82,393 research outputs found
On the exposure to mobile phone radiation in trains
This report presents theoretical estimates of the Power Density levels which
may be reached inside trains. Two possible sources of high levels of radiation
are discussed. The first one arises since the walls of the wagons are metallic
and therefore bounce back almost all radiation impinging on them. The second is
due to the simultaneous emission of a seemingly large number of nearby
telephones. The theoretical study presented here shows that Power Densities
stay at values below reference levels always.Comment: 9 pages, 1 figur
Non-analyticities in three-dimensional gauge theories
Quantum fluctuations generate in three-dimensional gauge theories not only
radiative corrections to the Chern-Simons coupling but also non-analytic terms
in the effective action. We review the role of those terms in gauge theories
with massless fermions and Chern-Simons theories. The explicit form of
non-analytic terms turns out to be dependent on the regularization scheme and
in consequence the very existence of phenomena like parity and framing
anomalies becomes regularization dependent. In particular we find
regularization regimes where both anomalies are absent. Due to the presence of
non-analytic terms the effective action becomes not only discontinuous but also
singular for some background gauge fields which include sphalerons. The
appearence of this type of singularities is linked to the existence of nodal
configurations in physical states and tunneling suppression at some classical
field configurations. In the topological field theory the number of physical
states may also become regularization dependent. Another consequence of the
peculiar behaviour of three-dimensional theories under parity odd
regularizations is the existence of a simple mechanism of generation of a mass
gap in pure Yang-Mills theory by a suitable choice of regularization scheme.
The generic value of this mass does agree with the values obtained in
Hamiltonian and numerical analysis. Finally, the existence of different
regularization regimes unveils the difficulties of establishing a Zamolodchikov
c-theorem for three-dimensional field theories in terms of the induced
gravitational Chern-Simons couplings.Comment: 21 pages; Contribution to Ian Kogan Memorial Collection, ``From
Fields to Strings: Circumnavigating Theoretical Physics'
Survival and Nonescape Probabilities for Resonant and Nonresonant Decay
In this paper we study the time evolution of the decay process for a particle
confined initially in a finite region of space, extending our analysis given
recently (Phys. Rev. Lett. 74, 337 (1995)). For this purpose, we solve exactly
the time-dependent Schroedinger equation for a finite-range potential. We
calculate and compare two quantities: (i) the survival probability S(t), i.e.,
the probability that the particle is in the initial state after a time t; and
(ii) the nonescape probability P(t), i.e., the probability that the particle
remains confined inside the potential region after a time t. We analyze in
detail the resonant and nonresonant decay. In the former case, after a very
short time, S(t) and P(t) decay exponentially, but for very long times they
decay as a power law, albeit with different exponents. For the nonresonant case
we obtain that both quantities differ initially. However, independently of the
resonant and nonresonant character of the initial state we always find a
transition to the ground state of the system which indicates a process of
``loss of memory'' in the decay.Comment: 26 pages, RevTex file, figures available upon request from
[email protected] (To be published in Annals of Physics
Feynman diagams coupled to three-dimensional quantum gravity
A framework for quantum field theory coupled to three-dimensional quantum
gravity is proposed. The coupling with quantum gravity regulates the Feynman
diagrams. One recovers the usual Feynman amplitudes in the limit as the
cosmological constant tends to zero.Comment: 7 pages. v2: minor corrections, added re
Renormalization of Optical Excitations in Molecules near a Metal Surface
The lowest electronic excitations of benzene and a set of donor-acceptor
molecular complexes are calculated for the gas phase and on the Al(111) surface
using the many-body Bethe-Salpeter equation (BSE). The energy of the
charge-transfer excitations obtained for the gas phase complexes are found to
be around 10% lower than the experimental values. When the molecules are placed
outside the surface, the enhanced screening from the metal reduces the exciton
binding energies by several eVs and the transition energies by up to 1 eV
depending on the size of the transition-generated dipole. As a striking
consequence we find that close to the metal surface the optical gap of benzene
can exceed its quasiparticle gap. A classical image charge model for the
screened Coulomb interaction can account for all these effects which, on the
other hand, are completely missed by standard time-dependent density functional
theory.Comment: 4 pages, 3 figures; revised versio
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