Maamar Benkraouda

Magnetic fields and currents for two current-carrying parallel coplanar superconducting strips in a perpendicular magnetic field

We present general solutions for the Meissner-state magnetic-field and current-density distributions for a pair of parallel, coplanar superconducting strips carrying arbitrary but subcritical currents in a perpendicular magnetic field. From these solutions we calculate (a) the inductance per unit length when the strips carry equal and opposite currents, (b) flux focusing in an applied field—how much flux per unit length is focused into the slot between the two strips when each strip carries no net current, (c) the current distribution for the zero-flux quantum state when the strips are connected with superconducting links at the ends and (d) the current and field distributions around both strips when only one of the strips carries a net current. The solutions are closely related to those found recently for the magnetic-field and current-density distributions in a thin, bulk-pinning-free, type-II superconducting strip with a geometrical barrier when the strip carries a current in a perpendicular applied field.

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.

Electronic properties of a new structure of BC2N and its intercalation with magnesium: Possibility for a new high-Tc superconductor

We propose a new structure for the graphite-like BC2N, other than the one proposed in Liu et al. (Phys. Rev. B39, 1760, 1989). We compare it with the older structure and show that it has a lower energy. The band structure calculations of a single sheet of this new structure show that it is a semiconductor with a very small band gap of 0.25 eV, whereas an AA stacking of BC2N layers of this structure behaves like a metal. Because of the similarity of this structure with the boron layers in MgB2, we propose to intercalate the layers of the new structure of BC2N with the magnesium atoms to obtain Mg2BC2N. The band structure calculations of this new structure show an unusually large metallic density of states at the Fermi level, much higher than that of MgB2. This leads us to expect Mg2BC2N to be a superconductor with a higher Tc.

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.

Theoretical Studies of Positron Annihilation in Aluminum Bismuth Alloy

Electron and positron charge densities are calculated as a function of position in the unit cell for Aluminum Bismuth binary compound. Wave functions are derived from pseudopotential band structure calculations and the independent particle approximation (IPM), respectively, for the electrons and the positrons. It is observed that the positron density is maximum in the open interstices and is excluded not only, from the ion cores but also to a considerable degree from the valence bonds. Electron-positron momentum densities are calculated for (001, 110) planes. The results are used to analyze the positron effects in AlBi.

Pressure Effect on the Electronic Properties of Cerium Monochalcogenides CeX (X=S, Se, Te) Using Modified Becke-Johnson Exchange Potential and LDA+U

We present a systematic and comparative study of the electronic properties of CeX monochalcogenides, The density of state (DOS) and electronic band structure of CeX (X=S, Se, Te) have been calculated using the full-potential linearized augmented plane-wave (FP-LAPW) + local orbital (lo) method based on the density functional theory (DFT), which is implemented in WIEN2k code. The trends in the high pressure behavior of these systems are discussed. Four approximations for the exchange-correlation functional have been used, the GGAs of Perdew-Burke-Ernzherhof. (PBE08) , Engel-Vosko (EV93), a modified version of the exchange potential proposed by Becke and Johnson (MBJ), and LDA+U is used to calculate the band gaps at different pressures. All methods allow for a description of the Ce f electrons as either localized or delocalized, it is found that the underestimations of the bandgap by means of LDA-GGA and Engel-Vosko are considerably improved by using the modified Becke-Johnson (MBJ) potential for all compounds in the series, On the other hand, LDA+U, method gives good results for the lighter chalcogenides, but it fails to give good results for the heavier cerium monochalcogenides.

First principles calculations on the electronic properties of the II-VII semiconductors, radiation detectors, TlBr and TlCl

In this work, we have investigated the electronic properties of the III-VII binary compound semiconductors TiBr and TICI by means of first-principles density-functional total-energy calculation using the all-electron full potential linear augmented plane-wave method (FP-LAPW). The (FPLAPW) method was used within the density functional theory (DFT) along with the Engel-Vosko and (GGA96) exchange correlation potential. The energy bands along high symmetry directions, the density of states and valence charge density distributions cut through various planes are presented. The results have been discussed in terms of previously existing experimental and theoretical data, and comparisons with similar compounds have been made. Analysis of band structure suggests a direct and a pseudo-direct band gaps for both compounds.

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.