2,926 research outputs found
Radiative spacetimes approaching the Vaidya metric
We analyze a class of exact type II solutions of the Robinson-Trautman family
which contain pure radiation and (possibly) a cosmological constant. It is
shown that these spacetimes exist for any sufficiently smooth initial data, and
that they approach the spherically symmetric Vaidya-(anti-)de Sitter metric. We
also investigate extensions of the metric, and we demonstrate that their order
of smoothness is in general only finite. Some applications of the results are
outlined.Comment: 12 pages, 3 figure
On hamiltonian colorings of block graphs
A hamiltonian coloring c of a graph G of order p is an assignment of colors
to the vertices of G such that for every two
distinct vertices u and v of G, where D(u,v) denoted the detour distance
between u and v. The value hc(c) of a hamiltonian coloring c is the maximum
color assigned to a vertex of G. The hamiltonian chromatic number, denoted by
hc(G), is the min{hc(c)} taken over all hamiltonian coloring c of G. In this
paper, we present a lower bound for the hamiltonian chromatic number of block
graphs and give a sufficient condition to achieve the lower bound. We
characterize symmetric block graphs achieving this lower bound. We present two
algorithms for optimal hamiltonian coloring of symmetric block graphs.Comment: 12 pages, 1 figure. A conference version appeared in the proceedings
of WALCOM 201
Canonical theory of spherically symmetric spacetimes with cross-streaming null dusts
The Hamiltonian dynamics of two-component spherically symmetric null dust is
studied with regard to the quantum theory of gravitational collapse. The
components--the ingoing and outgoing dusts--are assumed to interact only
through gravitation. Different kinds of singularities, naked or "clothed", that
can form during collapse processes are described. The general canonical
formulation of the one-component null-dust dynamics by Bicak and Kuchar is
restricted to the spherically symmetric case and used to construct an action
for the two components. The transformation from a metric variable to the
quasilocal mass is shown to simplify the mathematics. The action is reduced by
a choice of gauge and the corresponding true Hamiltonian is written down.
Asymptotic coordinates and energy densities of dust shells are shown to form a
complete set of Dirac observables. The action of the asymptotic time
translation on the observables is defined but it has been calculated explicitly
only in the case of one-component dust (Vaidya metric).Comment: 15 pages, 3 figures, submitted to Phys. Rev.
Magnetothermoelectric transport in modulated and unmodulated graphene
We draw motivation from recent experimental studies and present a
comprehensive study of magnetothermoelectric transport in a graphene monolayer
within the linear response regime. We employ the modified Kubo formalism
developed for thermal transport in a magnetic field. Thermopower as well as
thermal conductivity as a function of the gate voltage of a graphene monolayer
in the presence of a magnetic field perpendicular to the graphene plane is
determined for low magnetic fields (~1 Tesla) as well as high fields (~8
Tesla). We include the effects of screened charged impurities on thermal
transport. We find good, qualitative as well as quantitative, agreement with
recent experimental work on the subject. In addition, in order to analyze the
effects of modulation, which can be induced by various means, on the thermal
transport in graphene, we evaluate the thermal transport coefficients for a
graphene monolayer subjected to a periodic electric modulation in a magnetic
field. The results are presented as a function of the magnetic field and the
gate voltage.Comment: 14 pages, 8 figure
The most general axially symmetric electrovac spacetime adimitting separable equations of motion
We obtain the most general solution of the Einstein electro - vacuum equation
for the stationary axially symmetric spacetime in which the Hamilton-Jacobi and
Klein - Gordon equations are separable. The most remarkable feature of the
solution is its invariance under the duality transformation involving mass and
NUT parameter, and the radial and angle coordinates. It is the general solution
for a rotating (gravitational dyon) particle which is endowed with both
gravoelectric and gravomagnetic charges, and there exists a duality
transformation from one to the other. It also happens to be a transform of the
Kerr - NUT solution. Like the Kerr family, it is also possible to make this
solution radiating which asymptotically conforms to the Vaidya null radiation.Comment: 9 pages, RevTex, Accepted by Class. Quantum Grav. Title, Abstract and
some expressions have been modified, typos corrected. The solution and main
result remain unaltere
Hawking Radiation of a Quantum Black Hole in an Inflationary Universe
The quantum stress-energy tensor of a massless scalar field propagating in
the two-dimensional Vaidya-de Sitter metric, which describes a classical model
spacetime for a dynamical evaporating black hole in an inflationary universe,
is analyzed. We present a possible way to obtain the Hawking radiation terms
for the model with arbitrary functions of mass. It is used to see how the
expansion of universe will affect the dynamical process of black hole
evaporation. The results show that the cosmological inflation has an
inclination to depress the black hole evaporation. However, if the cosmological
constant is sufficiently large then the back-reaction effect has the
inclination to increase the black hole evaporation. We also present a simple
method to show that it will always produce a divergent flux of outgoing
radiation along the Cauchy horizon where the curvature is a finite value. This
means that the Hawking radiation will be very large in there and shall modify
the classical spacetime drastically. Therefore the black hole evaporation
cannot be discussed self-consistently on the classical Vaidya-type spacetime.
Our method can also be applied to analyze the quantum stress-energy tensor in
the more general Vaidya-type spacetimes.Comment: Proper boundary will lead to anti-evaporation of schwarzschild-de
Sitter black holes, as corrected in Class. Quantum Grav. 11 (1994) 28
Room temperature spin coherence in ZnO
Time-resolved optical techniques are used to explore electron spin dynamics
in bulk and epilayer samples of n-type ZnO as a function of temperature and
magnetic field. The bulk sample yields a spin coherence time T2* of 20 ns at T
= 30 K. Epilayer samples, grown by pulsed laser deposition, show a maximum T2*
of 2 ns at T = 10 K, with spin precession persisting up to T = 280 K.Comment: 3 pages, 3 figure
Scalar Field Theory on Fuzzy S^4
Scalar fields are studied on fuzzy and a solution is found for the
elimination of the unwanted degrees of freedom that occur in the model. The
resulting theory can be interpreted as a Kaluza-Klein reduction of CP^3 to S^4
in the fuzzy context.Comment: 16 pages, LaTe
Biological dosimetry for breast cancer radiotherapy: a comparison of external beam and intraoperative radiotherapy
Fuzzy Torus via q-Parafermion
We note that the recently introduced fuzzy torus can be regarded as a
q-deformed parafermion. Based on this picture, classification of the Hermitian
representations of the fuzzy torus is carried out. The result involves
Fock-type representations and new finite dimensional representations for q
being a root of unity as well as already known finite dimensional ones.Comment: 12pages, no figur
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