A.N. Lykov

Disorder and vortex dynamics in high- superconductors

We have investigated V - I characteristics in the microvolt region of c-axis-oriented films in the temperature range 20 - 70 K. A change in the curvature of the log V - log I curves was observed and to explain it we have used a model based on the Anderson - Kim theory of thermally activated flux creep for the washboard-type pinning potential in the presence of a pinning strength distribution. This model gives quantitative agreement with our experimental data.

Thickness dependence of pinning mechanisms in granular Nb thin films

We have investigated the influence of thickness on the pinning properties of granular sputtered Nb films in a parallel magnetic field. The presence of a crossover in the pinning mechanisms at some critical thickness dc was established. Below dc the pinning is due to individual vortices. Above dc the commensurability between the vortex period and the film thickness occurs, causing a large increase in the critical current density due to surface pinning. It is shown that, in the vortex-free limit, when the critical current is determined by the depairing current, the effect of the commensurability is not present.

Numerical simulation of flux creep in high- superconductors

Anderson theory is modified to explain features of the flux creep in high- superconductors. The approach is based on a consideration of single-vortex pinning and creep in a washboard potential with the presence of a normal pinning strength distribution. It explains the scaling behaviour of the E-J curves, the logarithmic shape of the potential barrier as a function of the transport current, the decrease of the barrier as the temperature goes to zero and other results of the studies of flux creep in high- superconductors. Moreover, the method provides the possibility of estimating the main pinning parameters of superconductors by fitting the calculated and experimental data.

Asymmetry of the critical current and peak effect in superconducting multilayers

The critical current Ic in Nb/NbO and Nb/Pd multilayers with different periods has been investigated in parallel magnetic fields H. The Ic(H) curves were measured for two opposite directions of the bias current Ibias (always oriented perpendicularly to the magnetic field) which causes the motion of the vortices towards the free surface of the sample and the substrate, respectively. For both directions of the current the so-called peak effect has been observed in the Ic(H) dependencies but with a large difference in the absolute values of Ic for the positive and negative directions of Ibias. The position of the peak in the Ic(H) dependencies does not depend on the direction of Ibias and it is shifted towards higher H values when the period of the multilayered structures is increased. These experimental results can be explained by considering the superposition of the applied magnetic field and the field induced by the transport current along the layers which, if the superconducting properties in different Nb layers are non-homogeneous, causes an asymmetric redistribution of the current. The effect is more pronounced when only one superconducting layer has different properties.

Frequency effects of spatial dispersion in semiclassical model of conductivity

Effect of significant retardation of an electromagnetic wave in microstrip microwave systems, where the phase velocity becomes less than the Fermi velocity, opens a novel experimental approach to investigations of properties and pairing mechanisms of carriers in superconductors by measurements of spatial dispersion effects at low temperatures. In this study, we present for the first time the semiclassical model of the current carrier dynamics depending on the frequency and phase velocity of electromagnetic wave, which accounts the unlocal spatial dispersion effects. These equations make it possible to perform a comparative analysis of the spatial effects observed in high-temperature superconductors, conventional superconductors, and normal metals.

Critical State Simulation of the Superconducting Layered Structures Based on Numerical Solution of the Ginzburg‐Landau Equations

The critical state of the superconducting layered structures consisting of isolated superconducting plates is investigated. The critical current density, pinning force, the distribution of the local magnetic field, and the current in the structures are found via numerical solution of the Ginzburg‐Landau equations. The external magnetic field and the transport current are directed along plates perpendicularly to each other. The vortices are believed to be absent in the plates. A method for analyzing the critical state of superconducting layered structures in a parallel magnetic field based on simple transformation of the current distribution in a zero external magnetic field is found.

Angular Effects of the Critical Current in Nb/Pd Multilayer Structures

The critical current density dependencies on the angular orientation of the applied magnetic field (Jc(θ)) in superconducting multilayers based on Nb/Pd were investigated in this work. We have demonstrated that the procedure of measurements is an important factor for the Jc(θ). In rotating of the sample at a fixed magnetic field, it was found that the direction of the external field at which the maximum of Jc(θ) was obtained formed an angle with the planar direction. The results can be explained in terms of magnetic field suppression of percolation currents in a weak link network. Flux trapped within grains by flux pinning processes is relatively stable at low fields and its contribution to transport current suppression is added vectorially to the applied field.

Paraelectric Permittivity And Temperature Dependence Of Resistivity And Hall Coefficient In High‐Tc Metal Oxides

Local electric fields in the metal oxide superconductors, which are generally close to the metal‐insulator phase transition and possess the very short length of the mean free path for current carriers, order of a lattice constant, can result in the paraelectric permittivity for bound charges (soft dipoles), which obeys the Curie law. The Hall effect and dc resistivity temperature behaviours are explained here by the model of the paraelectric crystal close to the point of the Mott‐Hubbard instability, in the ground state of which the current is carried by a liquid of boson‐like pairs of carriers in upper and lower Hubbard bands. Fermion‐like carriers, temperature excited over the energy of boson‐like pair dissociation (pseudo gap), explain the temperature behaviour of Hall effect. Available data are compared with the model.

Generation of the electromagnetic radiation by superconducting films

Coherent microwave radiation has been directly detected from superconducting Nb films in the frequency range up to 600 MHz. The mixed state in the films has been influenced by a superposition of two alternating magnetic fields directed perpendicular to film surface. First of them, which slowly varies in time, sets up a vortex structure in the film, while the second high-frequency electromagnetic field provides a synchronization of Abrikosov vortex motion. The simultaneous action of the fields results in either the amplification or generation of electromagnetic radiation. Harmonic mixing of the radiation is also detected.

Scaling of I–V curves and flux creep in high-Tc superconductors

Abstract The scaling of I–V curves of high-Tc superconductors is explained in terms of thermally activated flux creep for the modified washboard type pinning potential. Our approach is based on the consideration of the uncorrelated interaction of isolated vortices with the pinning potential and taking into account flux flow motion. Quantitative agreement between the model and the experimental data was obtained.