4,970 research outputs found
Decay of accelerated particles
We study how the decay properties of particles are changed by acceleration.
It is shown that under the influence of acceleration (1) the lifetime of
particles is modified and (2) new processes (like the decay of the proton)
become possible. This is illustrated by considering scalar models for the decay
of muons, pions, and protons. We discuss the close conceptual relation between
these processes and the Unruh effect.Comment: Latex2e, 12 pages, 6 Postscript figures included with epsfig, to
appear in Phys. Rev.
Three-dimensional flux states as a model for the pseudogap phase of transition metal oxides
We propose that the pseudogap state observed in the transition metal oxides
can be explained by a three-dimensional flux state, which exhibits
spontaneously generated currents in its ground state due to electron-electron
correlations. We compare the energy of the flux state to other classes of mean
field states, and find that it is stabilized over a wide range of and
. The signature of the state will be peaks in the neutron diffraction
spectra, the location and intensity of which are presented. The dependence of
the pseudogap in the optical conductivity is calculated based on the parameters
in the model.Comment: submitted to Phys. Rev. B on January 8, 200
Enhancement of the electronic contribution to the low temperature specific heat of Fe/Cr magnetic multilayer
We measured the low temperature specific heat of a sputtered
magnetic multilayer, as well as separate
thick Fe and Cr films. Magnetoresistance and magnetization
measurements on the multilayer demonstrated antiparallel coupling between the
Fe layers. Using microcalorimeters made in our group, we measured the specific
heat for and in magnetic fields up to for the multilayer. The
low temperature electronic specific heat coefficient of the multilayer in the
temperature range is . This is
significantly larger than that measured for the Fe or Cr films (5.4 and respectively). No magnetic field dependence of was
observed up to . These results can be explained by a softening of the
phonon modes observed in the same data and the presence of an Fe-Cr alloy phase
at the interfaces.Comment: 20 pages, 5 figure
Spin density wave dislocation in chromium probed by coherent x-ray diffraction
We report on the study of a magnetic dislocation in pure chromium. Coherent
x-ray diffraction profiles obtained on the incommensurate Spin Density Wave
(SDW) reflection are consistent with the presence of a dislocation of the
magnetic order, embedded at a few micrometers from the surface of the sample.
Beyond the specific case of magnetic dislocations in chromium, this work may
open up a new method for the study of magnetic defects embedded in the bulk.Comment: 8 pages, 7 figure
Heavy Carriers and Non-Drude Optical Conductivity in MnSi
Optical properties of the weakly helimagnetic metal MnSi have been determined
in the photon energy range from 2 meV to 4.5 eV using the combination of
grazing incidence reflectance at 80 degrees (2 meV to 0.8 eV) and ellipsometry
(0.8 to 4.5 eV). As the sample is cooled below 100 K the effective mass becomes
strongly frequency dependent at low frequencies, while the scattering rate
developes a linear frequency dependence. The complex optical conductivity can
be described by the phenomenological relation \sigma(\omega,T) \propto
(\Gamma(T)+i\omega)^{-1/2} used for cuprates and ruthenates.Comment: 5 pages, ReVTeX 4, 5 figures in eps forma
Reorientation of Spin Density Waves in Cr(001) Films induced by Fe(001) Cap Layers
Proximity effects of 20 \AA thin Fe layers on the spin density waves (SDWs)
in epitaxial Cr(001) films are revealed by neutron scattering. Unlike in bulk
Cr we observe a SDW with its wave vector Q pointing along only one {100}
direction which depends dramatically on the film thickness t_{Cr}. For t_{Cr} <
250 \AA the SDW propagates out-of-plane with the spins in the film plane. For
t_{Cr} > 1000 \AA the SDW propagates in the film plane with the spins
out-of-plane perpendicular to the in-plane Fe moments. This reorientation
transition is explained by frustration effects in the antiferromagnetic
interaction between Fe and Cr across the Fe/Cr interface due to steps at the
interface.Comment: 4 pages (RevTeX), 3 figures (EPS
On the injectivity of the circular Radon transform arising in thermoacoustic tomography
The circular Radon transform integrates a function over the set of all
spheres with a given set of centers. The problem of injectivity of this
transform (as well as inversion formulas, range descriptions, etc.) arises in
many fields from approximation theory to integral geometry, to inverse problems
for PDEs, and recently to newly developing types of tomography. The article
discusses known and provides new results that one can obtain by methods that
essentially involve only the finite speed of propagation and domain dependence
for the wave equation.Comment: To appear in Inverse Problem
An excess power statistic for detection of burst sources of gravitational radiation
We examine the properties of an excess power method to detect gravitational
waves in interferometric detector data. This method is designed to detect
short-duration (< 0.5 s) burst signals of unknown waveform, such as those from
supernovae or black hole mergers. If only the bursts' duration and frequency
band are known, the method is an optimal detection strategy in both Bayesian
and frequentist senses. It consists of summing the data power over the known
time interval and frequency band of the burst. If the detector noise is
stationary and Gaussian, this sum is distributed as a chi-squared (non-central
chi-squared) deviate in the absence (presence) of a signal. One can use these
distributions to compute frequentist detection thresholds for the measured
power. We derive the method from Bayesian analyses and show how to compute
Bayesian thresholds. More generically, when only upper and/or lower bounds on
the bursts duration and frequency band are known, one must search for excess
power in all concordant durations and bands. Two search schemes are presented
and their computational efficiencies are compared. We find that given
reasonable constraints on the effective duration and bandwidth of signals, the
excess power search can be performed on a single workstation. Furthermore, the
method can be almost as efficient as matched filtering when a large template
bank is required. Finally, we derive generalizations of the method to a network
of several interferometers under the assumption of Gaussian noise.Comment: 22 pages, 6 figure
Magnetic phase diagram of cubic perovskites SrMn_1-xFe_xO_3
We combine the results of magnetic and transport measurements with Mossbauer
spectroscopy and room-temperature diffraction data to construct the magnetic
phase diagram of the new family of cubic perovskite manganites SrMn_1-xFe_xO_3.
We have found antiferromagnetic ordering for lightly and heavily Fe-substituted
material, while intermediate substitution leads to spin-glass behavior. Near
the SrMn_0.5Fe_0.5O_3 composition these two types of ordering are found to
coexist and affect one another. The spin glass behavior may be caused by
competing ferro- and antiferromagnetic interactions among Mn^4+ and observed
Fe^3+ and Fe^5+ ions.Comment: 8 pages, 10 figures, revtex, accepted to Phys. Rev.
The Chemorepulsive Protein Semaphorin 3A and Perineuronal Net-Mediated Plasticity.
During postnatal development, closure of critical periods coincides with the appearance of extracellular matrix structures, called perineuronal nets (PNN), around various neuronal populations throughout the brain. The absence or presence of PNN strongly correlates with neuronal plasticity. It is not clear how PNN regulate plasticity. The repulsive axon guidance proteins Semaphorin (Sema) 3A and Sema3B are also prominently expressed in the postnatal and adult brain. In the neocortex, Sema3A accumulates in the PNN that form around parvalbumin positive inhibitory interneurons during the closure of critical periods. Sema3A interacts with high-affinity with chondroitin sulfate E, a component of PNN. The localization of Sema3A in PNN and its inhibitory effects on developing neurites are intriguing features and may clarify how PNN mediate structural neural plasticity. In the cerebellum, enhanced neuronal plasticity as a result of an enriched environment correlates with reduced Sema3A expression in PNN. Here, we first review the distribution of Sema3A and Sema3B expression in the rat brain and the biochemical interaction of Sema3A with PNN. Subsequently, we review what is known so far about functional correlates of changes in Sema3A expression in PNN. Finally, we propose a model of how Semaphorins in the PNN may influence local connectivity.Peer Reviewe
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