A. S. Rybalko

Observation of mechanoelectric effect in He II

An experimental study of the electrical response in 4He in the temperature interval 1.5–Tλ is investigated experimentally by the torsion generator technique. It is shown that the motion of the normal component of He II, entrained by the wall of the torsion oscillator, causes dynamic polarization of the liquid. It is established that the induction potential is proportional to the square of the linear velocity of the oscillator wall.

Microwave experiments in He II. New features of undamped superfluid flows

The stability and oscillatory properties of superfluid ring flow arising around the cylindrical surface of a disk-shaped dielectric resonator immersed in liquid helium are studied experimentally. The velocity of superfluid flow is controlled with special heat guns, placed in He II and generating counterflows of the normal and superfluid components, directed along the tangent to the cylindrical surface of the resonator. In the experiment the amplitude of the microwave signal passing through the resonator is measured and the effect of the phase and dynamic states of the liquid on the signal amplitude is studied. It is found that periodic oscillations of the signal are observed in the He II state, and each period starts with a sharp spike of the amplitude. It is proposed that this behavior signifies instability of superfluid ring flow due to a change in the number of circulation quanta of the superfluid velocity over a very short time. Another effect is due to the appearance of new periodically repeating res...

Resonance excitation of single rotons in He II by an electromagnetic wave. Spectral line shape

The amplitude-frequency characteristic of the spectral line of electromagnetic absorption in liquid helium is measured in the frequency range 40–200GHz at temperatures in the interval 1.4–2.75K. It is found that in the roton frequency region a narrow resonance absorption line on a broad pedestal is observed. The results are compared with data on the roton spectrum found in neutron scattering experiments in liquid helium. The narrow line is due to the creation of a single roton. It is shown that the momentum conservation law is satisfied on account of the transfer of momentum to the superfluid component. The analogy of this effect with the Mossbauer effect is pointed out.

Interaction of microwaves with superfluid flow in HeII

The absorption of 40–200GHz electromagnetic waves in liquid helium at temperatures 1.4–2.8K is investigated experimentally. The spectrum of oscillations of the “whispering gallery” modes of a dielectric disk-shaped resonator immersed in liquid helium is recorded. Superfluid flows in HeII is produced artificially using two “heat guns.” A narrow microwave absorption line has been detected at a frequency corresponding to the excitation energy of a single roton. It has been determined that the character of the resonance line changes radically as the velocity vs of the superfluid flow increases: microwave absorption was replaced by induced microwave emission, which was seen as a sharp intensification of the high frequency signal. The effect can be explained qualitatively on the basis of a two-level model of HeII. An abrupt change in the velocity of the superfluid flow is observed with continual increase of the power delivered to the heat gun. This could attest to quantization of vs. It is shown that the superf...

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.

Formation of a glass phase in solid He4. Contribution to the pressure in the supersolid region

High-precision barometry is used to perform measurements of the temperature dependence of the pressure of helium crystals, located in a constant volume, in the temperature range 50–800mK on crystals of different quality ranging from freshly prepared to carefully annealed. It is found that an additional contribution to the pressure, proportional to T2, appears in on annealed samples. This contribution becomes dominant at T<300mK, i.e. the region where anomalous behavior of solid helium, ascribed to the super solid effect, was previously observed. Such behavior of the pressure is characteristic for the glass phase, and this phase was practically absent in carefully annealed samples. The observed effect shows that in solid helium containing a high concentration of nonequilibrium defects a glass phase forms easily on cooling. Anomalously high pressure relaxation in low-quality samples was also detected in the experiments at temperatures close to the melting temperature. The possible reasons for this effect ar...

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 ...

Observation of Andreev-Pushkarov vacancy clusters in phase-separated solid solutions of 4He in 3He

New features are observed for the pressure in a phase-separated dilute solid solution 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. The equation for determining the radius of a cluster of pure 4He in a solution of 4He in 3He is refined. This hypothesis is shown to provide quantitative agreement between the calculated and experimental data under the assumption that the homogenization of the phase-separated solution is accompanied by the formation of metastable vacancies with a concentration of ∼(4–5)×10−5.

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.