V. N. Grigor’ev

Contribution of phonon and vacancion excitations to the thermodynamic properties of solid helium

Precision measurements of the temperature dependence of the pressure are made on high-quality crystals of He4 and He3–He4 solutions grown at a constant volume. The phonon and vacancion contributions to the pressure are separated on the basis of the Debye model for the phonons and the model of wide-band vacancion excitations. This approach is also used to analyze all the other available thermodynamic data for the solid pure isotopes of helium and their solutions. This yields information about the Debye temperature and vacancy activation energy, and a universal dependence of these parameters on the molar volume is found for He3, He4, and He3–He4 solutions. The values found for the corresponding Gruneisen parameters turn out to be independent of the molar volume.

Formation of a glass phase in solid He4. Contribution to the pressure in the supersolid region

High-precision barometry is used to perform measurements of the temperature dependence of the pressure of helium crystals, located in a constant volume, in the temperature range 50–800mK on crystals of different quality ranging from freshly prepared to carefully annealed. It is found that an additional contribution to the pressure, proportional to T2, appears in on annealed samples. This contribution becomes dominant at T<300mK, i.e. the region where anomalous behavior of solid helium, ascribed to the super solid effect, was previously observed. Such behavior of the pressure is characteristic for the glass phase, and this phase was practically absent in carefully annealed samples. The observed effect shows that in solid helium containing a high concentration of nonequilibrium defects a glass phase forms easily on cooling. Anomalously high pressure relaxation in low-quality samples was also detected in the experiments at temperatures close to the melting temperature. The possible reasons for this effect ar...

Concentration dependence of the diffusion coefficient in separating dilute solid mixtures of 4He in 3He

The kinetics of the separation of dilute solid mixtures of 4He in 3He is investigated in the ranges of temperature 100–200 mK, 4He concentration x=2.2–3.3%, and pressure 32–35 bar. It is found that the characteristic time τ required for the separating mixture to come to equilibrium depends substantially on the degree of supercooling. When the mixture is supercooled by more than 40–50 mK relative to the separation temperature of the initial mixture, the characteristic time τ<103 s and remains practically unchanged as the temperature is lowered further. At low supercoolings the values of τ reach 4×104 s and decrease noticeably with further decrease in temperature. A relation between the measured values of τ and the effective coefficient of mass diffusion is established using the solution of the diffusion problem with allowance for the surface resistance arising when the 4He impurity atoms leave the solution and enter new-phase inclusions. It is shown that an adequate description of the experimental data in ...

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.

Giant asymmetry of the processes of separation and homogenization of 3He–4He solid mixtures

A comparison of the kinetics of the separation processes and homogenization of 3He–4He solid mixtures is made with the use of precision barometry for samples of three types—dilute mixtures of 3He in 4He and of 4He in 3He and concentrated 3He–4He mixtures. It is found that in all types of mixtures studied the rate of the initial stage of homogenization can exceed the rate of separation by more than 500 times. An appreciable rate of phase separation in the concentrated mixtures, where, according to existing ideas, the impurity atoms in quantum crystals should be localized, attests to a new, unknown mechanism of mass transfer under those conditions, while the fast homogenization indicates that this process is nondiffusional in nature.

Kinetics of the phase transition in solid solutions of 4He in 3He at different degrees of supersaturation

Precision measurements of the pressure during phase separation in samples of solid solutions of 4He in 3He have been used to obtain data on the characteristic times of the phase transition. A processing of the results gives additional evidence supporting the view that homogeneous nucleation is realized in 3He–4He solid solutions at significant supercoolings and heterogeneous nucleation at the smallest supercoolings. Two different ways are proposed for comparing the results with a theoretical calculation taking into account the processes at the boundary of a nucleus of the new phase. Both give roughly similar values of the coefficient of surface tension at the nucleus–matrix boundary, and those values agree with those obtained in other studies. It is conjectured that the bcc–hcp transition has a substantial influence on the kinetics of separation at the lowest supersaturations.

Phase separation curve of dilute 3He–4He hcp solid solutions

A method is proposed for constructing the separation phase diagram of 3He–4He solid solutions on the basis of precision measurement of the pressure jump due to phase separation at constant volume. The technique is implemented on high-quantity samples of the solid solutions, making it possible to obtain reliable and reproducible experimental data with no appreciable manifestation of hysteresis effects. The line of phase separation constructed from the experimental data is compared with the results of various theoretical approaches describing phase separation in solid solutions of helium isotopic mixtures. It is found that good agreement with experiment is observed only for the Edwards–Balibar model, which is an extension of the theory of regular solutions to take into account the differences of the crystal structures (hcp and bcc) of the phases existing in the system.

Detection of fluctuation effects near the phase separation temperature of concentrated 3He-4He solid solutions

A precision barometric study has revealed unusual behavior of the pressure of 3He−4He solid solutions with a concentration of around 30% 3He: in the pre-separation region the pressure increases with decreasing temperature long before the start of the phase transition. It is established that such an anomaly is due to correlation effects in the impurity subsystem which give rise to large-scale fluctuations of the impurity concentration, and the fluctuation contribution to the pressure is much greater than the phonon contribution. Quantitative agreement between the experimental data and the proposed theory is obtained, and it is shown that the observed temperature dependence of the pressure in the pre-separation and metastable regions can be explained only when the long-range character of the interaction between impurities is taken into account.

4He nanoclusters around vacancies in solid solutions of 4He in 3He

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, ...

Evidence for the edge magnetoplasmon “Acoustic” mode in an electron layer over liquid helium

The spectrum of new modes of edge magnetoplasmons (EMP) in an electron layer over liquid helium was studied. The experiments were carried out in the temperature range 0.5–0.7 K at frequencies 0.87–1 MHz and electron density (1.5–3.5) ⋅ 1012 m−2 in a magnetic field up to 0.5 T. Besides conventional edge magnetoplasmons and the EMP mode connected with the displacement of the electron sheet boundary, a new “ acoustic” edge magnetoplasmon mode was observed in an electron layer over liquid helium for the first time. Spectrum of all three types of the magnetoplasmon modes are compared with the existing theories.