7,408 research outputs found
Critical vortex line length near a zigzag of pinning centers
A vortex line passes through as many pinning centers as possible on its way
from one extremety of the superconductor to the other at the expense of
increasing its self-energy. In the framework of the Ginzburg-Landau theory we
study the relative growth in length, with respect to the straight line, of a
vortex near a zigzag of defects. The defects are insulating pinning spheres
that form a three-dimensional cubic array embedded in the superconductor. We
determine the depinning transition beyond which the vortex line no longer
follows the critical zigzag path of defects.Comment: 8 pages, 25 figures with low resolution option, 1 table. To be
published in Eur. Phys. Jour.
On Exact and Approximate Solutions for Hard Problems: An Alternative Look
We discuss in an informal, general audience style the da Costa-Doria conjecture about the independence of the P = NP hypothesis and try to briefly assess its impact on practical situations in economics. The paper concludes with a discussion of the Coppe-Cosenza procedure, which is an approximate, partly heuristic algorithm for allocation problems.P vs. NP , allocation problem, assignment problem, traveling salesman, exact solution for NP problems, approximate solutions for NP problems, undecidability, incompleteness
Topologically stable gapped state in a layered superconductor
We show that a layered superconductor, described by a two-component order
parameter, has a gapped state above the ground state, topologically protected
from decay, containing flow and counter flow in the absence of an applied
magnetic field. This state is made of skyrmions, breaks time reversal symmetry
and produces a weak local magnetic field below the present threshold of
detection by SR and NMR/NQR. We estimate the density of carriers that
condense into the pseudogap.Comment: 6 pages, 4 figure
Is the pseudogap a topological state?
We conjecture that the pseudogap is an inhomogeneous condensate above the
homogeneous state whose existence is granted by topological stability. We
consider the simplest possible order parameter theory that provides this
interpretation of the pseudogap and study its angular momentum states. The
normal state gap density, the breaking of the time reversal symmetry and the
checkerboard pattern are naturally explained under this view. The pseudogap is
a lattice of skyrmions and the inner weak local magnetic field falls below the
experimental threshold of observation given by NMR/NQR and SR experiments.Comment: 12 pages, six figures, one tabl
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