2,464 research outputs found
Nonlinear interaction between electromagnetic fields at high temperature
The electron-positron `box' diagram produces an effective action which is
fourth order in the electromagnetic field. We examine the behaviour of this
effective action at high-temperature (in analytically continued imaginary-time
thermal perturbation theory). We argue that there is a finite, nonzero limit as
(where is the temperature). We calculate this limit
in the nonrelativistic static case, and in the long-wavelength limit. We also
briefly discuss the self-energy in 2-dimensional QED, which is similar in some
respects.Comment: 13 pages, DAMTP 94/3
Towards a political economy framework for wind power: Does China break the mould?
We propose a general taxonomy of the political economy challenges to wind power development and integration, highlighting the implications in terms of actors, interests, and risks. Applying this framework to three functions in China's electricity sector - planning and project approval, generator cost recovery, and balancing area coordination - we find evidence of challenges common across countries with significant wind investments, despite institutional and industry characteristics that are unique to China. We argue that resolving these political economy challenges is as important to facilitating the role of wind and other renewable energies in a low carbon energy transition as providing dedicated technical and policy support. China is no exception
Molecular dynamics of folding of secondary structures in Go-type models of proteins
We consider six different secondary structures of proteins and construct two
types of Go-type off-lattice models: with the steric constraints and without.
The basic aminoacid-aminoacid potential is Lennard Jones for the native
contacts and a soft repulsion for the non-native contacts. The interactions are
chosen to make the target secondary structure be the native state of the
system. We provide a thorough equilibrium and kinetic characterization of the
sequences through the molecular dynamics simulations with the Langevin noise.
Models with the steric constraints are found to be better folders and to be
more stable, especially in the case of the -structures. Phononic spectra
for vibrations around the native states have low frequency gaps that correlate
with the thermodynamic stability. Folding of the secondary structures proceeds
through a well defined sequence of events. For instance, -helices fold
from the ends first. The closer to the native state, the faster establishment
of the contacts. Increasing the system size deteriorates the folding
characteristics. We study the folding times as a function of viscous friction
and find a regime of moderate friction with the linear dependence. We also
consider folding when one end of a structure is pinned which imitates
instantaneous conditions when a protein is being synthesized. We find that,
under such circumstances, folding of helices is faster and of the
-sequences slower.Comment: REVTeX, 14 pages, EPS figures included, JCP in pres
Cherenkov radiation by particles traversing the background radiatio n
High energy particles traversing the Universe through the cosmic microwave
backgroung radiation can, in principle, emit Cherenkov radiation. It is shown
that the energy threshold for this radiation is extremely high and its
intensity would be too low due to the low density of the "relic photons gas"
and very weak interaction of two photons.Comment: 6 pages, LATEX, no Figs.; to be published in JETP Lett. 75 (N4)
(2002
Ultraviolet and soft X--ray photon--photon elastic scattering in an electron gas
We have considered the processes which lead to elastic scattering between two
far ultraviolet or X--ray photons while they propagate inside a solid, modeled
as a simple electron gas. The new ingredient, with respect to the standard
theory of photon--photon scattering in vacuum, is the presence of low--energy,
nonrelativistic electron--hole excitations. Owing to the existence of
two--photon vertices, the scattering processes in the metal are predominantly
of second order, as opposed to fourth order for the vacuum case. The main
processes in second order are dominated by exchange of virtual plasmons between
the two photons. For two photons of similar energy , this gives
rise to a cross section rising like up to maximum of around
~cm, and then decreasing like . The maximal cross
section is found for the photon wavevector , the Fermi surface
size, which typically means a photon energy in the keV range.
Possible experiments aimed at checking the existence of these rare but
seemingly measurable elastic photon--photon scattering processes are discussed,
using in particular intense synchrotron sources.Comment: 33 pages, TeX, Version 3.1, S.I.S.S.A. preprint 35/93/C
QED Corrections to Planck's Radiation Law and Photon Thermodynamics
Leading corrections to Planck's formula and photon thermodynamics arising
from the pair-mediated photon-photon interaction are calculated. This
interaction is attractive and causes an increase in occupation number for all
modes. Possible consequences, including the role of the cosmic photon gas in
structure formation, are considered.Comment: 15 pages, Revtex 3.
Sequencing of folding events in Go-like proteins
We have studied folding mechanisms of three small globular proteins: crambin
(CRN), chymotrypsin inhibitor 2 (CI2) and the fyn Src Homology 3 domain (SH3)
which are modelled by a Go-like Hamiltonian with the Lennard-Jones
interactions. It is shown that folding is dominated by a well-defined
sequencing of events as determined by establishment of particular contacts. The
order of events depends primarily on the geometry of the native state.
Variations in temperature, coupling strengths and viscosity affect the
sequencing scenarios to a rather small extent. The sequencing is strongly
correlated with the distance of the contacting aminoacids along the sequence.
Thus -helices get established first. Crambin is found to behave like a
single-route folder, whereas in CI2 and SH3 the folding trajectories are more
diversified. The folding scenarios for CI2 and SH3 are consistent with
experimental studies of their transition states.Comment: REVTeX, 12 pages, 11 EPS figures, J. Chem. Phys (in press
One Loop Multiphoton Helicity Amplitudes
We use the solutions to the recursion relations for double-off-shell fermion
currents to compute helicity amplitudes for -photon scattering and
electron-positron annihilation to photons in the massless limit of QED. The
form of these solutions is simple enough to allow {\it all}\ of the
integrations to be performed explicitly. For -photon scattering, we find
that unless , the amplitudes for the helicity configurations (+++...+) and
(-++...+) vanish to one-loop order.Comment: 27 pages + 4 uuencoded figures (included), Fermilab-Pub-93/327-T,
RevTe
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