Eduard Ya. Rudavskii

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.

Fast Diffusion Process in Quenched hcp Dilute Solid 3He–4He Mixture

The study of phase structure of dilute 3He–4He solid mixtures of differing quality is performed by the spin echo NMR technique. The diffusion coefficient is determined for each coexistent phase. Two diffusion processes are observed in rapidly quenched (non-equilibrium) hcp samples: the first process has a diffusion coefficient corresponding to hcp phase and the second one has huge diffusion coefficient corresponding to liquid phase, which is evidence of liquid-like inclusions formation during fast crystal growing. It is established that these inclusions disappear in equilibrium crystals after careful annealing.

NMR Study of Disordered Inclusions in the Quenched Solid Helium

Phase structure of rapidly quenched solid helium samples is studied by the NMR technique on dilute 3He–4He mixtures. The pulse NMR method is used for measurements of spin–spin T2 relaxation time and spin diffusion coefficient D for all coexisting phases. It was found that quenched samples are two-phase systems consisting of the hcp matrix and some inclusions which are characterized by D and T2 values close to those in liquid phase. Such liquid-like inclusions undergo a spontaneous transition to a new state with anomalously short T2 times. It is found that inclusions observed in both the states disappear on careful annealing near the melting curve. It is assumed that the liquid-like inclusions transform into a new state—a glass or a crystal with a large number of dislocations. These disordered inclusions may be responsible for the anomalous phenomena observed in supersolid region.

Features of the temperature dependence of pressure of solid helium at low temperatures

A series of experiments has been performed to investigate the conditions of formation of a disordered (glass-like) state in crystals of 3He. With the help of precise measurements of pressure at constant volume it has been established that a glass phase is formed easily in rapidly cooled crystals grown under homogeneous temperature conditions in the presence of large numbers of nucleation centers. This phase can be removed only by careful annealing. This result has been found in both 3He and 4He, and is independent of type of quantum statistics and determined mainly by crystal growth conditions. An analysis of similar measurements has been performed using a different cell where during the crystal growth a directed temperature gradient was created. In this case, additional defects created as a result of deformation of the crystal were necessary to form a glass-like phase. The degree of deformation of a crystal, achievable in the experiment, was sufficient to form a glass-like phase in solid 4He, but not in ...

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.

Resonance Microwave Absorption in He II

Microwave (MW) absorption in liquid 4He is investigated in the frequency range of 40-200 GHz at T = 1.4 - 2.5 K. Whispering gallery of waves was generated by a dielectric disc resonator immersed into the liquid. Resonance absorption of MWs was detected at 180.3 GHz, which corresponds to the roton minimum of the liquid helium excitation spectrum. The creation of a single roton is possible because of the presence of the resonator wall which absorbs an extra momentum. The resonance frequency is shown to decreases with temperature in an excellent agreement with the temperature dependence of the roton gap obtained previously in the neutron scattering experiment. The temperature dependence of the MW absorption data indicates the anomalous behavior near the lambda-point and displays the hysteretic character.

Kinetics of Phase Transition at the BCC‐HCP‐Liquid Triple Points of 4He

The kinetics of BCC‐HCP phase transition along the 4He melting curve is studied by precise pressure measurements in the temperature range of 1.25–2.0 K in the vicinity of the triple points (BCC‐HCP‐He II and BCC‐HCP‐He I). It is found that the time dependence of the pressure change along the melting curve far from the triple point can be fitted to an exponential. During the crossing of the triple points, an extra contribution to the kinetics of pressure is found in addition to the exponential one. These anomalies can be explained by assumption a re‐melting of the crystal during the structural phase transition.

Temperature and concentration gradients in superfluid 3He–4He mixtures under steady-state conditions

Abstract The temperature and concentration gradients ∇T and ∇x are measured in a superfluid 3 He – 4 He mixture with a concentration 9.8% of 3 He in the temperature range 70– 500 mK . The gradients are produced by a steady thermal flow Q and registered by two thermometers and two capacitive concentration gauges. It is shown that the relationship between the steady values of ∇T and ∇x in the superfluid solutions is in agreement with a theoretical model derived from the temperature and concentration dependencies of the osmotic pressure. From the results obtained in the experiment we found the thermal diffusion ratio of the solution responsible for the thermal diffusion coefficient.

The first sound velocity and attenuation of supersaturated superfluid 3He–4He solutions under elevated pressure

Abstract The experimental study of concentration dependencies of the first sound velocity and attenuation coefficient in superfluid 3 He – 4 He mixtures for saturated and supersaturated regions at constant elevated pressure and constant temperature has been carried out. The metastable long-lived state is reached by original technique of continuous increasing of the 3 He concentration in situ due to changing of osmotic and fountain pressures. It was shown that the data obtained are in good agreement with the theory of sound propagation in a gas of fermi-excitations. Corresponding values of effective mass and relaxation time of 3 He quasi-particles were taken from the analysis of the experimental data available. The excess sound attenuation was not registered in the supersaturated region within the accuracy experiment.

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.