10,722 research outputs found
Dynamics of electromagnetic waves in Kerr geometry
Here we are interested to study the spin-1 particle i.e., electro-magnetic
wave in curved space-time, say around black hole. After separating the
equations into radial and angular parts, writing them according to the black
hole geometry, say, Kerr black hole we solve them analytically. Finally we
produce complete solution of the spin-1 particles around a rotating black hole
namely in Kerr geometry. Obviously there is coupling between spin of the
electro-magnetic wave and that of black hole when particles propagate in that
space-time. So the solution will be depending on that coupling strength. This
solution may be useful to study different other problems where the analytical
results are needed. Also the results may be useful in some astrophysical
contexts.Comment: 15 Latex pages, 4 Figures; Accepted for publication in Classical and
Quantum Gravit
All order covariant tubular expansion
We consider tubular neighborhood of an arbitrary submanifold embedded in a
(pseudo-)Riemannian manifold. This can be described by Fermi normal coordinates
(FNC) satisfying certain conditions as described by Florides and Synge in
\cite{FS}. By generalizing the work of Muller {\it et al} in \cite{muller} on
Riemann normal coordinate expansion, we derive all order FNC expansion of
vielbein in this neighborhood with closed form expressions for the curvature
expansion coefficients. Our result is shown to be consistent with certain
integral theorem for the metric proved in \cite{FS}.Comment: 27 pages. Corrected an error in a class of coefficients resulting
from a typo. Integral theorem and all other results remain unchange
DeWitt-Virasoro construction
We study a particular approach for analyzing worldsheet conformal invariance
for bosonic string propagating in a curved background using hamiltonian
formalism. We work in the Schrodinger picture of a single particle description
of the problem where the particle moves in an infinite-dimensional space.
Background independence is maintained in this approach by adopting DeWitt's
(Phys.Rev.85:653-661,1952) coordinate independent formulation of quantum
mechanics. This enables us to construct certain background independent notion
of Virasoro generators, called DeWitt-Virasoro (DWV) generators, and invariant
matrix elements of an arbitrary operator constructed out of them in spin-zero
representation. We show that the DWV algebra is given by the Witt algebra with
additional anomalous terms that vanish for Ricci-flat backgrounds. The actual
quantum Virasoro generators should be obtained by first introducing the vacuum
state and then normal ordering the DWV generators with respect to that. We
demonstrate the procedure in the simple cases of flat and pp-wave backgrounds.
This is a shorter version of arXiv:0912.3987 [hep-th] with many technical
derivations omitted.Comment: 18 pages, shorter version of arXiv:0912.3987 [hep-th] accepted for
publication in Pramana - Journal of Physic
The use of singular value gradients and optimization techniques to design robust controllers for multiloop systems
A method for designing robust feedback controllers for multiloop systems is presented. Robustness is characterized in terms of the minimum singular value of the system return difference matrix at the plant input. Analytical gradients of the singular values with respect to design variables in the controller are derived. A cumulative measure of the singular values and their gradients with respect to the design variables is used with a numerical optimization technique to increase the system's robustness. Both unconstrained and constrained optimization techniques are evaluated. Numerical results are presented for a two output drone flight control system
Application of matrix singular value properties for evaluating gain and phase margins of multiloop systems
For Abstract see A83-12457 (or A8312457/2
Application of constrained optimization to active control of aeroelastic response
Active control of aeroelastic response is a complex in which the designer usually tries to satisfy many criteria which are often conflicting. To further complicate the design problem, the state space equations describing this type of control problem are usually of high order, involving a large number of states to represent the flexible structure and unsteady aerodynamics. Control laws based on the standard Linear-Quadratic-Gaussian (LQG) method are of the same high order as the aeroelastic plant. To overcome this disadvantage of the LQG mode, an approach developed for designing low order optimal control laws which uses a nonlinear programming algorithm to search for the values of the control law variables that minimize a composite performance index, was extended to the constrained optimization problem. The method involves searching for the values of the control law variables that minimize a basic performance index while satisfying several inequality constraints that describe the design criteria. The method is applied to gust load alleviation of a drone aircraft
Inversion of magnetoresistance in magnetic tunnel junctions : effect of pinhole nanocontacts
Inverse magnetoresistance has been observed in magnetic tunnel junctions with
pinhole nanocontacts over a broad temperature range. The tunnel
magnetoresistance undergoes a change of sign at higher bias and temperature.
This phenomenon is attributed to the competition between the spin conserved
ballistic transport through the pinhole contact where the transmission
probability is close to unity and spin polarized tunneling across the
insulating spacer with weak transmittivity.Comment: Replaced with revised version and new figure, 6 figures, RevTex
The holographic spectral function in non-equilibrium states
We develop holographic prescriptions for obtaining spectral functions in
non-equilibrium states and space-time dependent non-equilibrium shifts in the
energy and spin of quasi-particle like excitations. We reproduce strongly
coupled versions of aspects of non-equilibrium dynamics of Fermi surfaces in
Landau's Fermi-liquid theory. We find that the incoming wave boundary condition
at the horizon does not suffice to obtain a well-defined perturbative expansion
for non-equilibrium observables. Our prescription, based on analysis of
regularity at the horizon, allows such a perturbative expansion to be achieved
nevertheless and can be precisely formulated in a universal manner independent
of the non-equilibrium state, provided the state thermalizes. We also find that
the non-equilibrium spectral function furnishes information about the
relaxation modes of the system. Along the way, we argue that in a typical
non-supersymmetric theory with a gravity dual, there may exist a window of
temperature and chemical potential at large N, in which a generic
non-equilibrium state can be characterized by just a finitely few operators
with low scaling dimensions, even far away from the hydrodynamic limit.Comment: revtex; 43 pages, 2 figures; typos corrected, accepted for
publication in PR
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