1,174,663 research outputs found
Doppelganger defects
We study k-defects - topological defects in theories with more than two
derivatives and second-order equations of motion - and describe some striking
ways in which these defects both resemble and differ from their analogues in
canonical scalar field theories. We show that, for some models, the homotopy
structure of the vacuum manifold is insufficient to establish the existence of
k-defects, in contrast to the canonical case. These results also constrain
certain families of DBI instanton solutions in the 4-dimensional effective
theory. We then describe a class of k-defect solutions, which we dub
doppelgangers, that precisely match the field profile and energy density of
their canonical scalar field theory counterparts. We give a complete
characterization of Lagrangians which admit doppelganger domain walls. By
numerically computing the fluctuation eigenmodes about domain wall solutions,
we find different spectra for doppelgangers and canonical walls, allowing us to
distinguish between k-defects and the canonical walls they mimic. We search for
doppelgangers for cosmic strings by numerically constructing solutions of DBI
and canonical scalar field theories. Despite investigating several examples, we
are unable to find doppelganger cosmic strings, hence the existence of
doppelgangers for defects with codimension >1 remains an open question.Comment: 27 pages, 4 figure
Semilocal Defects
I analyze the interplay of gauge and global symmetries in the theory of
topological defects. In a two-dimensional model in which both gauge symmetries
and {\it exact} global symmetries are spontaneously broken, stable vortices may
fail to exist even though magnetic flux is topologically conserved. Following
Vachaspati and Ach\'ucarro, I formulate the condition that must be satisfied by
the pattern of symmetry breakdown for finite-energy configurations to exist in
which the conserved magnetic flux is spread out instead of confined to a
localized vortex. If this condition is met, vortices are always unstable at
sufficiently weak gauge coupling. I also describe the properties of defects in
models with an ``accidental'' symmetry that is partially broken by gauge boson
exchange. In some cases, the spontaneously broken accidental symmetry is not
restored inside the core of the defect. Then the structure of the defect can be
analyzed using an effective field theory; the details of the physics
responsible for the spontaneous symmetry breakdown need not be considered.
Examples include ``semilocal'' domain walls and vortices that are classically
unstable, but are stabilized by loop corrections, and ``semilocal'' magnetic
monopoles that have an unusual core structure. Finally, I examine the general
theory of the ``electroweak strings'' that were recently discussed by
Vachaspati. These arise only in models with gauge boson ``mixing,'' and can
always end on magnetic monopoles. Cosmological implications are briefly
discussed.Comment: 41 pages, CALT-68-178
Topological defects of N\'eel order and Kondo singlet formation for Kondo-Heisenberg model on a honeycomb lattice
Heavy fermion systems represent a prototypical setting to study magnetic
quantum phase transitions. A particular focus has been on the physics of Kondo
destruction, which captures quantum criticality beyond the Landau framework of
order-parameter fluctuations. In this context, we study the spin one-half
Kondo-Heisenberg model on a honeycomb lattice at half filling. The problem is
approached from the Kondo destroyed, antiferromagnetically ordered insulating
phase. We describe the local moments in terms of a coarse grained quantum
non-linear sigma model, and show that the skyrmion defects of the
antiferromagnetic order parameter host a number of competing order parameters.
In addition to the spin Peierls, charge and current density wave order
parameters, we identify for the first time Kondo singlets as the competing
orders of the antiferromagnetism. We show that the antiferromagnetism and
various competing singlet orders can be related to each other via generalized
chiral transformations of the underlying fermions. We also show that the
conduction electrons acquire a Berry phase through their coupling to the
hedgehog configurations of the N\'eel order, which cancels the Berry phase of
the local moments. Our results demonstrate the competition between the
Kondo-singlet formation and spin-Peierls order when the antiferromagnetic order
is suppressed, thereby shedding new light on the global phase diagram of heavy
fermion systems at zero temperature.Comment: 14 pages, 4 figure
Regrowth-related defect formation and evolution in 1āMeV amorphized (001) Ge
Geimplanted with 1MeV Siāŗ at a dose of 1Ć10Ā¹āµcmā»Ā² creates a buried amorphous layer that, upon regrowth, exhibits several forms of defectsāend-of-range (EOR), regrowth-related, and clamshell defects. Unlike Si, no planar {311} defects are observed. The minimal EOR defects are small dotlike defects and are very unstable, dissolving between 450 and 550Ā°C. This is in contrast to Si, where the EOR defects are very stable. The amorphous layer results in both regrowth-related defects and clamshell defects, which were more stable than the EOR damage.This work is supported by Semiconductor Research Corporation
Contract No. 00057787
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Predicting Software Defects Based on Cognitive Error Theories
As the primary cause of software defects, human error is the key to understanding and perhaps to predicting and preventing software defects. However, little research has been done to forecast software defects based on defects' cognitive nature. This paper proposes an idea for predicting software defects by applying the current scientific understanding of human error mechanisms. This new prediction method is based on the main causal mechanism underlying software defects: an error-prone scenario triggers a sequence of human error modes. Preliminary evidence for supporting this idea is presented
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