7,849 research outputs found
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
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