22,315 research outputs found
Non-perturbative approach to backscattering off a dynamical impurity in 1D Fermi systems
We investigate the problem of backscattering off a time-dependent impurity in
a one-dimensional electron gas. By combining the Schwinger-Keldysh method with
an adiabatic approximation in order to deal with the corresponding out of
equilibrium Dirac equation, we compute the total energy density (TED) of the
system. We show how the free fermion TED is distorted by the backscattering
amplitude and the geometry of the impurity.Comment: 5 pages, 2 figures, RevTex4. Appendix and some text added. Results
and conclusions did not change. Version accepted for publication in Phys.
Rev.
Cosmology with Varying Constants
The idea of possible time or space variations of the `fundamental' constants
of nature, although not new, is only now beginning to be actively considered by
large numbers of researchers in the particle physics, cosmology and
astrophysics communities. This revival is mostly due to the claims of possible
detection of such variations, in various different contexts and by several
groups. Here, I present the current theoretical motivations and expectations
for such variations, review the current observational status, and discuss the
impact of a possible confirmation of these results in our views of cosmology
and physics as a whole.Comment: 14 pages, no figures. Essay to appear in Phil. Trans. Roy. Soc. Lond.
A Triennial Series (Christmas 2002 Issue
Collective electromagnetic relaxation in crystals of molecular magnets
We study the magnetization reversal and electromagnetic radiation due to
collective Landau-Zener relaxation in a crystal of molecular magnets.
Analytical and numerical solutions for the time dependence of the relaxation
process are obtained. The power of the radiation and the total emitted energy
are computed as functions of the crystal parameters and the field sweep rate.Comment: 7 pages, 9 figure
Modal Analysis of a Two-Parachute System
The Orion capsule is designed to land under a nominal configuration of three main parachutes; however, the system is required to be fault tolerant and land successfully if one of the main parachutes fails to open. The Capsule Parachute Assembly System (CPAS) Team performed a series of drop tests in order to characterize the performance of the system with two main parachutes. During the series of drop tests, several distinct dynamical modes were observed. The most consequential of these is the pendulum mode. Three other modes are benign: flyout (scissors), maypole, and breathing. The actual multi-body system is nonlinear, flexible, and possesses significant cross-coupling. Rather than perform analysis of this highly complex system directly, we conduct analysis of each dynamical mode observed during flight, based on first principles. This approach is analogous to traditional aircraft flight dynamics analysis in which the full nonlinear behavior of the airframe is decomposed into longitudinal dynamics (phugoid and short-period modes) and lateral dynamics (spiral, roll-subsidence, and dutch-roll modes). This analysis is intended to supplement multi-body nonlinear simulations in order to provide further insight into the system
A Bell pair in a generic random matrix environment
Two non-interacting qubits are coupled to an environment. Both coupling and
environment are represented by random matrix ensembles. The initial state of
the pair is a Bell state, though we also consider arbitrary pure states.
Decoherence of the pair is evaluated analytically in terms of purity; Monte
Carlo calculations confirm these results and also yield the concurrence of the
pair. Entanglement within the pair accelerates decoherence. Numerics display
the relation between concurrence and purity known for Werner states, allowing
us to give a formula for concurrence decay.Comment: 4 pages, 3 figure
An alternative formulation of classical electromagnetic duality
By introducing a doublet of electromagnetic four dimensional vector
potentials, we set up a manifestly Lorentz covariant and SO(2) duality
invariant classical field theory of electric and magnetic charges. In our
formulation one does not need to introduce the concept of Dirac string.Comment: 14 pages, no figures, Latex, minor corrections, references and
acknowledgements adde
Comparison of the Spherical Averaged Pseudopotential Model with the Stabilized Jellium Model
We compare Kohn-Sham results (density, cohesive energy, size and effect of
charging) of the Spherical Averaged Pseudopotential Model with the Stabilized
Jellium Model for clusters of sodium and aluminum with less than 20 atoms. We
find that the Stabilized Jellium Model, although conceptually and practically
more simple, gives better results for the cohesive energy and the elastic
stiffness. We use the Local Density Approximation as well as the Generalized
Gradient Approximation to the exchange and correlation energies.Comment: 13 pages, latex, 8 figures, compressed postscript version available
at http://www.fis.uc.pt/~vieir
Field Dependence of the Electron Spin Relaxation in Quantum Dots
Interaction of the electron spin with local elastic twists due to transverse
phonons has been studied. Universal dependence of the spin relaxation rate on
the strength and direction of the magnetic field has been obtained in terms of
the electron gyromagnetic tensor and macroscopic elastic constants of the
solid. The theory contains no unknown parameters and it can be easily tested in
experiment. At high magnetic field it provides parameter-free lower bound on
the electron spin relaxation in quantum dots.Comment: 4 PR pages, no figure
Decoherence of an -qubit quantum memory
We analyze decoherence of a quantum register in the absence of non-local
operations i.e. of non-interacting qubits coupled to an environment. The
problem is solved in terms of a sum rule which implies linear scaling in the
number of qubits. Each term involves a single qubit and its entanglement with
the remaining ones. Two conditions are essential: first decoherence must be
small and second the coupling of different qubits must be uncorrelated in the
interaction picture. We apply the result to a random matrix model, and
illustrate its reach considering a GHZ state coupled to a spin bath.Comment: 4 pages, 2 figure
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