5,354 research outputs found
Gravitational Couplings, Orientifolds and M-Planes
We examine string-theory orientifold planes of various types including the Sp
and SO-odd planes, and deduce the gravitational Chern-Simons couplings on their
world-volumes. Consistency checks are carried out in different spacetime
dimensions using various dualities, including those relating string theory with
F-theory and M-theory. It is shown that when an orientifold 3-plane crosses a
5-brane, the jump in the charge is accompanied by a corresponding change in the
gravitational couplings.Comment: 14 pages, harvmac (b), 2 figures, referencing improved and a
reference added, no changes in conten
Gauge-Invariant Couplings of Noncommutative Branes to Ramond-Ramond Backgrounds
We derive the couplings of noncommutative D-branes to spatially varying
Ramond-Ramond fields, extending our earlier results in hep-th/0009101. These
couplings are expressed in terms of *n products of operators involving open
Wilson lines. Equivalence of the noncommutative to the commutative couplings
implies interesting identities as well as an expression for the Seiberg-Witten
map that was previously conjectured. We generalise our couplings to include
transverse scalars, thereby obtaining a Seiberg-Witten map relating commutative
and noncommutative descriptions of these scalars. RR couplings for unstable
non-BPS branes are also proposed.Comment: harvmac, 22 pages, v2: typos corrected, references and
acknowledgements adde
Brane-Antibrane Constructions
In type II string theories, we examine intersecting brane constructions
containing brane-antibrane pairs suspended between 5-branes, and more general
non-BPS constructions. The tree-level spectra are obtained in each case. We
identify various models with distinct physics: parallel brane-antibrane pairs,
adjacent pairs, non-adjacent pairs, and configurations which break all
supersymmetry even though any pair of branes preserves some supersymmetry. In
each case we examine the possible decay modes. Some of these configurations
turn out to be tachyon-free, stable non-BPS states. We use T-duality to map
some of our brane constructions to brane-antibrane pairs at ALE singularities.
This enables us to explicitly derive the spectra by the analogue of the quiver
construction, and to compute the sign of the brane-antibrane force in each
case.Comment: 40 pages, harvmac (b), 14 eps figures (included), v2: references
added and report no. corrected, no other change
Size-dependent magnetization fluctuations in NiO nanoparticles
The finite size and surface roughness effects on the magnetization of NiO
nanoparticles is investigated. A large magnetic moment arises for an
antiferromagnetic nanoparticle due to these effects. The magnetic moment
without the surface roughness has a non-monotonic and oscillatory dependence on
, the size of the particles, with the amplitude of the fluctuations varying
linearly with . The geometry of the particle also matters a lot in the
calculation of the net magnetic moment. An oblate spheroid shape particle shows
an increase in net magnetic moment by increasing oblateness of the particle.
However, the magnetic moment values thus calculated are very small compared to
the experimental values for various sizes, indicating that the bulk
antiferromagnetic structure may not hold near the surface. We incorporate the
surface roughness in two different ways; an ordered surface with surface spins
inside a surface roughness shell aligned due to an internal field, and a
disordered surface with randomly oriented spins inside surface roughness shell.
Taking a variational approach we find that the core interaction strength is
modified for nontrivial values of which is a signature of
multi-sublattice ordering for nanoparticles. The surface roughness scale
is also showing size dependent fluctuations, with an envelope decay
. The net magnetic moment values calculated using
spheroidal shape and ordered surface are close to the experimental values for
different sizes.Comment: 19 pages, 8 figures, Accepted for publication in Int. J. Mod. Phys.
Jordan-Schwinger realizations of three-dimensional polynomial algebras
A three-dimensional polynomial algebra of order is defined by the
commutation relations ,
where is an -th order polynomial in
with the coefficients being constants or central elements of the algebra.
It is shown that two given mutually commuting polynomial algebras of orders
and can be combined to give two distinct -th order polynomial
algebras. This procedure follows from a generalization of the well known
Jordan-Schwinger method of construction of and algebras from
two mutually commuting boson algebras.Comment: 10 pages, LaTeX2
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