Vortex line lattice: structure and dynamics

This thesis deals with a number of properties of the vortex lattice in high-temperature superconducting materials. Novel vortex lattice structures, other than the conventional one, are investigated. They are found to be energetically more stable for layered superconductors and for moderately anisotropic superconductors containing columnar defects;The dynamics of the vortex lattice, and in particular the magnetic hysteresis which arises because of the geometrical barrier in superconducting strips, is also studied. Its effect on the magnetization in both the absence and presence of the bulk pinning, as well as the time evolution of the vortex penetration into superconducting strips are investigated

First Principles Calculations on The Electronic and Optical Properties of Zincblende Bxga1-Xas and Bxin1-Xas Semiconductor Alloys.

A theoretical study of the electronic and optical properties of zincblende BxGa1-xAs and BxIn1-xAs  semiconductor alloys is presented, using the full potential linearized augmented plane wave method. In this approach, the generalized gradient approximation was used for the exchange correlation potential. Ground state properties such as lattice parameter and band structure are calculated as a function of the mole fraction. We have also analyzed the optical properties (refractive index, dielectric function, real and imaginary), the 4x4 Kanes interaction matrix is calculated in order to ease simulations of optoelectronic devices. The results have been discussed in terms of previously existing experimental and theoretical data, and comparisons with similar compounds have been made

Distribution of the magnetic field and current density in superconducting films of finite thickness

A one-dimensional equation describing the distribution of the effective vector potential $\bar A(y)$ across a film width, which holds for thin ($d\lambda$) films alike, is derived based on the analysis of a 2D Maxwell-Londons equation for superconducting films in a perpendicular magnetic field. The validity of this equation for a finite-thickness film is verified by a numerical analysis. An approximation dependence $\bar A(y)$, finite (with all of its derivatives) across the entire film width, is found for films, being in the Meissner state. The flux-entry field is evaluated for a film of arbitrary thickness. An approximation expression is obtained for the distribution of the sheet current density in the mixed state of a pin-free superconducting film with an edge barrier. The latter approximation allows to estimate magnetic field concentration factor at the film edge as a function of external magnetic field and geometrical parameters of the sample.Comment: 12 pages, 10 figure

Full potential linear augmented plane wave calculations of Electronic and Optical properties in ZnO

 In this work we present self-consistent calculations for the electronic and optical properties of Zinc oxide. A theoretical investigation of the electronic properties (band structure, density of charge and contour map) and optical properties (refractive index, absorption coefficient, dielectric constants and reflectivity) of Zinc oxide semiconductor ZnO. A full-potential linearized augmented plane-wave (FPLAPW) method was used within the density functional theory (DFT) along with the generalized gradient approximation (GGA96) exchange correlation potential. The results are compared with the experimental data available and some other theoretical work. We found that the GGA approximation yields only a small improvement to the band gap, however, if we allow for a rigid shift of the band structure, the so-called scissors operator, the optical properties are excellently reproduced

Geometrical edge barriers and magnetization in superconducting strips with slits

We theoretically investigate the magnetic-field and current distributions for coplanar superconducting strips with slits in an applied magnetic field H_a. We consider ideal strips with no bulk pinning and calculate the hysteretic behavior of the magnetic moment m_y as a function of H_a due solely to geometrical edge barriers. We find that the m_y-H_a curves are strongly affected by the slits. In an ascending field, the m_y-H_a curves exhibit kink or peak structures, because the slits prevent penetration of magnetic flux. In a descending field, m_y becomes positive, because magnetic flux is trapped in the slits, in contrast to the behavior of a single strip without slits, for which m_y =0.Comment: 11 pages, 5 figures, revtex

Dynamics of the superconducting condensate in the presence of a magnetic field. Channelling of vortices in superconducting strips at high currents

On the basis of the time-dependent Ginzburg-Landau equation we studied the dynamics of the superconducting condensate in a wide two-dimensional sample in the presence of a perpendicular magnetic field and applied current. We could identify two critical currents: the current at which the pure superconducting state becomes unstable ($J_{c2}$ \cite{self1}) and the current at which the system transits from the resistive state to the superconducting state ($J_{c1}<J_{c2}$). The current $J_{c2}$ decreases monotonically with external magnetic field, while $J_{c1}$ exhibits a maximum at $H^*$. For sufficient large magnetic fields the hysteresis disappears and $J_{c1}=J_{c2}=J_c$. In this high magnetic field region and for currents close to $J_c$ the voltage appears as a result of the motion of separate vortices. With increasing current the moving vortices form 'channels' with suppressed order parameter along which the vortices can move very fast. This leads to a sharp increase of the voltage. These 'channels' resemble in some respect the phase slip lines which occur at zero magnetic field.Comment: 5 pages, 4 figures, Proceedings of Third European Conference on Vortex Matter in Superconductor

Study on anisotropies and momentum densities in AlN, GaN and InN by positron annihilation

The independent particle model (IPM) coupled with empirical pseudopotential method (EPM) was used to compute the thermalized positron charge densities in specific family of binary tetrahedrally coordinated crystals of formula ANB8-N. Initial results show a clear asymmetrical positron charge distribution relative to the bond center. 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 the (001,110) planes. The results are used to analyze the positron effects in AlN, GaN and InN compounds. Our computational technique provides the theoretical means of interpreting the k-space densities obtained experimentally using the twodimensional angular correlation of annihilation radiation (2D-ACAR)

Newly Discovered Topological Insulator Sr3SnO for Spintronics, Optical and Electronic Properties

A theoretical study of the electronic and optical properties of dilute magnetic semiconductor Sr3SnO is presented, using the full potential linearized augmented plane wave method. The Perdew Burke Ernzerhof (GGA08) (generalized gradient approximation) is used for the total energy calculations, while the Modified Becke Johnson (MBJ) is used for electronic structure calculations since this functional was designed to reproduce as well as possible the exact exchange correlation potential rather than the total energy, and as a result gives significantly improved results such as band gap and electronic structure. In this study, we have investigated the optical properties by means of first-principles density-functional total-energy calculation using the all-electron full potential linear augmented plane-wave method (FPLAPW)