11,049 research outputs found
Axisymmetric Stationary Solutions as Harmonic Maps
We present a method for generating exact solutions of Einstein equations in
vacuum using harmonic maps, when the spacetime possesses two commutating
Killing vectors. This method consists in writing the axisymmetric stationry
Einstein equations in vacuum as a harmonic map which belongs to the group
SL(2,R), and decomposing it in its harmonic "submaps". This method provides a
natural classification of the solutions in classes (Weil's class, Lewis' class
etc).Comment: 17 TeX pages, one table,( CINVESTAV- preprint 12/93
Pseudospin excitations in coaxial nanotubes
In a 2DEG confined to two coaxial tubes the `tube degree of freedom' can be
described in terms of pseudospin-1/2 dynamics. The presence of tunneling
between the two tubes leads to a collective oscillation known as pseudospin
resonance. We employ perturbation theory to examine the dependence of the
frequency of this mode with respect to a coaxial magnetic field for the case of
small intertube distances. Coulomb interaction leads to a shift of the
resonance frequency and to a finite lifetime of the pseudospin excitations. The
presence of the coaxial magnetic field gives rise to pronounced peaks in the
shift of the resonance frequency. For large magnetic fields this shift vanishes
due to the effects of Zeeman splitting. Finally, an expression for the
linewidth of the resonance is derived. Numerical analysis of this expression
suggests that the linewidth strongly depends on the coaxial magnetic field,
which leads to several peaks of the linewidth as well as regions where damping
is almost completely suppressed.Comment: 11 pages, 7 figure
On the Space Time of a Galaxy
We present an exact solution of the averaged Einstein's field equations in
the presence of two real scalar fields and a component of dust with spherical
symmetry. We suggest that the space-time found provides the characteristics
required by a galactic model that could explain the supermassive central object
and the dark matter halo at once, since one of the fields constitutes a central
oscillaton surrounded by the dust and the other scalar field distributes far
from the coordinate center and can be interpreted as a halo. We show the
behavior of the rotation curves all along the background. Thus, the solution
could be a first approximation of a ``long exposition photograph'' of a galaxy.Comment: 8 pages REVTeX, 11 eps figure
Spin-orbit coupling induced by a mass gradient
The existence of a spin-orbit coupling (SOC) induced by the gradient of the
effective mass in low-dimensional heterostructures is revealed. In structurally
asymmetric quasi-two-dimensional semiconductor heterostructures the presence of
a mass gradient across the interfaces results in a SOC which competes with the
SOC created by the electric field in the valence band. However, in graded
quantum wells subjected to an external electric field, the mass-gradient
induced SOC can be finite even when the electric field in the valence band
vanishes.Comment: 4 pages, 2 figures, 1 tabl
Oscillatons revisited
In this paper, we study some interesting properties of a spherically
symmetric oscillating soliton star made of a real time-dependent scalar field
which is called an oscillaton. The known final configuration of an oscillaton
consists of a stationary stage in which the scalar field and the metric
coefficients oscillate in time if the scalar potential is quadratic. The
differential equations that arise in the simplest approximation, that of
coherent scalar oscillations, are presented for a quadratic scalar potential.
This allows us to take a closer look at the interesting properties of these
oscillating objects. The leading terms of the solutions considering a quartic
and a cosh scalar potentials are worked in the so called stationary limit
procedure. This procedure reveals the form in which oscillatons and boson stars
may be related and useful information about oscillatons is obtained from the
known results of boson stars. Oscillatons could compete with boson stars as
interesting astrophysical objects, since they would be predicted by scalar
field dark matter models.Comment: 10 pages REVTeX, 10 eps figures. Updated files to match version
published in Classical and Quantum Gravit
Generation of Closed Timelike Curves with Rotating Superconductors
The spacetime metric around a rotating SuperConductive Ring (SCR) is deduced
from the gravitomagnetic London moment in rotating superconductors. It is shown
that theoretically it is possible to generate Closed Timelike Curves (CTC) with
rotating SCRs. The possibility to use these CTC's to travel in time as
initially idealized by G\"{o}del is investigated. It is shown however, that
from a technology and experimental point of view these ideas are impossible to
implement in the present context.Comment: 9 pages. Submitted to Classical and Quantum Gravit
Revivals, collapses and magnetic-pulse generation in quantum rings
Using a microscopic theory based on the density matrix formalism we
investigate quantum revivals and collapses of the charge polarization and
charge current dynamics in mesoscopic rings driven by short asymmetric
electromagnetic pulses. The collapsed state is utilized for sub-picosecond
switching of the current and associated magnetization, enabling thus the
generation of pulsed magnetic fields with a tunable time structure and shape
asymmetry which provides a new tool to study ultrafast spin-dynamics and
ratchet-based effects.Comment: 4 pages, 2 figure
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