Viktor N. Grigorev

Influence of Supercooling Level on Kinetics of Phase Separation in Solid Mixtures of 4He in 3He

We studied the kinetics of phase separation in the solid mixture of 4He in 3He at various supercooling in the temperature range of 100–200 mK through precise pressure measurements. The time dependences of the pressure change during phase separation were exponential. At small supercooling the characteristic time constant τ is almost independent of the final temperature Tf and is about 10 hours, which is considered to result from heterogeneous nucleation. In a narrow range of Tf τ decreases more than an order of magnitude. At larger supercooling τ is independent of Tf again. This behaviour agrees qualitatively with the theoretical consideration of the phase separation kinetics at homogeneous nucleation taking into account the finiteness of a cooling rate. The value of interphase surface tension has been obtained from comparison of the theory with the experimental results.

Formation of Vacancion-Impurity Complexes in Phase-Separated Solid Mixtures of 4He in 3He

New features are observed for the pressure in a phase-separated dilute solid mixtures 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.

Threshold Effect During Dissolution of 3He Inclusions in Solid 4He

A pressure jump has been found at the onset of the dissolution of bcc inclusions in separated solid3He - 4He mixture if the crystal is overheated above a certain critical value. This effect can be explained in the framework of a multistage dissolution process model.

2D Wigner Solid over Liquid Helium: Resonant and Non–Linear Phenomena

Results are presented of an experimental study of coupled phonon–ripplon resonances in the Wigner crystal over a liquid helium surface in high driving fields. The resonant frequencies are shown to be in a good agreement with existing theoretical results in the limit of low exciting signal. Novel additional resonances appear at low frequencies for high driving fields. The mobility of the Wigner crystal at fixed frequency, measured as a function of driving field, is essentially nonmonotonic showing a mobility jump in high enough driving fields.

Phase Separation Line for Solid Mixtures of 3He in 4He

The excess pressure due to the phase separation of solid mixtures of 3He in 4He held at a constant volume was measured and used for constructing the phase separation diagram of this system. We obtained high-quality homogeneous samples of the solid mixtures after several cycles of cooling down and heating up the two-phase crystal. This gave reliable and reproducible experimental data without hysteresis efects. We compared the phase diagram line obtained with various theoretical approaches, which describe the phase separation of the helium isotope mixtures. The regular solution model can not describe the experimental data well and neither can the asymmetrical Mullins model. Good agreement is observed only with the theory of Edwards and Balibar which takes into account the difference between the crystal symmetry (hcp and bcc) of the coexisting phases.

Influence of temperature cycling on the kinetics of new phase growth in two-phase solid 3He–4He mixtures

Abstract Precise pressure measurements at constant volume were carried out to study the influence of thermal cycling on the kinetics of new phase growth and dissolution in 3He–4He solid mixtures in the phase separation region. It is found that the equilibrium pressure in the sample reduces after cycling. This points to improvement of the crystal quality, which is supported by the acceleration of the growth and dissolution processes. These data indicate that cooling–warming cycles in the two-phase region are more effective in eliminating defects than the traditional annealing near the melting temperature.

Melting and Crystallization of 3He Inclusions in Two-phase Solid 3He-4He Mixtures

The peculiar features of the phase diagram for the 3He-4He system make it possible to melt the 3He inclusions formed during phase separation of the mixture by further cooling and to crystallize them in subsequent heating. The kinetics of these processes is studied on a sample with a molar volume of 20.54 cm3/mole (P=31.7 bar) using pressure measurements. The time dependence of the crystal pressure P(t) is measured on cooling at a rate of ∼10 mK/h followed by heating. The dependence P(t) has two distinct rises in pressure, the first rise being associated with the phase separation of the mixture and the second one with the melting of the 3He inclusions formed. It is shown that the melting of the 3He inclusions is almost complete after the fast cooling and the observed pressure jump is in good agreement with the corresponding change in the molar volume. The repeated crystallization of the inclusions is found to give rise to a large pressure gradient near the boundary of the inclusions, suppressing quantum diffusion considerably. This may result in an incomplete crystallization of the inclusions. The experimentally observed difference between the initial and final pressure in the sample corresponds to the fact that approximately 20% of the 3He remains in the liquid state.

Homogeneous nucleation in phase separation of solid 3He–4He mixtures

Abstract NMR and pressure have been measured during phase separation in solid 3 He – 4 He mixtures. Spin echoes were used to observe bounded diffusion and to estimate the diffusion coefficient, size and nuclei concentration in the 3 He -enriched phase. The characteristic phase separation time constant of the mixture was found from pressure measurements. The results argue convincingly for homogeneous nucleation. The surface tension of the nuclei is found independently from NMR and from pressure measurements; the two determinations agree well and yield a surface tension coefficient of 4.9×10 −6 J m −2 .

Andreev–Pushkarov's vacancion clusters in phase-separated solid 4He–3He mixtures

Abstract An anomalous behaviour of pressure found in phase-separated mixture of 4 He in 3 He at cycling the temperature explained by formation of pure 4 He clusters around vacancies. The thermodynamic calculation was carried out for the cluster parameters taking the change of the initial mixture concentration at finite amount of vacancies and the surface tension at the cluster boundary into account.

Vacancy formation volume in BCC 3He along the melting curve

Abstract The value of the vacancy formation volume is obtained near the melting curve in BCC solid 3He in the temperature range 0.5–2.7 K for molar volumes 19.32– 24.60 cm 3 / mol . The data are obtained both by measuring the temperature dependence of the self-diffusion coefficient in 3He crystals at various densities and by use of the NMR data obtained by H.A. Reich. The value of the vacancy formation volume is in disagreement with the values calculated from the standard thermodynamical relation.