The interaction between a superconducting vortex and an out-of-plane magnetized ferromagnetic disk: influence of the magnet geometry

The interaction between a superconducting vortex in a type II superconducting film (SC) and a ferromagnet (FM) with out-of-plane magnetization is investigated theoretically within the London approximation. The dependence of the interaction energy on the FM-vortex distance, film thickness and different geometries of the magnetic structures: disk, annulus(ring), square and triangle are calculated. Analytic expressions and vectorplots of the current induced in the SC due to the presence of the FM are presented. For a FM disk with a cavity, we show that different local minima for the vortex position are possible, enabling the system to be suitable to act as a qubit. For FMs with sharp edges, like e.g. for squares and triangles, the vortex prefers to enter its equilibrium position along the corners of the magnet.Comment: Preprint, 10 pages, 10 figures, submitted to Phys. Rev.

Field-enhanced critical parameters in magnetically nanostructured superconductors

Within the phenomenological Ginzburg-Landau theory, we demonstrate the enhancement of superconductivity in a superconducting film, when nanostructured by a lattice of magnetic particles. Arrays of out-of-plane magnetized dots (MDs) extend the critical magnetic field and critical current the sample can sustain, due to the interaction of the vortex-antivortex pairs and surrounding supercurrents induced by the dots and the external flux lines. Depending on the stability of the vortex-antivortex lattice, a peak in the Hext-T boundary is found for applied integer and rational matching fields, which agrees with recent experiments [Lange et al., Phys. Rev. Lett. 90, 197006 (2003)]. Due to compensation of MDs- and Hext-induced currents, we predict the field-shifted jc-Hext characteristics, as was actually realized in previous experiment but not commented on [Morgan and Ketterson, Phys. Rev. Lett. 80, 3614 (1998)].Comment: 8 pages, 5 figures, to appear in Europhysics Letter

Super-slippery Carbon Nanotubes: Symmetry Breaking breaks friction

The friction between the walls of multi-wall carbon nanotubes is shown to be extremely low in general, with important details related to the specific choice of the walls. This is governed by a simple expression revealing that the phenomenon is a profound consequence of the specific symmetry breaking: super-slippery sliding of the incommensurate walls is a Goldstone mode. Three universal principles of tribology, offering a recipe for the lubricant selection are emphasized.Comment: 4 pages, 2 figures, 1 table; pdf available from: http://www.ff.bg.ac.yu/qmf/qsg_e.ht

Fluxonic Cellular Automata

We formulate a new concept for computing with quantum cellular automata composed of arrays of nanostructured superconducting devices. The logic states are defined by the position of two trapped flux quanta (vortices) in a 2x2 blind-hole-matrix etched on a mesoscopic superconducting square. Such small computational unit-cells are well within reach of current fabrication technology. In an array of unit-cells, the vortex configuration of one cell influences the penetrating flux lines in the neighboring cell through the screening currents. Alternatively, in conjoined cells, the information transfer can be strengthened by the interactions between the supercurrents in adjacent cells. Here we present the functioning logic gates based on this fluxonic cellular automata (FCA), where the logic operations are verified through theoretical simulations performed in the framework of the time-dependent Ginzburg-Landau theory. The input signals are defined by current loops placed on top of the two diagonal blind holes of the input cell. For given current-polarization, external flux lines are attracted or repelled by the loops, forming the '0' or '1' configuration. The read-out technology may be chosen from a large variety of modern vortex imaging methods, transport and LDOS measurements.Comment: Featured on the cover page of APL, November 2007 issu

Irreducible Representations of Diperiodic Groups

The irreducible representations of all of the 80 diperiodic groups, being the symmetries of the systems translationally periodical in two directions, are calculated. To this end, each of these groups is factorized as the product of a generalized translational group and an axial point group. The results are presented in the form of the tables, containing the matrices of the irreducible representations of the generators of the groups. General properties and some physical applications (degeneracy and topology of the energy bands, selection rules, etc.) are discussed.Comment: 30 pages, 5 figures, 28 tables, 18 refs, LaTex2.0