Індекс рубрикатора НБУВ: В368.3е(0)

Шифр НБУВ: Ж14063 Пошук видання у каталогах НБУВ 

Stationary and nonstationary, in particular, chaotic states in long Josephson junctions are investigated. Bifurcation lines on the parametric bias current-external magnetic field plane are calculated. The chaos strip along the bifurcation line is observed. It is shown that transitions between stationary states are the transitions from metastable to stable states and that the thermodynamical Gibbs potential of these stable states may be larger than for some metastable states. The definition of a dynamical critical magnetic field characterizing the stability of the stationary states is given.

A theory is offered for the ballistic 4-terminal Josephson junction. The studied system consists of a mesoscopic two-dimensional normal rectangular layer which іs attached on each side to the bulk superconducting banks (terminals). A relation is obtained between the currents through the different terminals, that is valid for arbitrary temperatures and junction sizes. The nonlocal coupling of the supercurrents leads to a new effect, specific for the mesoscopic weak link between two superconducting rings; an applied magnetic flux through one of the rings produces a magnetic flux in the other ring even in the absence of an external flux through the other one. The phase dependent distributions of the local density of Andreev states, of the supercurrents and of the induced order parameter are obtained. The "interference pattern" for the anomalous average inside the two-dimensional region can be regulated by the applied magnetic fluxes or the transport currents. For some values of the phase differences between the terminals, the current vortex state and the two-dimensional phase slip center appear.

We report the first observation of the lock-in transition in artificial superconducting superlattices, which takes place in tilted magnetic fields. The measurements were carried out on the Mo/Si layered system. The temperature dependence of the critical angle for the trapping of the vortices in the orientation parallel to the layer planes is determined by the previously known resistive method and by a new method based on the effect of commensurability between the intervortex distance and the superlattice wavelength. The temperature dependences of the critical angle obtained by the two methods practically coincide. The experimental results are consistent with the theoretical predictions of Feinberg and Villard.

The behavior in an external magnetic field is studied exactly for a wide class of multichain quantum spin models. It is shown that the magnetic field together with the interchain couplings cause commensurate - incommensurate phase transitions between the gapless phases in the ground state. The conformal limit of these models is studied and it is shown that the low-lying excitations for the incommensurate phases are not independent, because they are governed by the same magnetic field (chemical potential for excitations). A scenario for the transition from one to two space dimensions for the exactly integrable multichain quantum spin models is proposed, and it is shown that the incommensurate phases in an external magnetic field disappear in the limit of an infinite number of coupled spin chains. The similarities in the external field behavior for the quantum multichain spin models and a wide class of quantum field theories are discussed. The scaling exponents for the appearence of the gap in the spectrum of a low-lying excitations of the quantum multichain models due to the relevant perturbations of the integrable theories are calculated.

The problem of the robust de Haas-van Alphen oscillations seen experimentally in mixed superconducting state is discussed. The new threshold mechanism is suggested to explain persistence of the quantum oscillations deep in the superconducting state.

The collective electromagnetic wave excitations in layered conductors in an external quantizing magnetic field are studied theoretically. A set of coupled equations for the electric field components and the Greens functions describing collective electromagnetic modes in such a system within the linear-response approach are obtained on the basis of a model of conducting planes embedded into a dielectric matrix. These equations, written in the layer-site representation, provide a basis for studies of the electromagnetic waves in the general case which does not imply either the spatial uniformity across the layers or the homogenity within the conducting planes. The dispersion equations are obtained for the uniform and homogeneous layered conductors, which give a rich structure of collective electromagnetic modes in layered conductors in an external magnetic field. The dispersion and damping of helicons in layered conductors in the regimes of conventional and quantum Hall effects have been studied, both analytically and numerically. Two new modes propagating perpendicular to the magnetic field with frequencies of the order of the plasma frequency have been found.

Dynamical chaos, states stability in long Josephson junctions are investigated from the point of view of the flux quantization. It is shown that the stationary Meissner and fluxon states having integer number of fluxons are stable. Stationary antifluxon states also having integer number of the flux quanta and all other states with half-integer number of flux quenta are unstable. The transitions between all states - Meissner, the states having the integer and half-integer number of the flux quanta - take place in the nonstationary case, and all these states are dynamically equivalent, but the number of the flux quanta is a nonregular time-dependent functions for the chaotic states and regular for the regular ones.

The thermal conductivity of solid parahydrogen with orthodeuterium impurity at the impurity concentration from 0.01 to 1% has been studied in the temperature range from 1.8 to 9 K. The nonlinear concentration dependence of the thermal resistivity of the investigated samples is explained by the fact that the intensity of normal three-phonon scattering processes is comparable to that of resistive processes. The analysis of the experimental results has been made in the framework of the Callaway model by using different models for the impurity phonon scattering. It has been found that the intensity of phonon impurity scattering in parahydrogen with orthodeuterium impurity is essentially higher than classical theory predicts and it is close to analogous scattering in solutions of helium isotopes with the equal reduced molar volume given. It is considered that additional impurity scattering of phonons is connected with the change of force constants and lattice distortion in the vicinity of impurity molecules. The quantitative estimation of the mentioned effects has been made.

The tunnel resistance of highly stable, mechanically controlled break junctions of Al, Au, Cu, Pb, Ni, Pt, and Pt -Ir, have been recorded as a function of the electrode spacing over 6-7 decades. Clear deviations from the expected exponential behavior have been observed. Comparison with previous experimental and theoretical studies indicate that the discussed deviations in some cases are most probably due to the shape of, rather than to interactions between, the two electrodes.

Індекс рубрикатора НБУВ: В368.313

Рубрики:

Магнітні властивості надпровідників

Шифр НБУВ: Ж14063 Пошук видання у каталогах НБУВ 

The growth of clusters in an overcooled atomic jet is calculated with the simultaneous allowance for two processes: the condensation of supersaturated gas and the cluster-cluster aggregation. These processes are consistently described within a quasi-equilibrium approach for large enough clusters, while their nucleation stage is allowed for phenomenologically by the use of a fitting parameter. The theory quantitatively describes the experimental dependence of the mean cluster size on the initial gas temperature within a broad range of sizes. The mean size of clusters, their distribution over sizes and the contribution of clusters to the total mass of the jet are calculated versus the distance from the gas ejection point (nozzle entry). It is shown that these characteristics are mainly determined by the initial gas entropy which is equivalent to the empirical law of corresponding jets. The size distribution function has a halfwidth exceeding the mean cluster size and is close (on a relative scale) to an universal curve. The broad size distribution of clusters should be taken into account when analyzing spectroscopic data on the size dependence of electronic processes in a cluster.

We have analyzed the quantum interference effects in the macroscopic "superconducting molecule". The composite system consists of two massive superconducting rings, each interrupted by a Josephson junction, which are at the same time weakly coupled with one another. The special case of coupling via the Josephson four-terminal junction is considered. The structure of the macroscopic quantum states in an applied magnetic field is calculated. It is shown that, depending on the values of the magnetic fluxes through each ring, the system displays two groups of states, the "orthostates" with both induced currents going in the same direction, and the "parastates" with the opposite currents and with the total induced flux locked to zero value. The transition to the flux locked state with changing of the total applied flux is sudden and is preserved in a certain interval which is determined by the difference of the fluxes applied through each ring. It makes the system sensitive to small gradients of the external magnetic field.

We have studied the force in mesoscopic wires in the presence of an external magnetic field along the wire using a free electron model. We show that the applied magnetic field can be used to affect the force in the wire. The magnetic field breaks the degeneracy of the eigenenergies of the conduction modes, resulting in more structure in the force as a function of wire length. The use of an external magnetic field is an equilibrium method of controlling the number of transporting channels. Under the least favorable circumstances (on the middle of a low conduction step) one needs about 1.3 T to see an abrupt change in the force at fixed wire length for a mesoscopic bismuth wire.

In an approach of low transparency of the barrier the tunneling of electrons through doubly degenerate local state has been considered with allowance for the Coulomb and electron-phonon interactions. It is shown that in the case of weak electron-phonon and strong electron-electron interactions the dependence of tunneling current on the applied voltage has a step-like character at low temperature. The threshold value of the current was measured for small applied bias. The bistable state of the tunneling current is possible in the region of large bias. In the case of strong electron-phonon and weak electron-electron interactions, the threshold of tunneling current can be bistable. This result is a direct consequence of the electron pairing in local states.

Two different approaches (presented in the literature as alternative approximations) to the problem of carrier-induced ferromagnetism in the system of disordered magnetic ions of a diluted magnetic semiconductor are analyzed. They are based on a self-consistent procedure for the mean exchange fields and the Ruderman - Kittel - Kasuya - Yosida interaction. Calculations in the framework of exactly solvable model are carried out, and it shows that these approaches stem from two different contributions to the magnetic susceptibility. One stems from the diagonal part of the carrier - ion exchange interaction and corresponds to mean field approximation. The other one stems from the off-diagonal part of the same interaction and describes the indirect interaction between localized spins via free carries. These two contributions can be responsible for the different magnetic properties. Thus, the aforementioned contributions are complementary but not alternative to each other. A general approach is proposed and compared with different approximations to the problem under consideration.

The interaction of electromagnetic radiation with a low-density disordered system of fine metallic particles located in a dielectric matrix is considered in a quasi-static approach with allowance for dipole-dipole interaction. The electric dipole moment of the system is calculated in a pair approximation. The case of nonidentical particles differing in their sizes or dielectric functions is presented. Allowance for particle nonidentity leads to doubling of the number of their surface modes and to the appearance of band gaps in the spectrum. The spectrum peculiarities are revealed in the fine structure of electromagnetic absorption.

A new type of quantum size effect in metal-semiconductor superlattices is predicted. Giant oscillations of the transverse tunnel conductivity arise if size quantization of the electron spectrum in the metal layers takes place. This effect is due to the fact that the probability of metal electron tunneling through a semiconductor layer depends sharply on the electron incidence angle. The oscillations have been found to exist even in disordered systems, provided the electrons in metal layers undergo low-angle scattering on imperfections.

Surface polaritons in a semi-infinite insulator in a constant magnetic field at the boundary with an ideal metal or superconductor have been considered. These polaritons are induced by dynamic magnetoelectric interaction, which exists in any insulator in the presence of a magnetic field. The modes of these surface polaritons differ appreciably in opposite directions of the magnetic field or the propagation of the wave. As a result polaritons of the given optical or IR frequency propagate only in one direction, which is the effect of rectification of surface electromagnetic waves. The inversion of the magnetic field results in "switching on" or "switching off" of surface polaritons.

There are rather exotic semiconducting superlattices (SL) consisting of monochalcogenides of Pb, Sn and rare-earth metals which exhibit superconductivity at temperature as high as 6 K. Here we report the results of precision x-ray diffractometry and TEM investigations as well as Auger spectroscopy data obtained on some of the epitaxially grown superconducting semiconducting SLs. It is established that essential features of the SL structure determining the appearance of superconductivity are the perfect single-crystallinity of the samples and the presence of continuous dense grids of misfit dislocations on the interfaces between two semiconductors. The segregation of free Pb which was observed in some cases does not correlate, according to experimental data, with the appearance of superconductivity.