated and experimental data under the assumption that the homogenization of the phase-separated solution is accompanied by the formation of metastable vacancies with a concentration of ∼(4–5)×10−5.

Viscosity and Mean Free Path of Very Diluted Solutions of 3He in 4He

We have measured the laminar friction of various diluted 3 He- 4 He mixtures, of natural 4 He and of isotopically pure 4 He on an oscillating sphere below 1 K. For 3 He concentrations x 3 ranging from 10 −2 to 10 −4 we find a reduction of the drag above 0.5 K when compared to the pure liquid and a large enhancement below, which is almost independent of x 3 . At low concentrations 5·10 −5 >x 3 ≥5·10 −7 the drag becomes proportional to x 3 which implies a transition from a hydrodynamic to a ballistic regime. This is confirmed by deducing the mean free path of the 3 He atoms from the data. The temperature dependence of the drag in the ballistic regime, however, is found to be proportional to T and therefore different from the expected T 1/2 behaviour.

Correlation effects in the heat capacity of separating 3He-4He solid solutions

We propose to describe the impurity heat capacity of solid 3He-4He mixtures both below and above the phase separation temperature Ts by an extension of the Bethe-Guggenheim approximation for the lattice gas model. It is shown that at T > Ts, the temperature behavior of the heat capacity is completely defined by correlation effects in the impurity subsystem. The developed theory enables us to explain from the common stand-point the experimental data by Edwards, McWilliams, and Daunt for all concentrations of 4He and make some conclusions about the structure of second phase nuclei.

Self-cooling of dilute solutions of 3He in 4He during adiabatic melting

A new single-shot method for cooling dilute solutions of 3He in 4He is described. Cooling factors (ratio of initial-to-final temperature) were measured for mixtures with 9.6 and 1.6% of 3He in 4He. A cooling factor of about 2 was obtained in the range of initial temperatures of 40–60 mK for 9.6% concentration (theoretical value is 2.9). For a solution with 1.6% 3He a cooling factor of 1.6–2.2 (theoretical value is 8.4) was obtained in the range 40–170 mK. Possible reasons for the discrepancy between theoretical and experimental results are discussed.

Superfluid transition temperature in a Fermi gas with repulsion allowing for higher orders of perturbation theory

Higher-order perturbation-theory corrections to the superfluid transition temperature in a weakly nonideal Fermi gas with repulsion are determined. This involves calculating the contribution of third-and fourth-order diagrams in terms of the gas parameter ap F to the effective interaction which determines the super-fluid transition temperature and also allowing for effects associated with retardation and renormalization of the polar part of the Green’s function. The expressions obtained provide evidence in support of attraction in the effective interaction in the second, third, and fourth orders of perturbation theory. It is shown that the critical temperature is mainly determined by second-and third-order terms of perturbation theory. Calculations are made of the superfluid transition temperature for a gas comprising neutral Fermi particles in a magnetic field. The limits of validity of the theory are analyzed and the possibility of applying the results to dilute solutions of 3He in 4He and neutral-particle Fermi gases in magnetic traps is discussed.

Coherent spin precession in a separated solution of 3He in 4He

It is suggested that a coherently precessing spin structure in a normal Fermi liquid should be used to study the interface of two Fermi liquids. It is shown that the interface makes an additional contribution to the attenuation of the precessing structure. This contribution is determined by the kinetic coefficient which relates the magnetization flux across the interface to the magnetization jump at the interface. A relationship is established between this kinetic coefficient and the nature of the scattering of Fermi quasiparticles at the interface. Results of numerical calculations of the attenuation of the structure are presented for a specific object, a separated solution of 3He in 4He.

Phase-separation kinetics of 3He-4He solid solutions at various degrees of supersaturation

The phase-separation kinetics of 3He-4He solid solutions at various degrees of supersaturation Δx are studied using precise pressure measurements. It is found that the characteristic phase-separation times do not depend on the degree of supersaturation but are determined by the final temperature to which the crystal is cooled. The behavior of the effective diffusion coefficient, which governs the corresponding mass transfer, cannot be described within the impuriton model of the mobility of 3He atoms in a 4He crystal.

Optical generation of the first and second sounds in superfluid 3He–4He solutions

An optical method of generation of the first and second sounds in superfluid 3He–4He solutions is proposed. The method is based on illumination by amplitude-modulated laser beam with wavelength corresponding to a region of absorption. Numerical calculations of pressure and temperature as a function of time are carried out. It was found that the presence of 3He isotopes in the solution provides strong coupling between these parameters. As a result, the waves of the first sound involve oscillations of the temperature, while the waves of the second sound involve oscillations of the pressure.

Mesoscopic behaviour of the neutral Fermi gas 3He confined in quantum wires

The behaviour of electron gases in restricted geometries provides a means to explore the fundamental quantum-mechanical properties of fermion gases at mesoscopic length scales1. But the existence of Coulomb repulsion between electrons unavoidably complicates the physics. Quantum gases of neutral fermions — such as 3He quasiparticles in a dilute solution of 3He in 4He, cooled to millikelvin temperatures2 — therefore offer a means of probing regimes completely inaccessible to electronic systems. Here we demonstrate the quantum exclusion of a 3He fermion gas from a network of narrow channels, connected to a reservoir of 3He/4He solution. The effect is expected from simple quantum-mechanical arguments, which predict that the 3He atoms cannot enter the channels when their wavelength exceeds √2 times the channel width. By adjusting the temperature of the solution, the energy of the particles and hence their average wavelength can becontrolled. In this way, we observe temperature-dependent changes in the penetration of the 3He quasiparticles into the channels. Our results demonstrate the macroscopic response of an atomic gas to basic quantum-mechanical restrictions at the mesoscopic level.

Supersaturation and Nucleation in Superfluid 3He–4He Solutions under Elevated Pressures

Nucleation and phase separation kinetics of superfluid 3 He– 4 He solutions at various pressures and concentrations were investigated. The measurements of the first sound velocity and the attenuation as well as the liquid dielectric constant determination were used. The study was performed at a constant temperature and pressure and the phase transition was initiated by continuous increasing the 3 He concentration. It is shown that the pressure dependence of the attainable supersaturations is non-monotonic when passing through a maximum at pressure of about 0.1 Bar. The comparison has discovered great differences between the results obtained and theoretical predictions.

The Effective Mass and Binding Energy of 4He in 3He in the Fermi Liquid Region

We have measured the solubility of4He in liquid3He down to about 40 mK and at pressures from zero up to ∼ 24 atm. The solubility was obtained from the thickness of the superfluid film in contact with unsaturated solutions of4He in3He as a function of temperature. By fitting the solubility data to Fermi liquid theory, we obtained the parameters m 4 * and Δ as a function of pressure. Here, m 4 * is the effective mass of4He in liquid3He and Δ the difference in binding energy between4He in pure4He and4He in liquid3He. This difference has a minimum near 10 atm. The average of the results for m 4 * , at different pressures, is (1.3 ± 0.2)m4. This agrees with the many body calculations of de Saavedra et al., and with the Stokes hydrodynamic mass using the partial volume of4He in3He, v 4 * . The partial volume was obtained by taking the derivative of Δ with respect to pressure.

Self-cooling of weak solutions of 3He in 4He during adiabatic melting

A new method of pressure regularization in the measuring cell is proposed for experiments on self-cooling of weak solutions. Preliminary results are presented for solutions with concentrations of 2% and 7%. Cooling by a factor of 1.5–2 was observed in the starting-temperature range 40–140 mK.