7,481 research outputs found
Lattice constant variation and complex formation in zincblende Gallium Manganese Arsenide
We perform high resolution X-ray diffraction on GaMnAs mixed crystals as well
as on GaMnAs/GaAs and GaAs/MnAs superlattices for samples grown by low
temperature molecular beam epitaxy under different growth conditions. Although
all samples are of high crystalline quality and show narrow rocking curve
widths and pronounced finite thickness fringes, the lattice constant variation
with increasing manganese concentration depends strongly on the growth
conditions: For samples grown at substrate temperatures of 220 and 270 degrees
C the extrapolated relaxed lattice constant of Zincblende MnAs is 0.590 nm and
0.598 nm respectively. This is in contrast to low temperature GaAs, for which
the lattice constant decreases with increasing substrate temperature.Comment: pdf onl
Self-forces on extended bodies in electrodynamics
In this paper, we study the bulk motion of a classical extended charge in
flat spacetime. A formalism developed by W. G. Dixon is used to determine how
the details of such a particle's internal structure influence its equations of
motion. We place essentially no restrictions (other than boundedness) on the
shape of the charge, and allow for inhomogeneity, internal currents,
elasticity, and spin. Even if the angular momentum remains small, many such
systems are found to be affected by large self-interaction effects beyond the
standard Lorentz-Dirac force. These are particularly significant if the
particle's charge density fails to be much greater than its 3-current density
(or vice versa) in the center-of-mass frame. Additional terms also arise in the
equations of motion if the dipole moment is too large, and when the
`center-of-electromagnetic mass' is far from the `center-of-bare mass' (roughly
speaking). These conditions are often quite restrictive. General equations of
motion were also derived under the assumption that the particle can only
interact with the radiative component of its self-field. These are much simpler
than the equations derived using the full retarded self-field; as are the
conditions required to recover the Lorentz-Dirac equation.Comment: 30 pages; significantly improved presentation; accepted for
publication in Phys. Rev.
Absence of a consistent classical equation of motion for a mass-renormalized point charge
The restrictions of analyticity, relativistic (Born) rigidity, and negligible
O(a) terms involved in the evaluation of the self electromagnetic force on an
extended charged sphere of radius "a" are explicitly revealed and taken into
account in order to obtain a classical equation of motion of the extended
charge that is both causal and conserves momentum-energy. Because the
power-series expansion used in the evaluation of the self force becomes invalid
during transition time intervals immediately following the application and
termination of an otherwise analytic externally applied force, transition
forces must be included during these transition time intervals to remove the
noncausal pre-acceleration and pre-deceleration from the solutions to the
equation of motion without the transition forces. For the extended charged
sphere, the transition forces can be chosen to maintain conservation of
momentum-energy in the causal solutions to the equation of motion within the
restrictions of relativistic rigidity and negligible O(a) terms under which the
equation of motion is derived. However, it is shown that renormalization of the
electrostatic mass to a finite value as the radius of the charge approaches
zero introduces a violation of momentum-energy conservation into the causal
solutions to the equation of motion of the point charge if the magnitude of the
external force becomes too large. That is, the causal classical equation of
motion of a point charge with renormalized mass experiences a high acceleration
catastrophe.Comment: 13 pages, No figure
Sites of Biosynthesis of Outer and Inner Membrane Proteins of Neurospora crassa Mitochondria
Outer and inner membranes of Neurospora crassa mitochondria were separated by the combined swelling, shrinking, sonication procedure. Membranes were characterized by electron microscopy and by marker enzyme activities. A red carotenoid pigment was found to be concentrated in the outer membrane. The inner mitochondrial membrane was resolved into about 20 protein bands on polyacrylamide gel electrophoresis, whereas the outer membrane shows essentially one single protein band. Only negligible incorporation of radioactive amino acids occurs into outer membrane when isolated mitochondria are synthesizing polypeptide chains. In agreement with this observation labeling of outer membrane protein is almost entirely blocked, when whole Neurospora cells are incubated with radioactive amino acids in the presence of cycloheximide, an inhibitor of cytoplasmic protein synthesis. Finally, the essential electrophoretic protein band from outer membrane does not become labeled when mitochondria are incubated with radioactive amino acids either in vitro or in vivo in the presence of cycloheximide. It is concluded that the vast majority, if not all, of the outer membrane protein is synthesized by the cytoplasmic system and that polypeptide chains formed by the mitochondrial ribosomes are integrated into the inner mitochondrial membrane
Self-forces from generalized Killing fields
A non-perturbative formalism is developed that simplifies the understanding
of self-forces and self-torques acting on extended scalar charges in curved
spacetimes. Laws of motion are locally derived using momenta generated by a set
of generalized Killing fields. Self-interactions that may be interpreted as
arising from the details of a body's internal structure are shown to have very
simple geometric and physical interpretations. Certain modifications to the
usual definition for a center-of-mass are identified that significantly
simplify the motions of charges with strong self-fields. A derivation is also
provided for a generalized form of the Detweiler-Whiting axiom that pointlike
charges should react only to the so-called regular component of their
self-field. Standard results are shown to be recovered for sufficiently small
charge distributions.Comment: 21 page
Internal Friction and Vulnerability of Mixed Alkali Glasses
Based on a hopping model we show how the mixed alkali effect in glasses can
be understood if only a small fraction c_V ofthe available sites for the mobile
ions is vacant. In particular, we reproduce the peculiar behavior of the
internal friction and the steep fall (''vulnerability'') of the mobility of the
majority ion upon small replacements by the minority ion. The single and mixed
alkali internal friction peaks are caused by ion-vacancy and ion-ion exchange
processes. If c_V is small, they can become comparable in height even at small
mixing ratios. The large vulnerability is explained by a trapping of vacancies
induced by the minority ions. Reasonable choices of model parameters yield
typical behaviors found in experiments.Comment: 4 pages, 4 figure
RooStatsCms: a tool for analyses modelling, combination and statistical studies
The RooStatsCms (RSC) software framework allows analysis modelling and
combination, statistical studies together with the access to sophisticated
graphics routines for results visualisation. The goal of the project is to
complement the existing analyses by means of their combination and accurate
statistical studies.Comment: Proceedings of the 11th Topical Seminar on Innovative Particle and
Radiation Detectors. 4 pages and 5 figure
High energy neutrino yields from astrophysical sources II: Magnetized sources
We calculate the yield of high energy neutrinos produced in astrophysical
sources for arbitrary interaction depths and magnetic field strengths
. We take into account energy loss processes like synchrotron radiation and
diffusion of charged particles in turbulent magnetic fields as well as the
scattering of secondaries on background photons and the direct production of
charm neutrinos. Meson-photon interactions are simulated with an extended
version of the SOPHIA model. Diffusion leads to an increased path-length before
protons leave the source of size R_s and therefore magnetized sources lose
their transparency below the energy , with and 1 for Kolmogorov and
Bohm diffusion, respectively. Moreover, the neutrino flux is suppressed above
the energy where synchrotron energy losses become important for charged
particles. As a consequence, the energy spectrum and the flavor composition of
neutrinos are strongly modified both at low and high energies even for sources
with \tau_0\lsim 1.Comment: 15 pages, 16 figure
Electromagnetic self-forces and generalized Killing fields
Building upon previous results in scalar field theory, a formalism is
developed that uses generalized Killing fields to understand the behavior of
extended charges interacting with their own electromagnetic fields. New notions
of effective linear and angular momenta are identified, and their evolution
equations are derived exactly in arbitrary (but fixed) curved spacetimes. A
slightly modified form of the Detweiler-Whiting axiom that a charge's motion
should only be influenced by the so-called "regular" component of its
self-field is shown to follow very easily. It is exact in some interesting
cases, and approximate in most others. Explicit equations describing the
center-of-mass motion, spin angular momentum, and changes in mass of a small
charge are also derived in a particular limit. The chosen approximations --
although standard -- incorporate dipole and spin forces that do not appear in
the traditional Abraham-Lorentz-Dirac or Dewitt-Brehme equations. They have,
however, been previously identified in the test body limit.Comment: 20 pages, minor typos correcte
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