A. A. Lisunov

Creep in solid 4He at temperatures below 1 K

Creep in solid 4He at temperatures of ∼100–1000 mK is studied experimentally by detecting the flow of helium through a frozen porous membrane under a constant external force. Creep curves are measured for different temperatures and mechanical stresses. This method has made it possible to detect low creep rates in helium down to the lowest temperatures in these experiments. It is found that throughout this temperature range, creep is thermally activated and the activation energy decreases with falling temperature and increasing mechanical stress. An analysis shows that for temperatures above ≈500 mK, Nabarro-Herring diffusive creep takes place in solid helium with mass transfer by self diffusion of atoms and a counterflow of vacancies. The experimental data have been used to obtain the self-diffusion coefficient as a function of temperature for different stresses. At temperatures below ≈500 mK creep takes place at a very low flow rate (∼10−13 cm/s) and a very low activation energy (∼0.5–0.7 K), while the c...

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 ...

Search for a disordered phase in solidHe3deformedin situ

A disordered (glassy) state has been searched in solid 3He deformed in the course of experiment employing precise measurements of pressure. The analysis of the temperature dependence of the crystal pressure measured at a constant volume shows that the main contribution to the pressure is made by the phonon subsystem, the influence of the disordered phase being very weak. Annealing of the deformed crystal does not affect this state. The results obtained differ greatly from the corresponding data for solid 4He measured in the region of supersolid effects where a pressure excessive in comparison to the phonon one was registered. The excess pressure had a quadratic dependence on temperature, which is typical of a disordered system. Absence of the excess pressure in solid 3He is unclear yet, some speculative interpretations are suggested.

Formation of a disordered (Glassy) phase in deformed solid 4He in the region of supersolid state

A method has been proposed to create disorder in helium crystals by their deformation immediately during the experiment. Precise measurements of the pressure have been performed at a constant volume in samples of various qualities. It has been revealed that excess pressure, which is characterized by the quadratic temperature dependence typical of the disordered glassy phase and of the dislocation contribution to the pressure, is observed in the deformed crystals along with the phonon contribution to the pressure. The effect is observed in the supersolid-state region and disappears after the careful annealing of the crystals. The ultraslow relaxation of the pressure also characteristic of the glassy phase has been observed in the process of annealing of the crystals. The obtained experimental results have been analyzed in the framework of the dislocation model and the model of two-level tunneling states.

Plastic flow of solid 3He through a porous elastic film

A study of plastic flow of solid 3He through a metalized porous elastic polymer film frozen into the crystal was carried out in the temperature range of 0.1–1 K. The flow was due to mechanical stresses in the crystal induced by external electrical forces. The plastic flow rate of solid helium was determined by measuring the capacitance changes of a capacitor in which the metalized surface of the film served as a movable electrode. Two different regions could be clearly identified on the temperature dependence of the plastic flow rate V(T). Above ∼200 mK, V drops exponentially with decreasing temperature, which corresponds to the thermally activated regime of plastic flow. At lower temperatures, the rate V is temperature independent, indicating quantum plastic flow. A detailed analysis of the experimental data was performed in the thermally activated region. The empirical values of the following parameters were defined: the activation volume and energy and the yield strength, corresponding to the onset of ...

Phase separation and diffusion processes in concentrated He3–He4 solid mixtures

The kinetics of phase separation in quantum crystals of concentrated He3–He4 mixtures containing 34.0, 61.3, and 89.9% He3 are investigated experimentally by the method of precision barometry. The variation of the pressure P(T) of the system upon stepwise cooling and heating is registered. The P(T) curves observed in the experiment are compared with a theory based on a diffusion description of the kinetics of isotropic phase separation. This yields the first information about the mass diffusion coefficient in the whole concentration range. It is shown that in concentrated mixtures the mass diffusion coefficient is several orders of magnitude lower than in dilute mixtures, owing to the interaction of impurities and the presence of elastic strains in the crystal. At the same time, because of the high concentration of new-phase nuclei and the small diffusion length, the phase transition process takes place over a comparatively short time.