91 research outputs found
Weak Field Magnetoresistance in Quasi-One-Dimensional Systems
Theoretical studies are presented on weak localization effects and
magnetoresistance in quasi-one-dimensional systems with open Fermi surfaces.
Based on the Wigner representation, the magnetoresistance in the region of weak
field has been studied for five possible configurations of current and field
with respect to the one-dimensional axis. It has been indicated that the
anisotropy and its temperature dependences of the magnetoresistance will give
information on the degree of one-dimensionality and the phase relaxation time.Comment: pages 11, LaTeX, 5 figures, uses jpsj.sty. To be published in J.
Phys. Soc. Jpn. (Vol.67(1998) No.4); Added some references and a Note at Feb.
13 199
Dimensional Crossover of Weak Localization in a Magnetic Field
We study the dimensional crossover of weak localization in strongly
anisotropic systems. This crossover from three-dimensional behavior to an
effective lower dimensional system is triggered by increasing temperature if
the phase coherence length gets shorter than the lattice spacing . A similar
effect occurs in a magnetic field if the magnetic length becomes shorter
than , where \D_{||}/D_\perp is the ratio of the
diffusion coefficients parallel and perpendicular to the planes or chains.
depends on the direction of the magnetic field, e.g. or
1/2 for a magnetic field parallel or perpendicular to the planes in a quasi
two-dimensional system. We show that even in the limit of large magnetic field,
weak localization is not fully suppressed in a lattice system. Experimental
implications are discussed in detail.Comment: RevTeX, 11 pages, 4 figures; three references added and discusse
Efficacy of sulphadoxine-pyrimethamine with or without artesunate for the treatment of uncomplicated Plasmodium falciparum malaria in southern Mozambique: a randomized controlled trial
BACKGROUND: An artemisinin-based combination therapy, artesunate (AS) plus sulphadoxine-pyrimethamine (SP), was compared to SP monotherapy to provide evidence of further treatment options in southern Mozambique. METHODS: Between 2003 and 2005, 411 patients over one year and 10 kg with uncomplicated Plasmodium falciparum malaria were randomly allocated SP (25/1.25 mg per kg day 0) or AS/SP (as above plus 4 mg/kg artesunate days 0, 1 and 2). Allocation was concealed, but treatment was open-label except to microscopists. The primary objective was the relative risk of treatment failure, which was assessed using World Health Organization response definitions modified to a 42-day follow-up. RESULTS: Of the 411 subjects enrolled, 359 (87.3%) completed the follow up period (SP n = 175, AS/SP n = 184). A survival analysis including 408 subjects showed that the polymerase chain reaction-adjusted cure rates were 90.4% (95% confidence interval [CI] 84.9%-93.9%) and 98.0% (95% CI 94.8%-99.3%) for SP and AS/SP respectively. Multivariable analysis showed that treatment with AS/SP decreased the relative hazard of treatment failure by 80% compared to SP (hazard ratio [HR] 0.2; 95% CI 0.1-0.6) and age over seven years decreased the relative hazard of failure by 70% (HR 0.3; 95% CI 0.1-0.9), when compared to younger age. However, having a quintuple dhfr/dhps mutation increased the relative hazard of failure compared to fewer mutations (HR 3.2; 95% CI 1.3-7.5) and baseline axillary temperature increased the relative hazard of failure by 50% for each degreesC increase (HR 1.5; 95% CI 1.1-2.2). CONCLUSION: While both treatments were efficacious, AS plus SP significantly decreased the relative hazard of treatment failure compared to SP monotherapy Artesunate plus sulphadoxine-pyrimethamine, but not sulphadoxine-pyrimethamine monotherapy, met the current WHO criteria of >95% efficacy for policy implementation.TRIAL REGISTRATION:NCT00203736 and NCT0020381
Effect of intersubband scattering on weak localization in 2D systems
The theory of weak localization is generalized for multilevel 2D systems
taking into account intersubband scattering. It is shown that weak intersubband
scattering which is negligible in a classical transport, affects strongly the
weak-localization correction to conductivity. The anomalous magnetoresistance
is calculated in the whole range of classically low magnetic fields. This
correction to conductivity is shown to depend strongly on the ratios of
occupied level concentrations. It is demonstrated that at relatively low
population of the excited subband, it is necessary to use the present theory
because the high-field limit asimptotics is shown to be achieved only in
classical magnetic fields.Comment: 18 pages, 4 figures. Accepted to Phys. Rev. B 6
The hydrodynamics of the supernova remnant Cas A: The influence of the progenitor evolution on the velocity structure and clumping
There are large differences in the proposed progenitor models for the Cas A
SNR. One of these differences is the presence or absence of a Wolf-Rayet (WR)
phase of the progenitor star. The mass loss history of the progenitor star
strongly affects the shape of the Supernova remnant (SNR). In this paper we
investigate whether the progenitor star of Cas A had a WR phase or not and how
long it may have lasted. We performed two-dimensional multi-species
hydrodynamical simulations of the CSM around the progenitor star for several WR
life times, each followed by the interaction of the supernova ejecta with the
CSM. We then looked at the influence of the length of the WR phase and compared
the results of the simulations with the observations of Cas A. The difference
in the structure of the CSM, for models with different WR life times, has a
strong impact on the resulting SNR. With an increasing WR life time the reverse
shock velocity of the SNR decreases and the range of observed velocities in the
shocked material increases. Furthermore, if a WR phase occurs, the remainders
of the WR shell will be visible in the resulting SNR. Comparing our results
with the observations suggests that the progenitor star of Cas A did not have a
WR phase. We also find that the quasi-stationary flocculi (QSF) in Cas A are
not consistent with the clumps from a WR shell that have been shocked and
accelerated by the interaction with the SN ejecta. We can also conclude that
for a SN explosion taking place in a CSM that is shaped by the wind during a
short < 15000 yr WR phase, the clumps from the WR shell will be visible inside
the SNR.Comment: 11 figures, 11 pages, accepted for publication in A&
Magnetic fields in supernova remnants and pulsar-wind nebulae
We review the observations of supernova remnants (SNRs) and pulsar-wind
nebulae (PWNe) that give information on the strength and orientation of
magnetic fields. Radio polarimetry gives the degree of order of magnetic
fields, and the orientation of the ordered component. Many young shell
supernova remnants show evidence for synchrotron X-ray emission. The spatial
analysis of this emission suggests that magnetic fields are amplified by one to
two orders of magnitude in strong shocks. Detection of several remnants in TeV
gamma rays implies a lower limit on the magnetic-field strength (or a
measurement, if the emission process is inverse-Compton upscattering of cosmic
microwave background photons). Upper limits to GeV emission similarly provide
lower limits on magnetic-field strengths. In the historical shell remnants,
lower limits on B range from 25 to 1000 microGauss. Two remnants show
variability of synchrotron X-ray emission with a timescale of years. If this
timescale is the electron-acceleration or radiative loss timescale, magnetic
fields of order 1 mG are also implied. In pulsar-wind nebulae, equipartition
arguments and dynamical modeling can be used to infer magnetic-field strengths
anywhere from about 5 microGauss to 1 mG. Polarized fractions are considerably
higher than in SNRs, ranging to 50 or 60% in some cases; magnetic-field
geometries often suggest a toroidal structure around the pulsar, but this is
not universal. Viewing-angle effects undoubtedly play a role. MHD models of
radio emission in shell SNRs show that different orientations of upstream
magnetic field, and different assumptions about electron acceleration, predict
different radio morphology. In the remnant of SN 1006, such comparisons imply a
magnetic-field orientation connecting the bright limbs, with a non-negligible
gradient of its strength across the remnant.Comment: 20 pages, 24 figures; to be published in SpSciRev. Minor wording
change in Abstrac
Radio emission from Supernova Remnants
The explosion of a supernova releases almost instantaneously about 10^51 ergs
of mechanic energy, changing irreversibly the physical and chemical properties
of large regions in the galaxies. The stellar ejecta, the nebula resulting from
the powerful shock waves, and sometimes a compact stellar remnant, constitute a
supernova remnant (SNR). They can radiate their energy across the whole
electromagnetic spectrum, but the great majority are radio sources. Almost 70
years after the first detection of radio emission coming from a SNR, great
progress has been achieved in the comprehension of their physical
characteristics and evolution. We review the present knowledge of different
aspects of radio remnants, focusing on sources of the Milky Way and the
Magellanic Clouds, where the SNRs can be spatially resolved. We present a brief
overview of theoretical background, analyze morphology and polarization
properties, and review and critical discuss different methods applied to
determine the radio spectrum and distances. The consequences of the interaction
between the SNR shocks and the surrounding medium are examined, including the
question of whether SNRs can trigger the formation of new stars. Cases of
multispectral comparison are presented. A section is devoted to reviewing
recent results of radio SNRs in the Magellanic Clouds, with particular emphasis
on the radio properties of SN 1987A, an ideal laboratory to investigate
dynamical evolution of an SNR in near real time. The review concludes with a
summary of issues on radio SNRs that deserve further study, and analyzing the
prospects for future research with the latest generation radio telescopes.Comment: Revised version. 48 pages, 15 figure
Six Years of Chandra Observations of Supernova Remnants
We present a review of the first six years of Chandra X-ray Observatory
observations of supernova remnants. From the official "first-light" observation
of Cassiopeia A that revealed for the first time the compact remnant of the
explosion, to the recent million-second spectrally-resolved observation that
revealed new details of the stellar composition and dynamics of the original
explosion, Chandra observations have provided new insights into the supernova
phenomenon. We present an admittedly biased overview of six years of these
observations, highlighting new discoveries made possible by Chandra's unique
capabilities.Comment: 82 pages, 28 figures, for the book Astrophysics Update
Molecular and atomic gas in the young TeV \gamma-ray SNRs RX J1713.7-3946 and RX J0852.0-4622; evidence for the hadronic production of \gamma-rays
The interstellar molecular clouds are the site of star formation and also the
target for the cosmic ray protons to produce \gamma-rays via the hadronic
process. The interstellar atomic gas is enveloping the molecular clouds and may
also be dense enough to affect the \gamma-ray production. In this Chapter, some
of the basic properties of the interstellar gas both in molecular and atomic
forms will be reviewed. Then, it is presented that two young TeV \gamma-ray
SNRs, RX J1713.7-3946 and RX J0852.0-4622, show good spatial correspondence
between the \gamma-rays and the interstellar protons. The good spatial
correspondence provides a support for the hadronic origin of the \gamma-rays in
these SNRs. It is emphasized that both molecular and atomic hydrogen plays a
role as targets for cosmic ray (CR) protons. The clumpy distribution of the
target interstellar medium (ISM) protons is crucial in the interaction of the
supernova shocks with the ISM, whereas models with uniform ISM distribution are
not viable. Finally, it is suggested that the dense atomic gas without
molecules may occupy the dominant part of the dark gas in the local ISM.Comment: 23 pages, 15 figures, To appear in "Cosmic Rays in Star-Forming
Environments: Proceedings of the Second Session of the Sant Cugat Forum on
Astrophysics" (April 16-19, 2012), Torres, Diego F. and Reimer, Olaf (Eds.
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