I.M. Obaidat

The effect of temperature and pinning density on the critical current of a superconductor with a square periodic array of pinning sites

We have studied the effect of temperature and pinning density on the critical depinning force for several values of pinning strength through an extensive series of molecular dynamic simulations on driven vortex lattices interacting with square periodic arrays of pinning sites. We have solved the overdamped equation of vortex motion taking into account the vortex–vortex repulsion interaction, the attractive vortex pinning interaction, the thermal force, and the driving Lorentz force. We have found that the critical depinning force, as a function of the pinning density, may decrease, increase, or even remain constant, depending on the temperature and pinning strength values.

Dynamic phases of low-temperature low-current driven vortex matter in superconductors

Using molecular dynamics simulations of vortices in a high-temperature superconductor with square periodic arrays of pinning sites, dynamic phases of the low-current driven vortices are studied at low temperatures. A rough vortex phase diagram of three distinct regimes of vortex flow is proposed. At zero temperature, we obtain a coupled-channel regime where rows of vortices flow coherently in the direction of the driving force. As the temperature is increased, a smooth crossover into an uncoupled-channel regime occurs where the coherence between the flowing rows of vortices becomes weaker. Increasing the temperature further leads to a plastic vortex regime, where the channels of flowing vortices completely disappear. The temperatures of the crossovers between these regimes were found to decrease with the driving force.

Roles of pinning strength and density in vortex melting

We have investigated the role of pinning strength and density on the equilibrium vortex-lattice to vortex-liquid phase transition under several applied magnetic fields. This study was conducted using a series of molecular dynamic simulations on several samples with different strengths and densities of pinning sites which are arranged in periodic square arrays. We have found a single solid–liquid vortex transition when the vortex filling factor n>1. We have found that, for fixed pinning densities and strengths, the melting temperature, Tm, decreases almost linearly with increasing magnetic field. Our results provide direct numerical evidence for the significant role of both the strength and density of pinning centers on the position of the melting line. We have found that the vortex-lattice to vortex-liquid melting line shifts up as the pinning strength or the pinning density was increased. The effect on the melting line was found to be more pronounced at small values of strength and density of pinning sites.

γ-irradiation dose effect on the critical current density in a Bi1.6Pb0.4Sr2Ca2Cu3O10polycrystal

The effect of different γ-irradiation doses on the critical current density, Jc, in a Bi1.6Pb0.4Sr2Ca2Cu3O10 superconducting polycrystal has been investigated under several values of applied magnetic field and at temperatures from 80 to 105 K using I–V measurements. Two doses of γ-irradiation were used: 50 and 300 MR. The specimen was prepared using the solid-state reaction technique. We found that for γ-irradiation dose of 50 MR the values of Jc were enhanced through the whole temperature range for all applied fields. This enhancement of Jc values is significant at low temperatures. We also found that for a γ-irradiation dose of 300 MR Jc values were suppressed at all temperatures for all applied fields. This reduction of Jc values is significant at low temperatures. At high temperatures both doses of γ-irradiation were found to have a very small effect on Jc values. These results are discussed in terms of the effect of defects produced by γ-rays and grain boundaries on the vortex pinning forces in the samples.

DEPENDENCE OF THE PEAK EFFECT ON THE DENSITY OF PINNING SITES

We have investigated the role of pinning density on the properties of the peak effect in the critical current density in superconducting systems. This study was conducted using a series of molecular dynamic simulations on two systems of nanostructures of periodic square arrays of pinning sites with different pining densities. We have found that the peak occurred in both systems only at zero temperature and for specific values of pinning strength. The most interesting result was that in both systems, the peak was found to occur only at nearly 0.5 fraction of the first matching field for all values of pinning strength. The properties of the peak were found to depend mainly on the initial positions of the vortices with respect to the positions of the pinning sites. The critical dipping force at the peak was found to increase linearly with the pinning strength and to have larger values for the system with the smaller density of pinning sites. The dependence of the relative height of the peak on the pinning strength was found be nearly the same in both systems. One-dimensional linear channels of moving vortices along the direction of the driving force were observed at the dip just before the peak.

INVESTIGATING DYNAMIC VORTEX TRANSITIONS IN 2D SUPERCONDUCTORS

Extensive molecular dynamics simulations were performed on superconducting samples with periodic square arrays of pinning centers where several strengths, sizes, and densities of the pinning centers were considered. By calculating the average speed of the vortices as the driving force increases, we defined two critical currents that divide the states of the vortices into three different states with respect to the magnitude of the driving current. These vortex states are a pinned state, a disordered flow state where only some of the vortices flow in some vortex channels, and an ordered flow state where all the vortices flow in an ordered and collective manner. We have clarified the roles of the number of vortices, the size, the strength, and the number of pinning centers in these three states at several temperatures.

Periodic Nanostructured Pins and the Anomaly in the Critical Depinning Force

For the first time, molecular dynamic simulations based on dense nanostructured periodic arrays of pinning sites were carried out. The variables in the simulations were the vortex density, the temperature, the pinning strength, the size of pinning sites. An interesting dip was found to occur in the critical current density but only at zero temperature and for specific values of pinning strength. The properties of the dip were found to depend strongly on the initial positions of the vortices with respect to the positions of the pinning sites. The occurrence of the dip is attributed to the formation of one dimensional linear channels of moving vortices in the direction of the applied current. The pinning strength was also found to have an important role in determining the relative depth of the dip.