369 research outputs found
A trick for passing degenerate points in Ashtekar formulation
We examine one of the advantages of Ashtekar's formulation of general
relativity: a tractability of degenerate points from the point of view of
following the dynamics of classical spacetime. Assuming that all dynamical
variables are finite, we conclude that an essential trick for such a continuous
evolution is in complexifying variables. In order to restrict the complex
region locally, we propose some `reality recovering' conditions on spacetime.
Using a degenerate solution derived by pull-back technique, and integrating the
dynamical equations numerically, we show that this idea works in an actual
dynamical problem. We also discuss some features of these applications.Comment: 9 pages by RevTeX or 16 pages by LaTeX, 3 eps figures and epsf-style
file are include
Topological 2-form Gravity in Four Dimensions
A kind of topological field theory is proposed as a candidate to describe the
global structure of the 2-form Einstein gravity with or without a cosmological
constant. Indeed in the former case, we show that a quantum state in the
candidate gives an exact solution of the Wheeler-DeWitt equation. The BRST
quantization based on the Batalin-Fradkin-Vilkovisky (BFV) formalism is carried
out for this topological version of the 2-form Einstein gravity.Comment: 15 page
Local virial relation for self-gravitating system
We demonstrate that the quasi-equilibrium state in self-gravitating -body
system after cold collapse are uniquely characterized by the local virial
relation using numerical simulations. Conversely assuming the constant local
virial ratio and Jeans equation for spherically steady state system, we
investigate the full solution space of the problem under the constant
anisotropy parameter and obtain some relevant solutions. Especially, the local
virial relation always provides a solution which has a power law density
profile in both the asymptotic regions and . This type of
solutions observed commonly in many numerical simulations. Only the anisotropic
velocity dispersion controls this asymptotic behavior of density profile.Comment: 9 pages, 15 eps figures, RevTex, submitted to PR
Gravitational Instantons and Moduli Spaces of Topological 2-form Gravity
A topological version of four-dimensional (Euclidean) Einstein gravity which
we propose regards anti-self-dual 2-forms and an anti-self-dual part of the
frame connections as fundamental fields. The theory describes the moduli spaces
of conformally self-dual Einstein manifolds for the non-zero cosmological
constant case and Einstein-Kahlerian manifold with the vanishing real first
Chern class for the zero cosmological constant. In the non-zero cosmological
constant case, we evaluate the index of the elliptic complex associated with
the moduli space and calculate the partition function. We also clarify the
moduli space and its dimension for the zero cosmological constant case which
are related to the Plebansky's heavenly equations.Comment: 36pages, LaTex, TIT/HEP-247/COSMO-4
Noncommutative Topological Half-flat Gravity
We formulate a noncommutative description of topological half-flat gravity in
four dimensions. BRST symmetry of this topological gravity is deformed through
a twisting of the usual BRST quantization of noncommutative gauge theories.
Finally it is argued that resulting moduli space of instantons is characterized
by the solutions of a noncommutative version of the Plebanski's heavenly
equation.Comment: 12+1 pages, revtex4, no figure
Half-Metallic Ferrimagnetism in Mn_2VAl
We show that Mn_2VAl is a compound for which the generalized gradient
approximation (GGA) to the exchange-correlation functional in density
functional theory makes a qualitative change in predicted behavior compared to
the usual local density approximation (LDA). Application of GGA leads to
prediction of Mn_2VAl being a half-metallic ferrimagnet, with the minority
channel being the conducting one. The electronic and magnetic structure is
analyzed and contrasted with the isostructural enhanced semimetal Fe_2VAl.Comment: 5 pages, Latex, 6 postscript figures. Description and figures of the
(minority) Fermi surfaces have been adde
Symmetries and observables in topological gravity
After a brief review of topological gravity, we present a superspace approach
to this theory. This formulation allows us to recover in a natural manner
various known results and to gain some insight into the precise relationship
between different approaches to topological gravity. Though the main focus of
our work is on the vielbein formalism, we also discuss the metric approach and
its relationship with the former formalism.Comment: 34 pages; a few explanations added in subsection 2.2.1, published
version of pape
Magnetoresistance of antidot lattices with grain boundaries
The magnetotransport properties of antidot lattices containing artificially
designed grain boundaries have been measured. We find that the grain boundaries
broaden the commensurability resonances and displace them anisotropically.
These phenomena are unexpectedly weak but differ characteristically from
isotropic, Gaussian disorder in the antidot positions. The observations are
interpreted in terms of semiclassical trajectories which tend to localize along
the grain boundaries within certain magnetic field intervals. Furthermore, our
results indicate how the transport through superlattices generated by
self-organizing templates may get influenced by grain boundaries.Comment: 6 pages, 3 figure
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