15,290 research outputs found
Proof of a generalized Geroch conjecture for the hyperbolic Ernst equation
We enunciate and prove here a generalization of Geroch's famous conjecture
concerning analytic solutions of the elliptic Ernst equation. Our
generalization is stated for solutions of the hyperbolic Ernst equation that
are not necessarily analytic, although it can be formulated also for solutions
of the elliptic Ernst equation that are nowhere axis-accessible.Comment: 75 pages (plus optional table of contents). Sign errors in elliptic
case equations (1A.13), (1A.15) and (1A.25) are corrected. Not relevant to
proof contained in pape
Differentially rotating disks of dust: Arbitrary rotation law
In this paper, solutions to the Ernst equation are investigated that depend
on two real analytic functions defined on the interval [0,1]. These solutions
are introduced by a suitable limiting process of Backlund transformations
applied to seed solutions of the Weyl class. It turns out that this class of
solutions contains the general relativistic gravitational field of an arbitrary
differentially rotating disk of dust, for which a continuous transition to some
Newtonian disk exists. It will be shown how for given boundary conditions (i.
e. proper surface mass density or angular velocity of the disk) the
gravitational field can be approximated in terms of the above solutions.
Furthermore, particular examples will be discussed, including disks with a
realistic profile for the angular velocity and more exotic disks possessing two
spatially separated ergoregions.Comment: 23 pages, 3 figures, submitted to 'General Relativity and
Gravitation
Determination of the Far-Infrared Cosmic Background Using COBE/DIRBE and WHAM Data
Determination of the cosmic infrared background (CIB) at far infrared
wavelengths using COBE/DIRBE data is limited by the accuracy to which
foreground interplanetary and Galactic dust emission can be modeled and
subtracted. Previous determinations of the far infrared CIB (e.g., Hauser et
al. 1998) were based on the detection of residual isotropic emission in skymaps
from which the emission from interplanetary dust and the neutral interstellar
medium were removed. In this paper we use the Wisconsin H-alpha Mapper (WHAM)
Northern Sky Survey as a tracer of the ionized medium to examine the effect of
this foreground component on determination of the CIB. We decompose the DIRBE
far infrared data for five high Galactic latitude regions into H I and H-alpha
correlated components and a residual component. We find the H-alpha correlated
component to be consistent with zero for each region, and we find that addition
of an H-alpha correlated component in modeling the foreground emission has
negligible effect on derived CIB results. Our CIB detections and 2 sigma upper
limits are essentially the same as those derived by Hauser et al. and are given
by nu I_nu (nW m-2 sr-1) < 75, < 32, 25 +- 8, and 13 +- 3 at 60, 100, 140, and
240 microns, respectively. Our residuals have not been subjected to a detailed
anisotropy test, so our CIB results do not supersede those of Hauser et al. We
derive upper limits on the 100 micron emissivity of the ionized medium that are
typically about 40% of the 100 micron emissivity of the neutral atomic medium.
This low value may be caused in part by a lower dust-to-gas mass ratio in the
ionized medium than in the neutral medium, and in part by a shortcoming of
using H-alpha intensity as a tracer of far infrared emission.Comment: 38 pages, 8 figures. Accepted for publication in Ap
Evidence of breakdown of the spin symmetry in diluted 2D electron gases
Recent claims of an experimental demonstration of spontaneous spin
polarisation in dilute electron gases \cite{young99} revived long standing
theoretical discussions \cite{ceper99,bloch}. In two dimensions, the
stabilisation of a ferromagnetic fluid might be hindered by the occurrence of
the metal-insulator transition at low densities \cite{abra79}. To circumvent
localisation in the two-dimensional electron gas (2DEG) we investigated the low
populated second electron subband, where the disorder potential is mainly
screened by the high density of the first subband. This letter reports on the
breakdown of the spin symmetry in a 2DEG, revealed by the abrupt enhancement of
the exchange and correlation terms of the Coulomb interaction, as determined
from the energies of the collective charge and spin excitations. Inelastic
light scattering experiments and calculations within the time-dependent local
spin-density approximation give strong evidence for the existence of a
ferromagnetic ground state in the diluted regime.Comment: 4 pages, 4 figures, Revte
Dirichlet Boundary Value Problems of the Ernst Equation
We demonstrate how the solution to an exterior Dirichlet boundary value
problem of the axisymmetric, stationary Einstein equations can be found in
terms of generalized solutions of the Backlund type. The proof that this
generalization procedure is valid is given, which also proves conjectures about
earlier representations of the gravitational field corresponding to rotating
disks of dust in terms of Backlund type solutions.Comment: 22 pages, to appear in Phys. Rev. D, Correction of a misprint in
equation (4
Unconventional resistivity at the border of metallic antiferromagnetism in NiS2
We report low-temperature and high-pressure measurements of the electrical
resistivity \rho(T) of the antiferromagnetic compound NiS_2 in its
high-pressure metallic state. The form of \rho(T) suggests that metallic
antiferromagnetism in NiS_2 is quenched at a critical pressure p_c=76+-5 kbar.
Near p_c the temperature variation of \rho(T) is similar to that observed in
NiS_{2-x}Se_x near the critical composition x=1 where the Neel temperature
vanishes at ambient pressure. In both cases \rho(T) varies approximately as
T^{1.5} over a wide range below 100 K. However, on closer analysis the
resistivity exponent in NiS_2 exhibits an undulating variation with temperature
not seen in NiSSe (x=1). This difference in behaviour may be due to the effects
of spin-fluctuation scattering of charge carriers on cold and hot spots of the
Fermi surface in the presence of quenched disorder, which is higher in NiSSe
than in stoichiometric NiS_2.Comment: 7 page
Observables for spacetimes with two Killing field symmetries
The Einstein equations for spacetimes with two commuting spacelike Killing
field symmetries are studied from a Hamiltonian point of view. The complexified
Ashtekar canonical variables are used, and the symmetry reduction is performed
directly in the Hamiltonian theory. The reduced system corresponds to the field
equations of the SL(2,R) chiral model with additional constraints.
On the classical phase space, a method of obtaining an infinite number of
constants of the motion, or observables, is given. The procedure involves
writing the Hamiltonian evolution equations as a single `zero curvature'
equation, and then employing techniques used in the study of two dimensional
integrable models. Two infinite sets of observables are obtained explicitly as
functionals of the phase space variables. One set carries sl(2,R) Lie algebra
indices and forms an infinite dimensional Poisson algebra, while the other is
formed from traces of SL(2,R) holonomies that commute with one another. The
restriction of the (complex) observables to the Euclidean and Lorentzian
sectors is discussed.
It is also shown that the sl(2,R) observables can be associated with a
solution generating technique which is linked to that given by Geroch.Comment: 23 pages (LateX-RevTeX), Alberta-Thy-55-9
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