T. V. Kalko

Relation between the temperature gradient and concentration gradient in superfluid 3He–4He mixtures

The temperature and concentration gradients ∇T and ∇x created by a heat flow in a superfluid 3He–4He mixture with a concentration of 9.8% 3He are measured in the steady state in the temperature range T=70–500 mK. It is found that the ratio ∇x/∇T obtained in the experiment can be described on the basis of a dependence of the osmotic pressure on the temperature and concentration. The data are used to find the thermodiffusion ratio and, hence, the thermodiffusion coefficient of this mixture.

Convective instability of 3He–4He superfluid mixtures heated from below

The convective thermal instability of 3He–4He superfluid mixtures on heating from below is studied in the temperature interval 100–500 mK in mixtures with an initial concentration of 9.8% 3He. It is found that at certain critical values of the power of the heat flux in the mixture there is a change in character of the heat transfer, indicating the onset of convective heat transfer. The critical temperature gradients associated with the onset of convection correspond to Rayleigh numbers many orders of magnitude greater than the Rayleigh numbers in the heating of the mixtures from above. The results are analyzed in the framework of the theory of the convective instability of binary mixtures.

Concentration dependence of the attenuation of first sound in supersaturated superfluid 3He–4He solutions under pressure

The concentration dependence of the attenuation coefficient of first sound in superfluid 3He–4He solutions in the saturation and supersaturation regions is investigated experimentally at pressures of 0–10 atm. An original technique of continuous variation of the concentration in situ by variation of the osmotic and thermomechanical pressures is used, permitting measurements to be made in the long-lived metastable phase of the superfluid solutions. It is shown that the data obtained are described well in terms of the theory of sound propagation in a gas of Fermi excitations without taking the Fermi-liquid corrections into account. The corresponding values of the effective mass and relaxation time of 3He quasiparticles are taken from an analysis of the existing experimental data. Within the experimental error, no excess sound attenuation was found in the region of supersaturated solutions.

Relaxation of temperature and concentration in superfluid 3He–4He mixtures. Effective thermal conductivity

The relaxation of temperature and concentration of a superfluid 3He–4He mixture with an initial molar concentration of 9.8% 3He is investigated in the temperature interval 100–500 mK for different values of the heat flux. It is shown that the kinetics of the change in temperature of the liquid can be approximated by an exponential function, and the time constants obtained depend weakly on temperature. The concentration relaxation processes are analogous to the temperature relaxation processes only in the region of the single-phase mixture, while below the phase separation temperature the change in concentration with time is of a nonmonotonic character and can be described by a superposition of two exponential functions. This kind of kinetic behavior of the mixture is explained in the framework of a simple model which illustrates the distribution of the concentration over the height of the cell at various times after a heat flux is turned on. In this case the concentration relaxation of the dilute phase co...

Phase separation and thermal instability of superfluid 3He-4He mixtures heated from below

The influence of phase separation on the thermal stability of superfluid 3He-4He mixtures heated from below by a heat flux with a power density Q up to 20 μW/cm2 is investigated in the temperature region 100–500 mK. It is found that instability is observed only in the presence of phase separation (in the high-temperature region) even at manifestly high heat flux densities. With increasing power of the heat flux at low temperatures the system undergoes phase separation and then, on further increase of Q, suffers thermal instability, which is manifested in a sharp increase in the effective thermal conductivity. These effects are apparently due to the destabilizing influence of the interface between the separated phases owing to capillary effects. The values of the Rayleigh and Marangoni numbers, which characterize the influence of the buoyancy and capillary forces, respectively, in relation to each other are analyzed, and it is noted that the concentrated phase is more sensitive to the action of capillary...