14,697 research outputs found
Stationary generalized Kerr-Schild spacetimes
In this paper we have applied the generalized Kerr-Schild transformation
finding a new family of stationary perfect-fluid solutions of the Einstein
field equations. The procedure used combines some well-known techniques of null
and timelike vector fields, from which some properties of the solutions are
studied in a coordinate-free way. These spacetimes are algebraically special
being their Petrov types II and D. This family includes all the classical
vacuum Kerr-Schild spacetimes, excepting the plane-fronted gravitational waves,
and some other interesting solutions as, for instance, the Kerr metric in the
background of the Einstein Universe. However, the family is much more general
and depends on an arbitrary function of one variable.Comment: 21 pages, LaTeX 2.09. To be published in Journal of Mathematical
Physic
Magnetic defects promote ferromagnetism in Zn1-xCoxO
Experimental studies of Zn1-xCoxO as thin films or nanocrystals have found
ferromagnetism and Curie temperatures above room temperature and that p- or
n-type doping of Zn1-xCoxO can change its magnetic state. Bulk Zn1-xCoxO with a
low defect density and x in the range used in experimental thin film studies
exhibits ferromagnetism only at very low temperatures. Therefore defects in
thin film samples or nanocrystals may play an important role in promoting
magnetic interactions between Co ions in Zn1-xCoxO. The electronic structures
of Co substituted for Zn in ZnO, Zn and O vacancies, substituted N and
interstitial Zn in ZnO were calculated using the B3LYP hybrid density
functional in a supercell. The B3LYP functional predicts a band gap of 3.34 eV
for bulk ZnO, close to the experimental value of 3.47 eV. Occupied minority
spin Co 3d levels are at the top of the valence band and unoccupied levels lie
above the conduction band minimum. Majority spin Co 3d levels hybridize
strongly with bulk ZnO states. The neutral O vacancy and interstitial Zn are
deep and shallow donors, respectively. The Zn vacancy is a deep acceptor and
the acceptor level for substituted N is at mid gap. The possibility that p- or
n-type dopants promote exchange coupling of Co ions was investigated by
computing total energies of magnetic states of ZnO supercells containing two Co
ions and an oxygen vacancy, substituted N or interstitial Zn in various charge
states. The neutral N defect and the singly-positively charged O vacancy are
the only defects which strongly promote ferromagnetic exchange coupling of Co
ions at intermediate range.Comment: 9 pages, 11 figure
Field quantization for chaotic resonators with overlapping modes
Feshbach's projector technique is employed to quantize the electromagnetic
field in optical resonators with an arbitray number of escape channels. We find
spectrally overlapping resonator modes coupled due to the damping and noise
inflicted by the external radiation field. For wave chaotic resonators the mode
dynamics is determined by a non--Hermitean random matrix. Upon including an
amplifying medium, our dynamics of open-resonator modes may serve as a starting
point for a quantum theory of random lasing.Comment: 4 pages, 1 figur
The permutation group S_N and large Nc excited baryons
We study the excited baryon states for an arbitrary number of colors Nc from
the perspective of the permutation group S_N of N objects. Classifying the
transformation properties of states and quark-quark interaction operators under
S_N allows a general analysis of the spin-flavor structure of the mass operator
of these states, in terms of a few unknown constants parameterizing the unknown
spatial structure. We explain how to perform the matching calculation of a
general two-body quark-quark interaction onto the operators of the 1/Nc
expansion. The inclusion of core and excited quark operators is shown to be
necessary. Considering the case of the negative parity L=1 states transforming
in the MS of S_N, we discuss the matching of the one-gluon and the
Goldstone-boson exchange interactions.Comment: 38 pages. Final version to be published in Physical Review
Rastall Cosmology and the \Lambda CDM Model
Rastall's theory is based on the non-conservation of the energy-momentum
tensor. We show that, in this theory, if we introduce a two-fluid model, one
component representing vacuum energy whereas the other pressureless matter
(e.g. baryons plus cold dark matter), the cosmological scenario is the same as
for the \Lambda CDM model, both at background and linear perturbative levels,
except for one aspect: now dark energy may cluster. We speculate that this can
lead to a possibility of distinguishing the models at the non-linear
perturbative level.Comment: 9 pages, 1 figure. Accepted for publication in Physical Review
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