Vladimir A. Mikheev

Nucleation and growth of the new phase in the supersaturated3He-4He superfluid solutions

Phase separation kinetics and growth of the new phase in supersaturated superfluid3He-4He solutions was studied using the method of continuous change of3He concentration in situ and developed theory. Simultaneous measurements of the first sound velocity and the dielectric constant were carried out during the change of concentration. The velocity and attenuation of the first sound and the dielectric constant in supersaturated metastable solutions were calculated on the assumption that the excess3He in the solution is in the form of both atoms and droplets of the new phase. It was shown that the sound velocity, unlike the dielectric constant, is very sensitive to the formation of droplets. Comparison of the theory and the experimental data allowed us to elucidate the growth dynamics of the nuclei of the new phase from the metastable supersaturated mixture.

Automated unit for physical research on quantum crystals at millikelvin temperatures

A cryogenic unit designed for physical research on quantum crystals at ultralow temperatures is described. The unit includes a dilution refrigerator with a hybrid 3He circulation system, an antivibration protection system, and a system of automated control and data acquisition and processing. The construction of the basic units, the thermometry system, the algorithms for measurements, and the software for conducting research on quantum crystals by the method of precision pressure measurements are described. The unit may be used for performing a wide range of tasks in research on various condensed systems at millikelvin temperatures.

Nucleation and phase separation in superfluid3He-4He solutions

The kinetics of the phase separation in3He-4He superfluid solutions is studied from anomaly in the first sound velocity and the dielectric constant. The histogram of experiments on solution supersaturation is obtained and used to study of phase separation probability. It was shown that above 50 mK the obtained temperature dependence of the supersaturation is qualitatively in accord with the thermal activation mechanism of nucleation. Below 50 mK the data may point towards a transition from classical to quantum mechanism of nucleation.

Equilibrium phase diagram and kinetics of isotopical phase separation of 3% 3 He in HCP 4 He mixture

The isotopical phase separation in a weak solid mixture of3He in4He is investigated by 250 kHz pulsed NMR under a pressure of 3.7 MPa in the temperature range 100 – 240 mK. The equilibrium concentrations of3 He in HCP phase are found to be in a good agreement with classical regular solution theory for phases with different crystal structures. The time constant of decay at steplike cooldown along the separation line are shown to monotonously decrease. The linear dependence of the time on the concentration may point towards an important role of quantum diffusion.

Continuously operating cryogenic cycle dilution refrigerator

Abstract A dilution refrigerator is described in which the continuous circulation of mixture is achieved by two cryopumps operated sequentially under automatic control. A base temperature of 8mK is achieved, with four discrete silver heat exchangers, and this is stable on switching between pumps. The design and performance of the refrigerator and their potential for improvement are discussed.