E. Rudavskii

Spin Diffusion Processes in Solid 3He-4He Mixtures Near the BCC-HCP Phase Transition at the Melting Curve

The spin diffusion coefficient of 1% 3He in solid of 4He has been measured in the vicinity of the BCC-HCP phase transition at the melting curve by pulsed NMR. The applied spin echo technique does allow to distinguish the contributions from all of coexisting phases. In addition to well-known diffusion in BCC, HCP, and bulk liquid phases the new fast diffusion process is observed and the diffusion coefficient of this process is shown to be close to that in liquid mixture being dependent on the time between the NMR pulses (bounded diffusion). The possible reason for the effect may be connected with formation of liquid droplets during the BCC-HCP transition.

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.

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.

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.

Kinetics of Liquid 3He Droplets in Solid 4He Matrix

Precise measurements of pressure in the crystal at constant volume were used to obtain the data on growth and dissolution kinetics of liquid 3He droplets formed as a result of isotopic phase separation of solid 3He-4He Mixtures. We studied several crystals with an initial 3He concentration of 2.05% in the pressure range of 26–27 bar. It is shown that the growth of the liquid droplets during the stepwise cooling of the two-phase crystal is correctly described by the superposition of two exponential processes: diffusion decomposition with a small time constant and strain relaxation with a big time constant. The strain layer near the droplet boundaries is due to a great difference in molar volume between the droplets and the matrix, and leads to a plastic deformation of the matrix and to a non-equilibrium 3He concentration in the matrix. Under such conditions quantum diffusion is significantly suppressed and 3He atom transport occurs only as the strain is relaxed.

Phase Separation Kinetics of Solid 3He-4He Mixtures

The kinetics of isotopic phase separation in solid mixture of3He in4He with the initial concentration 2.05 % at various molar volumes has been investigated by precise pressure measurements. It has been shown that during both stepped and fast cooldown into the metastable region the equilibrium of coexisting phases is described by the exponential law with a characteristic time constant τ, The value of τ is found to decrease as the molar volume increases and the temperature lowers. It confirms that the growth of the3He-rich phase is connected with nonthermally activated (quantum) diffusion in the gas of delocalized3He quasiparticles. The obtained experimental results can be described only qualitatively by current kinetic theory of binary quantum solid mixtures. The conditions permitting the realization of the isotopic phase separation during the time observed in the experiment are analyzed. The effective quantum diffusion coefficient providing required3He atoms transport is about an order of magnitude higher than the corresponding value measured in NMR experiments. These conditions are probably fulfilled at the big concentration gradient which takes place at isotopic phase separation. The corresponding kinetic theory should be developed.

Magnetic and Thermal Relaxation in Phase-Separated Solid 3He-4He Mixtures

Relaxation processes in phase-separated solid mixtures of3He in4He with the initial concentration about 3.18 % under the pressure about 3.7 MPa are investigated in the temperature range 1-250 mK. The system was subjected to the action of various sequences of NMR pulses and the character of relaxation was determined from restoration of the longitudinal magnetization to its equilibrium value. Above 100 mK two exponential curves corresponding to the daughter phases - concentrated and diluted - were distinctly observed, which permitted the spin-lattice relaxation time to be determined in these phases. Below 100 mK the NMR signal from the dilute phase practically disappears because of a very small amount of3He in the phase; in this case the Zeeman subsystem and the lattice are related both through magnetic and through thermal relaxation. The measurements using NMR pulses of different length and power demonstrate appreciable overheating of Zeeman subsystem. This shows up as nonmonotonic multi-exponential restoration of equilibrium magnetization whose parameters depend on the power of tipping NMR pulses. The results obtained are described within the proposed physical model.

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

Nucleation and phase separation kinetics of superfluid3He–4He 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 the3He 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.

Spin-lattice relaxation time in the phase separated HCP3He-4He mixtures

Spin-lattice relaxation time in a mixture of a 3.18%3He initial concentration is measured by a pulsed NMR technique at a frequency of 250 kHz during phase separation. The relaxation time in a concentrated BCC phase is found to differ substantially from that in the initial mixture and does not depend on temperature in the whole range of its existence. It is shown that relaxation is governed by the Zeeman-exchange mechanism. In a dilute HCP daughter phase relaxation time increases on cooling, what is treated as a manifestation of the inverse3He consentration dependence.

Time constant of isotopic phase separation in HCP3He−4He mixtures: mass quantum diffusion role

Experimental and theoretical study of the growth kinetics of BCC3He precipitates in isotopic phase separation of HCP3He−4He is carried out. Low frequency pulsed NMR is used to study the mixture with an initial concentration of 3.18% of3He at a pressure 3.7 MPa during stepped cool down into the separation region. The separation time constant is shown to decrease monotonically with cooling. the evolution of heterophase structure resulted from decay is investigated by solving the equation for spatially nonhomogeneous order parameter of decomposing mixture. The obtained temperature dependence of separation time constant is found to be in good agreement with experiment.