Vortex configurations and critical parameters in superconducting thin films containing antidot arrays: Nonlinear Ginzburg-Landau theory

Using the non-linear Ginzburg-Landau (GL) theory, we obtain the possible vortex configurations in superconducting thin films containing a square lattice of antidots. The equilibrium structural phase diagram is constructed which gives the different ground-state vortex configurations as function of the size and periodicity of the antidots for a given effective GL parameter $\kappa^{*}$. Giant-vortex states, combination of giant- and multi-vortex states, as well as symmetry imposed vortex-antivortex states are found to be the ground state for particular geometrical parameters of the sample. The antidot occupation number $n_o$ is calculated as a function of related parameters and comparison with existing expressions for the saturation number $n_s$ and with experimental results is given. For a small radius of antidots a triangular vortex lattice is obtained, where some of the vortices are pinned by the antidots and some of them are located between them. Transition between the square pinned and triangular vortex lattices is given for different values of the applied field. The enhanced critical current at integer and rational matching fields is found, where the level of enhancement at given magnetic field directly depends on the vortex-occupation number of the antidots. For certain parameters of the antidot lattice and/or temperature the critical current is found to be larger for higher magnetic fields. Superconducting/normal $H-T$ phase boundary exhibits different regimes as antidots are made larger, and we transit from a plain superconducting film to a thin-wire superconducting network. Presented results are in good agreement with available experiments and suggest possible new experiments.Comment: 15 pages and 20 figure

Resonant Raman-active localized vibrational modes in AlyGa{1-y}NxAs{1-x} alloys: Experiment and firstprinciples calculations

The localized vibrational modes associated with substitutional aluminium and nitrogen atoms in AlyGa1−yNxAs1−x have been studied within first-principles density functional theory using a supercell approach. Localized vibrational modes related to N-AlmGa4−m (1≤m≥4) complexes have been identified, which reveal the formation of N-Al4 units well above random abundance, in qualitative agreement with a large calculated value (391 meV) of the Al-N bond formation energy. We determine the resonant Raman-active modes from the selection rule obtained by calculating the electron-phonon coupling strength and optical transition matrix elements and compare them with resonant Raman spectroscopy measurements. The localized modes from Raman scattering measurements with frequencies around 325, 385, 400, 450, 500, and 540 cm−1 are found to be in good agreement with the calculated modes (326, 364, 384, 410, 456, 507, and 556 cm−1). The modes are classified as follows: the two modes at 326 and 556 cm−1 belong to the N-AlGa3 configuration; there are three modes which belong to N-Al2Ga2 with frequencies at 326, 364, and 507 cm−1; the N-Al3Ga configuration gives rise to modes whose frequencies are 384 and 456 cm−1; and the mode at a frequency of 410 cm−1 belongs to the N-Al4 complex. The comparison of line intensities from samples before and after rapid thermal annealing allows us to experimentally distinguish vibrational modes associated with different clusters, in agreement with the theoretical assignments

Nonlinear effects in resonant layers in solar and space plasmas

The present paper reviews recent advances in the theory of nonlinear driven magnetohydrodynamic (MHD) waves in slow and Alfven resonant layers. Simple estimations show that in the vicinity of resonant positions the amplitude of variables can grow over the threshold where linear descriptions are valid. Using the method of matched asymptotic expansions, governing equations of dynamics inside the dissipative layer and jump conditions across the dissipative layers are derived. These relations are essential when studying the efficiency of resonant absorption. Nonlinearity in dissipative layers can generate new effects, such as mean flows, which can have serious implications on the stability and efficiency of the resonance

Optimal representation of the polarization propagator for large-scale GW calculations

Quasiparticle calculations based on the GW approximation are enhanced by introducing an optimal basis set for the polarization propagator, based on a Wannier representation of the one-electron wave functions, thus allowing the treatment of substantially larger systems. Our method is validated by calculating the vertical ionization energies of the benzene molecule and the band structure of bulk silicon. Its potentials are then demonstrated by addressing the quasiparticle spectrum of a model structure of vitreous silica, as well as of the tetraphenylporphyrin molecule

Vortex Configurations in Mesoscopic Superconducting Nanowires

Beyond the well-known Abrikosov and giant vortex configurations, new solutions to the Ginzburg–Landau model corresponding to vortices of integer and half-integer winding number are described. Phase diagrams (Bext, Energy) and magnetization curves have been determined, aiming towards an understanding of the magnetic properties of lead nanowires and the possible consequences of such solutions with respect to the switching mechanism between vortex states in mesoscopic superconductors