B. I. Ivlev

The Flux-Flow Hall Effect in Type II Superconductors. An Explanation of the Sign Reversal

We consider a time-dependent Ginzburg-Landau (TDGL) model modified to take into account two mechanisms responsible for the Hall voltage in superconductors: the usual effect of the magnetic field on the normal current, and the vortex traction by the superflow. For the BCS model of superconductivity, the contribution of the vortex traction is proportional to the energy derivative of the quasiparticle density of states. Our theory gives the correct order of magnitude for the Hall angle in the mixed state. It predicts that the vortex-traction mechanism results in a negative Hall angle for the quasiparticle spectrum with a positive energy derivative of the density of states averaged over the Fermi surface, and vice versa. For the Fermi surface with a complicated shape, the sign of the Hall effect in the mixed state can be different from that in the normal state. If the signs are opposite, the Hall angle changes its sign as a function of the magnetic field belowHc2.

Electric currents and resistive states in thin superconductors

Abstract Properties of thin superconducting filaments carrying a d.c. electric current are considered. We study first the stability of the normal state in the current-carrying filament. It is shown that the phase transition from the normal into the superconducting or into the resistive state in the presence of a current is a first-order transition. The mechanism of the transition changes when the filament has a non-uniform structure; the normal state of a filament becomes unstable in this case below some critical current dependent on the type of inhomogeneity. We study then the resistive state in thin superconducting filaments. As is well known the usual superconducting state becomes impossible when the current exceeds the so-called Ginzburg-Landau critical current. Instead, either the normal or the so-called resistive state appears in the superconductor. In the resistive state, the mean electric field is non-zero in the superconductor. Nevertheless superconductivity is destroyed only in some separate poi...

Vortex motion in high-T c superconductors near the upper critical fieldH c2

The vortex dynamics in layered superconductors such as YBaCuO and BiSrCaCuO near the upper critical field Hc2 is considered. The magnetic field is parallel to the layers and a transport current flows along the layers perpendicular toH. There exists a finite pinning force for the vortex motion across the layers in a macroscopically homogeneous superconductor. The critical current vanishes exponentially in the limit of weakly layered structure, ξc(T)≫s. The current-voltage characteristic is calculated in case ξc(T)≫s. The initial part of the characteristic may have a negative slope depending on the magnetic fieldH.

Foam and cluster structure formation by latex particles at the air/water interface

We report the formation of two-dimensional foam and cluster structures by spherical polystyrene particles trapped at the air/water interface. The colloidal foam is a transient structure that evolves to the formation of clusters, but clusters can also be formed after deposition of the sample. We also observed the formation of small aggregates, whose formation along with the cluster stabilization can be explained in terms of a balance between electrostatic repulsive and van der Waals attractive interactions.

Computer simulations of confined colloidal systems at the air/water interface

We present Monte Carlo simulations in the NV T ensemble for spherical particles of diameter σ interacting via a hard-core potential with a squareshoulder (SS) repulsive barrier, a square-well (SW) attraction, and a second SS repulsion. This discrete potential is used to mimic the pair interaction in confined colloidal systems at the air/water interface. The SW attraction represents a secondary minimum for large interparticle distances, and the SS repulsion is a shallow secondary maximum after the secondary minimum. The effect of the SS range λr σ is studied for the cases λr = 0, 6, and 7. The simulation results for the last two cases indicate the important role of the presence of the secondary maximum in the interaction potential, since they are able to reproduce the main features observed in colloidal particles trapped at the air/water interface, such as clustering, chain formation, foams, and the presence of voids.

THE KINKED VORTICES IN LAYERED SUPERCONDUCTORS IN THE PRESENCE OF A TILTED MAGNETIC FIELD

In highly layered high-Tc superconductors in the presence of a magnetic field having an angle with respect to the planes, a system of vortex kinks is a more appropriate description of the structure than a set of rectilinear vortices. This description leads to a finite jump of the sample torque at zero angle between planes and magnetic field direction, to the nontrivial behaviour of the ratio at the perpendicular and parallel magnetization, and to strong anisotropy of the critical current.

Activated flux creep in layered high-temperature superconductors

We study thermally activated flux creep in layered superconductors in the case when the magnetic field is parallel to the layers and vortices move across the layers. We assume that the sample is without imperfections, so that the pinning is due to an interaction between vortices and the layered microstructure of the superconductors. The height of the energy barrier that vortices overcome during each activation process is calculated in the case when the vortex lattice period is commensurate with the interlayer spacing. This energy barrier depends on the transport current: it grows at small currents and goes to zero linearly whenj→jcwherejcis the depinning current.

Vortex Lattice in Layered Superconductors in the Presence of a Tilted Magnetic Field

In superconductors with a well-pronounced structure of order parameter, particularly in high-Tc superconductors, a magnetic field slightly tilted to superconducting planes, gives rise to a system of kinks onto vortex lines. This leads to a finite jump of the torque when passing by the orientation of the field parallel to the layers and also to strong anistropy of the critical current vs. the orientation of the field.

Superconducting tunnel junction detectors: Analysis of the linear regime

Abstract A theoretical analysis of the linear regime in a superconducting tunnel junction (STJ) has been carried out on the basis of a kinetic approach to the non-equilibrium superconductivity induced in the detector by an X-ray. A solution of the diffusive equation of the quasi-particles has been obtained assuming very slow recombination processes and an equilibrium 2Δ-phonon distribution. Under these conditions, the response of the detector is linear, that is the charge collected by the detector itself is proportional to the energy released. A relation between this released energy and the collected charge has also been obtained, thus achieving an expression of the mean energy ϵ per excess quasi-particle. A comparison with some experimental results taken from current literature has been also reported.

Effect of resonant pumping on activated decay rates

Abstract The problem of oscillations of a brownian particle at the bottom of a potential well under the action of a force E cos Ω t is considered. The rate of escapes out of the well is found to exhibit a threshold behavior when the resonance is crossed.