Ye. V. Syrnikov

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.

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.

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 ...

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.

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.

Nuclear spin–spin relaxation in 3He–4He two-phase solid solutions at ultralow temperatures

The spin–spin relaxation time in a 3He–4He solid solution is measured before and after phase separation in the temperature range 1–250 mK. The spin echo technique is used, which permits separating the contributions of the two separated phases to the magnetic relaxation. It is found that in the concentrated phase the spin–spin relaxation time is practically independent of temperature above 50 mK and is described by the same exchange mechanisms as in pure 3He. In the dilute phase the relaxation time is inversely proportional to the concentration and agrees with the corresponding values for homogeneous solutions. The dominant contribution to the spin–spin relaxation process is from 3He–4He tunneling exchange. At the lowest temperatures the spin echo exhibits anomalous behavior, which may be a manifestation of quasi-one-dimensional diffusion.

Spin-Spin Relaxation in Phase-Separated 3He-4He Solid Mixtures

The spin-spin relaxation time T2 in a 3He-4He solid mixture with an initial concentration of 3.18% 3He is measured during its phase separation in a temperature range of 1-250 mK. Cooling down to the region of separation was carried out by small steps (∼10 mK) followed by temperature stabilization for many hours. The time T2 was measured by using the pulsed NMR technique at a frequency of 250 kHz. The spin echo method makes it possible to distinguish the contributions to magnetic relaxation from both the concentrated and the dilute separated phases. The relaxation time T2 in the concentrated phase is found to be practically independent of temperature down to ∼50 mK and is determined by the same 3He-3He exchange interaction as in pure bulk solid 3He. It is found that the behavior of the spin echo signal at ultralow temperatures exhibits an anomaly, which may be connected with quasi-one-dimensional spin diffusion. In the dilute daughter phase the spin-spin relaxation time is inversely proportional to concentration and is described correctly by the Torrey model taping into account 3He-4He tunnel exchange. The values of T2 in this phase coincide with those for a homogeneous (non-separated) mixture of the same concentration.