V. A. Kashurnikov

On the mechanisms of transport losses in high-temperature superconductors

Monte Carlo simulations of a high-temperature superconductor carrying an alternating transport current have been performed. The influence of defect concentration and external magnetic field on the transport energy losses is investigated. We show that there are two components of the losses in a superconductor carrying an alternating transport current: a hysteresis component related to the superconductor remagnetization by the self-current field and a dynamic component related to the steady flow of vortices-antivortices and their annihilation in the superconductor.

Vortex system dynamics and energy losses in a current-carrying 2D superconducting wafer

The dynamic processes occurring in the vortex system of a 2D superconducting wafer carrying transport current are investigated using the model of the vortex system of high-temperature superconductors. Calculations are performed by the Monte Carlo method. For the first time, the dynamics of magnetic field penetration in a current-carrying HTSC wafer is demonstrated and the energy losses associated with a change in transport current are calculated. It is shown that changes in the transport current amplitude and in the number of defects lead to a change in the energy liberation mechanism: hysteresis energy losses are replaced by the losses in the saturated layer.

Magnetization reversal processes in layered high-temperature superconductors with ferromagnetic impurities

The self-consistent interaction of a vortex system of a high-temperature superconductor and ferromagnetic impurities, including single impurities and their clusters, has been considered in the model of a layered high-temperature superconductor. For different temperatures and concentrations of ferromagnetic impurities, the magnetization reversal loops have been calculated by the Monte Carlo method taking into account an ensemble of ferromagnetic particles with different orientations of their easy magnetization axes with respect to the direction of an external magnetic field and for different magnetic anisotropy energies. It has been demonstrated that there is a nonlinear interaction of the high-temperature superconductor with ferromagnetic impurities, in which the initially thermodynamically reversible character of the magnetization reversal of the ferromagnetic ensemble can become irreversible. For a periodic lattice of clusters of ferromagnetic impurities, the magnetization curves of the high-temperature superconductor have been calculated for different sizes and configurations of the clusters. It has been revealed that, when extended defects are oriented parallel to the direction of the entrance of vortices in the sample, the length of the defects does not affect the remanent magnetization. It has been shown that the inclusion of the interaction between the magnetic moments inside the impurity cluster leads to a decrease in the magnetization reversal loop, the coercivity, and, accordingly, the energy loss due to magnetization reversal.

Phase transitions in a two-dimensional vortex lattice with defects: Monte Carlo simulation

AbstractThe phase states and phase transitions in a system consisting of a two-dimensional vortex lattice with defects are studied by the Monte Carlo method. It is shown that a “rotating lattice” phase, which is an intermediate phase between the vortex crystal and vortex liquid phases, is present. The dependence of the temperature of the transition from the rotating lattice phase into a vortex liquid on the strength of the defect potential is determined. The current-voltage characteristics of the system are calculated at various temperatures for point, square, and linear defects. It is shown that the phase state of the system strongly affects its transport properties.

Nonlinear interaction of a ferromagnet with a high-temperature superconductor

The interaction of an Abrikosov vertex with a ferromagnetic substrate is taken into account in the model of a layered high-temperature superconductor (HTSC). The magnetization reversal loops are calculated by the Monte Carlo method for various values of the magnetic moment of the substrate and at various temperatures. The nonlinearity of the interaction of the superconductor with the ferromagnet is demonstrated. The magnetization of HTSC films on magnetic and nonmagnetic substrates is measured. It is found that the ferromagnetism of the substrate strongly affects the shape and magnitude of the magnetization of the HTSC-substrate composite. Experimental data are found to correlate with the results of calculations.

Magnetization of layered high-temperature superconductors with defects: Monte Carlo simulation

A new method of numerical simulation of the magnetization of layered high-temperature superconductors (HTSCs) with arbitrary pinning centres has been developed. A HTSC plate with a finite width in the x-direction and an infinite width in the y-direction was considered in a perpendicular external magnetic field. We have minimized a free energy functional by means of the Monte Carlo technique. This procedure allowed us to obtain an equilibrium number of vortices and vortex configurations inside the plate at the various external magnetic fields and so to calculate magnetic induction B(H,T) and magnetization M(H,T). Magnetization curves have been obtained for a cyclical change of the external magnetic field at different temperatures and defective states of the superconductor. The crossover from surface to bulk pinning was demonstrated. The magnetic flux profiles inside a superconductor were calculated. It was shown that the Bean model is true for strong pinning only.

Phase transitions in a two-dimensional vortex system with defects: Monte Carlo simulation

AbstractThe phase states and phase transitions in a system consisting of a two-dimensional vortex lattice with defects are studied by the Monte Carlo method. It is shown that a “rotating lattice” phase, which is an intermediate phase between the vortex crystal and vortex liquid phases, is present. The dependence of the temperature of the transition from the rotating lattice phase into a vortex liquid on the strength of the defect potential is determined. The current-voltage characteristics of the system are calculated at various temperatures for point, square, and linear defects. It is shown that the phase state of the system strongly affects its transport properties.

Features of the melting dynamics of a vortex lattice in a high-T c superconductor in the presence of pinning centers

The phase transition “triangular lattice-vortex liquid” in layered high-Tc superconductors in the presence of pinning centers is studied. A two-dimensional system of vortices simulating the superconducting layers in a high-Tc Shubnikov phase is calculated by the Monte Carlo method. It was found that in the presence of defects the melting of the vortex lattice proceeds in two stages: First, the ideal triangular lattice transforms at low temperature (≃3 K)into islands which are pinned to the pinning centers and rotate around them and then, at a higher temperature (≃8 K for Tc 584 K), the boundaries of the “islands” become smeared and the system transforms into a vortex liquid. As the pinning force increases, the temperatures of both phase transitions shift: The temperature of the point “triangular lattice-rotating lattice” decreases slightly (to ≃2 K)and the temperature of the phase transition “rotating lattice-vortex liquid” increases substantially (≃70 K).

Nonlinear effects at the remagnetization of layered high-temperature superconductors with ferromagnetic impurities under the action of a current and an external magnetic field

The processes of remagnetization by the self-field of a transport current in a high-temperature superconductor sample with ferromagnetic particles as defects in the presence of an external magnetic field have been calculated by the continuous Monte Carlo method. A new effect has been detected in the form of a nonlinear S-shaped character of current-voltage characteristics of the superconductor-ferromagnet in the external magnetic field. It has been shown that the S-shaped feature of the current-voltage characteristic is due to change in the effective interaction of magnetic moments of impurity particles with the vortex system of the superconductor induced by the local remagnetization of ferromagnetic impurities under the action of the self-field of the transport current and by the motion of the magnetic domain and the vortex density domain. The temperature, magnetic field, and imperfection parameters of the sample at which nonlinear effects are manifested have been analyzed. The conditions for the possible generation of radiation in the region of nonlinearity of the current-voltage characteristic have been obtained. The H-T phase diagram of the existence of the instability of the current-voltage characteristic has been plotted.

Calculation of density of states of transition metals: From bulk sample to nanocluster

A technique is presented of restoring the electronic density of states of the valence band from data of X-ray photoelectron spectroscopy (XPS). The originality of the technique consists in using a stochastic procedure to solve an integral equation relating the density of states and the experimental X-ray photoelectron spectra via the broadening function. To obtain the broadening function, only the XPS spectra of the core levels are needed. The results are presented for bulk sample of gold and tungsten and nanoclusters of tantalum; the possibility of using the results to determine the density of states of low-dimensional structures, including ensembles of metal nanoclusters, is demonstrated.