144 research outputs found
Enhancement of the Josephson current by an exchange field in superconductor-ferromagnet structures
We calculate the dc Josephson current for two superconductor-ferromagnet
(S/F) bilayers separated by a thin insulating film. It is demonstrated that the
critical Josephson current in the junction strongly depends on the
relative orientation of the effective exchange field of the bilayers. We
found that in the case of an antiparallel orientation, increases at low
temperatures with increasing and at zero temperature has a singularity when
equals the superconducting gap . This striking behavior contrasts
suppression of the critical current by the magnetic moments aligned in parallel
and is an interesting new effect of the interplay between superconductors and
ferromagnets.Comment: to be published in PR
SU(3) dibaryons in the Einstein-Skyrme model
SU(3) collective coordinate quantization to the regular solution of the B=2
axially symmetric Einstein-Skyrme system is performed. For the symmetry
breaking term, a perturbative treatment as well as the exact diagonalization
method called Yabu-Ando approach are used. The effect of the gravity on the
mass spectra of the SU(3) dibaryons and the symmetry breaking term is studied
in detail. In the strong gravity limit, the symmetry breaking term
significantly reduces and exact SU(3) flavor symmetry is recovered.Comment: 9 pages, 14 figure
Unified picture of Q-balls and boson stars via catastrophe theory
We make an analysis of Q-balls and boson stars using catastrophe theory, as
an extension of the previous work on Q-balls in flat spacetime. We adopt the
potential for Q-balls and
that with for boson stars. For solutions with at
its peak, stability of Q-balls has been lost regardless of the potential
parameters. As a result, phase relations, such as a Q-ball charge versus a
total Hamiltonian energy, approach those of boson stars, which tell us an
unified picture of Q-balls and boson stars.Comment: 10 pages, 13 figure
Free boundary problems involving singular weights
In this paper we initiate the investigation of free boundary minimization
problems ruled by general singular operators with weights. We show
existence and boundedness of minimizers. The key novelty is a sharp
regularity result for solutions at their singular free boundary
points. We also show a corresponding non-degeneracy estimate
Exact String Theory Instantons by Dimensional Reduction
We identify exact gauge-instanton-like solutions to (super)-string theory
using the method of dimensional reduction. We find in particular the Polyakov
instanton of 3d QED, and a class of generalized Yang-Mills merons. We discuss
their marginal deformations, and show that for the instanton they
correspond to a dissociation of vector- and axial-magnetic charges.Comment: LateX, 15pp., CERN-TH.7100/93, CPTh-A276.11.93 (Minor Errors
Corrected
Topological Symmetry Breaking on Einstein Manifolds
It is known that if gauge conditions have Gribov zero modes, then topological
symmetry is broken. In this paper we apply it to topological gravity in
dimension . Our choice of the gauge condition for conformal
invariance is , where is the Ricci scalar curvature. We find
when , topological symmetry is not broken, but when
and solutions of the Einstein equations exist then topological symmetry is
broken. This conditions connect to the Yamabe conjecture. Namely negative
constant scalar curvature exist on manifolds of any topology, but existence of
nonnegative constant scalar curvature is restricted by topology. This fact is
easily seen in this theory. Topological symmetry breaking means that BRS
symmetry breaking in cohomological field theory. But it is found that another
BRS symmetry can be defined and physical states are redefined. The divergence
due to the Gribov zero modes is regularized, and the theory after topological
symmetry breaking become semiclassical Einstein gravitational theory under a
special definition of observables.Comment: 16 pages, Late
The semigroup structure of Gaussian channels
We investigate the semigroup structure of bosonic Gaussian quantum channels.
Particular focus lies on the sets of channels which are divisible, idempotent
or Markovian (in the sense of either belonging to one-parameter semigroups or
being infinitesimal divisible). We show that the non-compactness of the set of
Gaussian channels allows for remarkable differences when comparing the
semigroup structure with that of finite dimensional quantum channels. For
instance, every irreversible Gaussian channel is shown to be divisible in spite
of the existence of Gaussian channels which are not infinitesimal divisible. A
simpler and known consequence of non-compactness is the lack of generators for
certain reversible channels. Along the way we provide new representations for
classes of Gaussian channels: as matrix semigroup, complex valued positive
matrices or in terms of a simple form describing almost all one-parameter
semigroups.Comment: 20 page
A parabolic free boundary problem with Bernoulli type condition on the free boundary
Consider the parabolic free boundary problem For a
realistic class of solutions, containing for example {\em all} limits of the
singular perturbation problem we prove that one-sided
flatness of the free boundary implies regularity.
In particular, we show that the topological free boundary
can be decomposed into an {\em open} regular set (relative to
) which is locally a surface with H\"older-continuous space
normal, and a closed singular set.
Our result extends the main theorem in the paper by H.W. Alt-L.A. Caffarelli
(1981) to more general solutions as well as the time-dependent case. Our proof
uses methods developed in H.W. Alt-L.A. Caffarelli (1981), however we replace
the core of that paper, which relies on non-positive mean curvature at singular
points, by an argument based on scaling discrepancies, which promises to be
applicable to more general free boundary or free discontinuity problems
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