5,824 research outputs found
Speckle-visibility spectroscopy: A tool to study time-varying dynamics
We describe a multispeckle dynamic light scattering technique capable of
resolving the motion of scattering sites in cases that this motion changes
systematically with time. The method is based on the visibility of the speckle
pattern formed by the scattered light as detected by a single exposure of a
digital camera. Whereas previous multispeckle methods rely on correlations
between images, here the connection with scattering site dynamics is made more
simply in terms of the variance of intensity among the pixels of the camera for
the specified exposure duration. The essence is that the speckle pattern is
more visible, i.e. the variance of detected intensity levels is greater, when
the dynamics of the scattering site motion is slow compared to the exposure
time of the camera. The theory for analyzing the moments of the spatial
intensity distribution in terms of the electric field autocorrelation is
presented. It is demonstrated for two well-understood samples, a colloidal
suspension of Brownian particles and a coarsening foam, where the dynamics can
be treated as stationary. However, the method is particularly appropriate for
samples in which the dynamics vary with time, either slowly or rapidly, limited
only by the exposure time fidelity of the camera. Potential applications range
from soft-glassy materials, to granular avalanches, to flowmetry of living
tissue.Comment: review - theory and experimen
Interplay between carrier and impurity concentrations in annealed GaMnAs intrinsic anomalous Hall Effect
Investigating the scaling behavior of annealed GaMnAs anomalous
Hall coefficients, we note a universal crossover regime where the scaling
behavior changes from quadratic to linear, attributed to the anomalous Hall
Effect intrinsic and extrinsic origins, respectively. Furthermore, measured
anomalous Hall conductivities when properly scaled by carrier concentration
remain constant, equal to theoretically predicated values, spanning nearly a
decade in conductivity as well as over 100 K in T. Both the qualitative
and quantitative agreement confirms the validity of new equations of motion
including the Berry phase contributions as well as tunablility of the intrinsic
anomalous Hall Effect.Comment: 4 pages, 5 figure
Analysis of White Dwarfs with Strange-Matter Cores
We summarize masses and radii for a number of white dwarfs as deduced from a
combination of proper motion studies, Hipparcos parallax distances, effective
temperatures, and binary or spectroscopic masses. A puzzling feature of these
data is that some stars appear to have radii which are significantly smaller
than that expected for a standard electron-degenerate white-dwarf equations of
state. We construct a projection of white-dwarf radii for fixed effective mass
and conclude that there is at least marginal evidence for bimodality in the
radius distribution forwhite dwarfs. We argue that if such compact white dwarfs
exist it is unlikely that they contain an iron core. We propose an alternative
of strange-quark matter within the white-dwarf core. We also discuss the impact
of the so-called color-flavor locked (CFL) state in strange-matter core
associated with color superconductivity. We show that the data exhibit several
features consistent with the expected mass-radius relation of strange dwarfs.
We identify eight nearby white dwarfs which are possible candidates for strange
matter cores and suggest observational tests of this hypothesis.Comment: 11 pages, 6 figures, accepted for publication in J. Phys. G: Nucl.
Part. Phy
Putative spin liquid in the triangle-based iridate BaIrTiO
We report on thermodynamic, magnetization, and muon spin relaxation
measurements of the strong spin-orbit coupled iridate BaIrTiO,
which constitutes a new frustration motif made up a mixture of edge- and
corner-sharing triangles. In spite of strong antiferromagnetic exchange
interaction of the order of 100~K, we find no hint for long-range magnetic
order down to 23 mK. The magnetic specific heat data unveil the -linear and
-squared dependences at low temperatures below 1~K. At the respective
temperatures, the zero-field muon spin relaxation features a persistent spin
dynamics, indicative of unconventional low-energy excitations. A comparison to
the isostructural compound BaRuTiO suggests that a concerted
interplay of compass-like magnetic interactions and frustrated geometry
promotes a dynamically fluctuating state in a triangle-based iridate.Comment: Physical Review B accepte
Study of the neutron star structure in strong magnetic fields including the anomalous magnetic moments
We study the effects of strong magnetic fields on the neutron star structure.
If the interior field of a star is on the same order of the surface field
currently observed, the influences of the magnetic field on the star mass and
radius are negligible. If one assumes that the internal magnetic field can be
as large as that estimated from the scalar virial theorem, considerable effects
can be induced. The maximum mass of stars is arisen substantially while the
central density is largely suppressed. For two equal-mass stars the radius of
the magnetic star can be larger by about 10% 20% than the nonmagnetic
star.Comment: 26 pages, 5 postscript figures; replaced by the revised version,
Chin. J. Astron. Astrophys., accepte
Structures of Staphylococcus aureus peptide deformylase in complex with two classes of new inhibitors
Peptide deformylase (PDF) catalyzes the removal of the formyl group from the N-terminal methionine residue in newly synthesized polypeptides, which is an essential process in bacteria. Four new inhibitors of PDF that belong to two different classes, hydroxamate/pseudopeptide compounds [PMT387 (7a) and PMT497] and reverse-hydroxamate/nonpeptide compounds [PMT1039 (15e) and PMT1067], have been developed. These compounds inhibited the growth of several pathogens involved in respiratory-tract infections, such as Streptococcus pneumoniae, Moraxella catarrhalis and Haemophilus influenzae, and leading nosocomial pathogens such as Staphylococcus aureus and Klebsiella pneumoniae with a minimum inhibitory concentration (MIC) in the range 0.1-0.8 mg ml(-1). Interestingly, the reverse-hydroxamate/nonpeptide compounds showed a 250-fold higher antimicrobial activity towards S. aureus, although the four compounds showed similar K-i values against S. aureus PDF enzymes, with Ki values in the 11-85 nM range. To provide a structural basis for the discovery of additional PDF inhibitors, the crystal structures of S. aureus PDF in complex with the four inhibitors were determined at resolutions of 1.90-2.30 angstrom. The inhibitor-bound structures displayed distinct deviations depending on the inhibitor class. The distance between the Zn2+ ion and the carbonyl O atom of the hydroxamate inhibitors (or the hydroxyl O atom of the reverse-hydroxamate inhibitors) appears to be correlated to S. aureus inhibition activity. The structural information reported in this study should aid in the discovery of new PDF inhibitors that can be used as novel antibacterial drugs
Electronic structures of ZnCoO using photoemission and x-ray absorption spectroscopy
Electronic structures of ZnCoO have been investigated using
photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). The
Co 3d states are found to lie near the top of the O valence band, with a
peak around eV binding energy. The Co XAS spectrum provides
evidence that the Co ions in ZnCoO are in the divalent Co
() states under the tetrahedral symmetry. Our finding indicates that the
properly substituted Co ions for Zn sites will not produce the diluted
ferromagnetic semiconductor property.Comment: 3 pages, 2 figure
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