6,256 research outputs found
QED in strong, finite-flux magnetic fields
Lower bounds are placed on the fermionic determinants of Euclidean quantum
electrodynamics in two and four dimensions in the presence of a smooth,
finite-flux, static, unidirectional magnetic field , where
or , and is a point in the xy-plane.Comment: 10 pages, postscript (in uuencoded compressed tar file
Mass zeros in the one-loop effective actions of QED in 1+1 and 3+1 dimensions
It is known that the one-loop effective action of is a quadratic in
the field strength when the fermion mass is zero: all potential higher order
contributions beyond second order vanish. For nonzero fermion mass it is shown
that this behavior persists for a general class of fields for at least one
value of the fermion mass when the external field's flux satisfies
. For the mass-shell renormalized one-loop effective
action vanishes for at least one value of the fermion mass for a class of
smooth, square integrable background gauge fields provided a plausible
zero-mass limit exists.Comment: Section IV has been amende
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Global and regional temperature-change potentials for near-term climate forcers
We examine the climate effects of the emissions of near-term climate forcers (NTCFs) from 4 continental regions (East Asia, Europe, North America and South Asia) using radiative forcing from the task force on hemispheric transport of air pollution source-receptor global chemical transport model simulations. These simulations model the transport of 3 aerosol species (sulphate, particulate organic matter and black carbon) and 4 ozone precursors (methane, nitric oxides (NOx), volatile organic compounds and carbon monoxide). From the equilibrium radiative forcing results we calculate global climate metrics, global warming potentials (GWPs) and global temperature change potentials (GTPs) and show how these depend on emission region, and can vary as functions of time. For the aerosol species, the GWP(100) values are â37±12, â46±20, and 350±200 for SO2, POM and BC respectively for the direct effects only. The corresponding GTP(100) values are â5.2±2.4, â6.5±3.5, and 50±33.
This analysis is further extended by examining the temperature-change impacts in 4 latitude bands. This shows that the latitudinal pattern of the temperature response to emissions of the NTCFs does not directly follow the pattern of the diagnosed radiative forcing. For instance temperatures in the Arctic latitudes are particularly sensitive to NTCF emissions in the northern mid-latitudes. At the 100-yr time horizon the ARTPs show NOx emissions can have a warming effect in the northern mid and high latitudes, but cooling in the tropics and Southern Hemisphere. The northern mid-latitude temperature response to northern mid-latitude emissions of most NTCFs is approximately twice as large as would be implied by the global average
Anomalous quantum confined Stark effects in stacked InAs/GaAs self-assembled quantum dots
Vertically stacked and coupled InAs/GaAs self-assembled quantum dots (SADs)
are predicted to exhibit a strong non-parabolic dependence of the interband
transition energy on the electric field, which is not encountered in single SAD
structures nor in other types of quantum structures. Our study based on an
eight-band strain-dependent Hamiltonian indicates that
this anomalous quantum confined Stark effect is caused by the three-dimensional
strain field distribution which influences drastically the hole states in the
stacked SAD structures.Comment: 4 pages, 4 figure
Theoretical interpretation of the experimental electronic structure of lens shaped, self-assembled InAs/GaAs quantum dots
We adopt an atomistic pseudopotential description of the electronic structure
of self-assembled, lens shaped InAs quantum dots within the ``linear
combination of bulk bands'' method. We present a detailed comparison with
experiment, including quantites such as the single particle electron and hole
energy level spacings, the excitonic band gap, the electron-electron, hole-hole
and electron hole Coulomb energies and the optical polarization anisotropy. We
find a generally good agreement, which is improved even further for a dot
composition where some Ga has diffused into the dots.Comment: 16 pages, 5 figures. Submitted to Physical Review
Long-distance Bell-type tests using energy-time entangled photons
Long-distance Bell-type experiments are presented. The different experimental
challenges and their solutions in order to maintain the strong quantum
correlations between energy-time entangled photons over more than 10 km are
reported and the results analyzed from the point of view of tests of
fundamental physics as well as from the more applied side of quantum
communication, specially quantum key distribution. Tests using more than one
analyzer on each side are also presented.Comment: 22 pages including 7 figures and 5 table
Analysis of radiatively stable entanglement in a system of two dipole-interacting three-level atoms
We explore the possibilities of creating radiatively stable entangled states
of two three-level dipole-interacting atoms in a configuration by
means of laser biharmonic continuous driving or pulses. We propose three
schemes for generation of entangled states which involve only the lower states
of the system, not vulnerable to radiative decay. Two of them employ
coherent dynamics to achieve entanglement in the system, whereas the third one
uses optical pumping, i.e., an essentially incoherent process.Comment: Replaced with the final version; 14 pages, 6 figures; to appear in
Phys. Rev. A, vol. 61 (2000
Baryogenesis from Cosmic Strings at the Electroweak Scale
We explore the viability of baryogenesis from light scalar decays after the
electroweak phase transition. A minimal model of this kind is constructed with
new CP violating interactions involving a heavy fourth family. The departure
from thermal equilbrium must come from topological defects like cosmic strings,
and we show that almost any mechanism for producing the cosmic strings at the
electroweak scale results in a viable theory. Baryogenesis occurs in the fourth
generation but the baryon number is later transported to the visible
generations. This mechanism of indirect baryogenesis allows us to satisfy
experimental limits on the proton lifetime while still having perturbative
baryon number violation at low energies. The fourth family has very small
mixing angles which opens the possibility of distinct observable signatures in
collider experiments.Comment: 28 pages, Latex, two postscript figures, the model is simplified
slightly, experimental signatures are detailed, some typographical errors
corrected, no major change in conclusion
The compositional and evolutionary logic of metabolism
Metabolism displays striking and robust regularities in the forms of
modularity and hierarchy, whose composition may be compactly described. This
renders metabolic architecture comprehensible as a system, and suggests the
order in which layers of that system emerged. Metabolism also serves as the
foundation in other hierarchies, at least up to cellular integration including
bioenergetics and molecular replication, and trophic ecology. The
recapitulation of patterns first seen in metabolism, in these higher levels,
suggests metabolism as a source of causation or constraint on many forms of
organization in the biosphere.
We identify as modules widely reused subsets of chemicals, reactions, or
functions, each with a conserved internal structure. At the small molecule
substrate level, module boundaries are generally associated with the most
complex reaction mechanisms and the most conserved enzymes. Cofactors form a
structurally and functionally distinctive control layer over the small-molecule
substrate. Complex cofactors are often used at module boundaries of the
substrate level, while simpler ones participate in widely used reactions.
Cofactor functions thus act as "keys" that incorporate classes of organic
reactions within biochemistry.
The same modules that organize the compositional diversity of metabolism are
argued to have governed long-term evolution. Early evolution of core
metabolism, especially carbon-fixation, appears to have required few
innovations among a small number of conserved modules, to produce adaptations
to simple biogeochemical changes of environment. We demonstrate these features
of metabolism at several levels of hierarchy, beginning with the small-molecule
substrate and network architecture, continuing with cofactors and key conserved
reactions, and culminating in the aggregation of multiple diverse physical and
biochemical processes in cells.Comment: 56 pages, 28 figure
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