Serguei N. Burmistrov

Diffusion mechanism in the quantum decay of metastable3He-4He mixtures at low temperatures

We study the quantum phase-separation kinetics of a supersaturated liquid mixture at temperatures down to absolute zero. The energy dissipation effect of diffusion is incorporated into the calculation of the nucleation rate in normal and superfluid liquid mixtures. Unlike single-component systems, diffusion is the principal process which governs both the quantum nucleation rate and supercritical droplet growth rate in mixtures near the demixing curve. The low temperature phase-separation experiments in supersaturated liquid3He-4He mixtures of3He-dilute phase are analyzed and discussed. The critical degree of metastability and at the same time the critical droplet radius are shown to be temperature-independent below about 80 mK, suggesting the temperature of the thermal-to-quantum crossover to be of the same order of magnitude. The critical radius of a demixed droplet is estimated as about 20 Å, being within reasonable values.

Rayleigh-Taylor instability of crystallization waves at the superfluid-solid 4He interface.

At the superfluid-solid 4He interface there exist crystallization waves having much in common with gravitational-capillary waves at the interface between two normal fluids. The Rayleigh-Taylor instability is an instability of the interface which can be realized when the lighter fluid is propelling the heavier one. We investigate here the analogs of the Rayleigh-Taylor instability for the superfluid-solid 4He interface. In the case of a uniformly accelerated interface the instability occurs only for a growing solid phase when the magnitude of the acceleration exceeds some critical value independent of the surface stiffness. For the Richtmyer-Meshkov limiting case of an impulsively accelerated interface, the onset of instability does not depend on the sign of the interface acceleration. In both cases the effect of crystallization wave damping is the reduction in the perturbation growth rate of the Taylor unstable interface.

Visual observation of the bubble dynamics in normal 4 He, superfluid 4 He and superfluid 3 He- 4 He mixtures

In order to compare the bubble dynamics of various quantum liquids, we performed the visual observation of a sound-induced bubble in a normal liquid 4 He, pure superfluid 4 He, and superfluid 3 He- 4 He liquid mixtures of saturated and unsaturated 3 He concentrations. When an acoustic wave pulse was applied to these liquids under saturated vapour pressure, a macroscopic bubble was generated on the surface of a piezoelectric transducer. For all liquids, the size of the bubble increased, as a higher voltage was applied to the transducer at a fixed temperature. In the normal 4 He we observed a primary bubble surrounded with many small bubbles which ascended upward together. In contrast to normal phase, only one bubble was generated in the superfluid 4 He, and its shape proved to be highly irregular with an ill-defined surface. In the 3 He saturated superfluid mixture, we also observed a solitary bubble but with a nearly perfect spherical shape. The bubble in this mixture expanded on the transducer surface, grew to a maximum size of the order of 1 mm and then started shrinking. As the bubble detached from the transducer with shrinking, we clearly detected an origination of the upward jet flow which penetrated the bubble. The jet velocity in the liquid mixture was approximately 10 2 -10 3 times smaller than in water. At the final stage of the process we could sometimes observe a vortex ring generation. It is interesting that, though the bubble size and time scale of the phenomenon differ from those in water, the behaviour in the collapsing process had much in common with the simulation study of the vortex ring generation in water. In addition, for the mixture with the unsaturated 3 He concentration of about 25 % at 600 mK, the shape of the upward jet was observed distinctly, using more precise measurement with shadowgraph method.

Electrically driven 4He-circulating dilution refrigerator for cooling aboard spacecraft

Abstract So far, 3 He– 4 He dilution refrigerators in terrestrial laboratories are the most effective way for obtaining steady low temperatures down to ∼10 mK. Gravity plays a key role as a force that separates the heavier 3 He-dilute (d) phase and the lighter 3 He-concentrated (c) phase into two distinct bulks with a well-defined horizontal flat interface. An equivalent for gravity must be found if 3 He– 4 He dilution refrigerators are planned for operating in space. We give a concrete technical design of 4 He-circulating dilution refrigerator ( 4 He-DR) for the use in a microgravity environment. This is done on the basis of a detailed consideration of the various features of 3 He– 4 He liquid mixtures.

Tangential instability of the superfluid–normal interface in a phase-separated 3He–4He liquid mixture

Abstract We study small oscillations of the interface between 3He-dilute superfluid and 3He-concentrated normal phases of a liquid 3He–4He mixture. A tangential instability specific only for the superfluid–normal interface is found. The interface becomes unstable provided the superfluid component flows in the direction tangential to the interface at a velocity exceeding the critical one of about 6.3 cm/s. The wavelength of the ripple structure equals the capillary constant of about 1.3 mm. The effect of the boundary conditions and phase-separation kinetics at the interface upon the oscillation spectrum is discussed.

On the structure of quantized vortices in a saturated liquid 3He–4He mixture

Abstract The recent experiments on the phase-separation kinetics in liquid 3 He– 4 He mixtures are discussed. The nucleus of the stable 3 He-concentrated phase is assumed to be located at the core of a quantized vortex. The energy of a nucleus is analysed for sufficiently large radii as an expansion in inverse powers of radius. The fact that the 3 He concentration in the 3 He-dilute phase cannot exceed the value at the spinodal or λ -line is involved. The results are compared with the bulk homogeneous nucleation of the stable phase.

Experimental and Theoretical Problems of the Quantum Phase-Separation Kinetics of Liquid 3He-4He Mixtures

The nucleation of the3He-concentrated phase is analyzed at the phase-separation of a supersaturated liquid3He–4He mixture in a wide range of temperatures. The crossover from the classical kinetics to the quantum kinetics is considered. Besides the homogeneous bulk nucleation the nucleation under heterogeneous conditions due to the possible presence of quantized vortices is examined.

On the electron spectrum of a type-II superconductor in magnetic field. The magnetization oscillations

We discuss the behavior of an extreme type-II superconductor in the magnetic field. In the very close vicinity of the upper critical fieldBc2 the electron spectrum is gapless and the de Haas—van Alphen effect reveals the resemblance with that of metals in the normal state, the oscillation frequency being proportional to the inverse magnetic field 1/B. The electron spectrum has a logarithmic singularity at the value of the superconducting gap δ=0. As the magnetic field decreases, the structure of the electron spectrum changes drastically. The electron spectrum proves to be independent of the magnetic field with the exception of the narrow region near the energies of about δ. This dependence results in the novel frequency of magnetization oscilations, being proportional toB−1/2

De Haas-van Alphen effect in a type-II superconductor

In extreme type-II superconductors subjected to an external magnetic field the magnetic quantum levels due to the electron orbital motion are shown to arise with the energy smaller than the magnitude of the superconducting gap. For the region of magnetic fields satisfying B2 < B < Bc2 and B2 = β(T)Bc2 these levels cross the Fermi level, β(T) being the known function of temperature and β(T) < 1. Like normal metals, the presence of such levels gives rise to various oscillation and resonance effects and, in particular, to the dHvA effect in type-II superconductors.

Compressibility effect on the quantum two-dimensional nucleation at low temperatures

We study the quantum two-dimensional nucleation of a stable solid phase during the first-order transition at temperatures down to absolute zero. The key role of the finite compressibility of a metastable liquid phase in calculating the quantum nucleation rate is emphasized. In particular, the nucleation rate proves to be dependent on temperature in the quantum tunneling regime. On the whole, the nucleation kinetics corresponds to dissipative tunnelung through a potential barrier. Energy dissipation is due to emitting sound waves during the growth of a solid nucleus. The features inherent to the quantum 2D growth of steps on the atomically smooth facets of a helium crystal are discussed as well.