1,888 research outputs found
Comments on "Vortex Glass and Lattice Melting Transitions in a YNi_2B_2C Single Crystal"
Recently, Mun et.al. (Phys. Rev. Lett., 76, 2790 (1996)) have published their
results on single crystal YNi_2B_2C, claiming that their experimental
observations can be explained in terms of formation of Vortex Glass and Lattice
melting. Our experiments, carried out on samples obtained from the SAME source,
reveal a much richer phase diagram and span wider regions of experimental
parameter space than Mun et. al. that encompasses most of their observations.
We speculate that this material has anomalous intrinsic properties and the
results cannot be explained by simple models about the flux lattice.Comment: 1 page, LaTeX type, 1 PostScript figure, Uses PRABIB.STY file, 600
dpi PS file available at http://sagar.physics.neu.edu/preprints.html To
appear in Physical Review Letter
Analysis of medium resolution spectra by automated methods - application to M55 and omega Centauri
We have employed feedforward neural networks trained on synthetic spectra in
the range 3800 to 5600 AA with resolutions of 2-3 AA to determine metallicities
from spectra of about 1000 main-sequence turn-off, subgiant and red giant stars
in the globular clusters M55 and omega Cen. The overall metallicity accuracies
are of the order of 0.15 to 0.2 dex. In addition, we tested how well the
stellar parameters logg and Teff can be retrieved from such data without
additional colour or photometric information. We find overall uncertainties of
0.3 to 0.4 dex for logg and 140 to 190 K for Teff. In order to obtain some
measure of uncertainty for the determined values of [Fe/H], logg and Teff, we
applied the bootstrap method for the first time to neural networks for this
kind of parametrization problem. The distribution of metallicities for stars in
omega Cen clearly shows a large spread in agreement with the well known
multiple stellar populations in this cluster.Comment: 13 pages, 9 figures, accepted for publication in A&
Critical State Flux Penetration and Linear Microwave Vortex Response in YBa_2Cu_3O_{7-x} Films
The vortex contribution to the dc field (H) dependent microwave surface
impedance Z_s = R_s+iX_s of YBa_2Cu_3O_{7-x} thin films was measured using
suspended patterned resonators. Z_s(H) is shown to be a direct measure of the
flux density B(H) enabling a very precise test of models of flux penetration.
Three regimes of field-dependent behavior were observed: (1) Initial flux
penetration occurs on very low field scales H_i(4.2K) 100Oe, (2) At moderate
fields the flux penetration into the virgin state is in excellent agreement
with calculations based upon the field-induced Bean critical state for thin
film geometry, parametrized by a field scale H_s(4.2K) J_c*d 0.5T, (3) for very
high fields H >>H_s, the flux density is uniform and the measurements enable
direct determination of vortex parameters such as pinning force constants
\alpha_p and vortex viscosity \eta. However hysteresis loops are in
disagreement with the thin film Bean model, and instead are governed by the low
field scale H_i, rather than by H_s. Geometric barriers are insufficient to
account for the observed results.Comment: 20 pages, LaTeX type, Uses REVTeX style files, Submitted to Physical
Review B, 600 dpi PostScript file with high resolution figures available at
http://sagar.physics.neu.edu/preprints.htm
The extended star formation history of omega Centauri
For the first time, the abundances of a large sample of subgiant and turn-off
region stars in omega Centauri have been measured, the data base being medium
resolution spectroscopy from FORS2 at the VLT. Absolute iron abundances were
derived for about 400 member stars from newly defined line indices with an
accuracy of +/-0.15 dex. The abundances range between -2.2<[Fe/H]<-0.7 dex,
resembling the large metallicity spread found for red giant branch stars. The
combination of the spectroscopic results with the location of the stars in the
colour magnitude diagram has been used to estimate ages for the individual
stars. Whereas most of the metal-poor stars are consistent with a single old
stellar population, stars with abundances higher than [Fe/H]=-1.3 dex are
younger. The total age spread in omega Cent is about 3 Gyr. The monotonically
increasing age-metallicity relation seems to level off above [Fe/H]=-1.0 dex.
Whether the star formation in omega Cen occured continuously or rather
episodically has to be shown by combining more accurate abundances with highest
quality photometry.Comment: 4 pages, 3 figures, accepted for publication in Astronomy &
Astrophysics (Letters
Microwave Current Imaging in Passive HTS Components by Low-Temperature Laser Scanning Microscopy (LTLSM)
We have used the LTLSM technique for a spatially resolved investigation of
the microwave transport properties, nonlinearities and material inhomogeneities
in an operating coplanar waveguide YBa_2Cu_3O_{7-\delta} (YBCO) microwave
resonator on an LaAlO_3 (LAO) substrate. The influence of twin-domain blocks,
in-plane rotated grains, and micro-cracks in the YBCO film on the nonuniform rf
current distribution were measured with a micrometer-scale spatial resolution.
The impact of the peaked edge currents and rf field penetration into weak links
on the linear device performance were studied as well. The LTLSM capabilities
and its future potential for non-destructive characterization of the microwave
properties of superconducting circuits are discussed.Comment: 8 pages, 9 figures, 2-column format, presented at High Temperature
Superconductors in High Frequency Fields 2004, Journal of Superconductivity
(in press
Beyond Prejudice as Simple Antipathy: Hostile and Benevolent Sexism Across Cultures
The authors argue that complementary hostile and benevolent componen:s of sexism exist ac ro.ss
cultures. Male dominance creates hostile sexism (HS). but men's dependence on women fosters
benevolent sexism (BS)-subjectively positive attitudes that put women on a pedestal but reinforce their
subordination. Research with 15,000 men and women in 19 nations showed that (a) HS and BS are
coherenl constructs th at correlate positively across nations, but (b) HS predicts the ascription of negative
and BS the ascription of positive traits to women, (c) relative to men, women are more likely to reject
HS than BS. especially when overall levels of sexism in a culture are high, and (d) national averages on
BS and HS predict gender inequal ity across nations. These results challenge prevailing notions of
prejudice as an antipathy in that BS (an affectionate, patronizing ideology) reflects inequality and is a
cross-culturally pervasive complement to HS
Biophysical Modeling of Mangrove Seedling Establishment and Survival Across an Elevation Gradient With Forest Zones
Mangrove forest development critically depends on the establishment and survival of seedlings. Mechanistic insights into how water levels, waves and bed level dynamics influence the establishment process of individual mangrove seedlings are increasing. However, little is known about how spatial and temporal changes in water levels, waves and bed level dynamics across elevation gradients in mangrove forests facilitate/limit seedling dynamics. For this study, a new seedling establishment and growth model was integrated into a process-based hydrodynamic and morphodynamic numerical model. This biophysical model was applied to a fringing mangrove forest located in the southern Firth of Thames, Aotearoa, New Zealand. This study quantifies the increasing establishment density and survival probability of mangrove seedlings from the lower-elevated unvegetated intertidal flat toward the higher-elevated mature mangrove forest. Three cross-shore zones with distinctive seedling dynamics were identified: (a) a zone with daily tidal inundation where seedling dynamics are episodic and limited by the dispersal of individual propagules that rapidly anchor to the substrate by root growth, (b) a zone with daily to bi-weekly tidal inundation where seedling dynamics respond to variations in spring-neap tidal cycles and, (c) a zone with less than bi-weekly inundation where seedling dynamics are governed by high propagule supply and seedling survival probability. The seedling establishment density and survival probability are dominated by annual extremes in tidal hydroperiod and bed shear stresses, respectively. The obtained parameterizations can be used to incorporate seedling dynamics in decadal-timescale mangrove forest development models that are instrumental for mangrove management and restoration
Systematics of c-axis Phonons in the Thallium and Bismuth Based Cuprate Superconductors
We present grazing incidence reflectivity measurements in the far infrared
region at temperatures above and below Tc for a series of thallium (Tl2Ba2CuO6,
Tl2Ba2CaCu2O8) and bismuth (Bi2Sr2CuO6, Bi2Sr2CaCu2O8, and
Bi(2-x)Pb(x)Sr2CaCu2O8) based cuprate superconductors. From the spectra, which
are dominated by the c-axis phonons, longitudinal frequencies (LO) are directly
obtained. The reflectivity curves are well fitted by a series of Lorentz
oscillators. In this way the transverse (TO) phonon frequencies were accurately
determined. On the basis of the comparative study of the Bi and Tl based
cuprates with different number of CuO2 layers per unit cell, we suggest
modifications of the assignment of the main oxygen modes. We compare the LO
frequencies in Bi2Sr2CaCu2O8 and Tl2Ba2Ca2Cu3O10 obtained from intrinsic
Josephson junction characteristics with our measurements, and explain the
discrepancy in LO frequencies obtained by the two different methods.Comment: 8 pages Revtex, 6 eps figures, 3 tables, to appear in Phys. Rev.
From the cell membrane to the nucleus: unearthing transport mechanisms for Dynein
Mutations in the motor protein cytoplasmic dynein have been found to cause Charcot-Marie-Tooth disease, spinal muscular atrophy, and severe intellectual disabilities in humans. In mouse models, neurodegeneration is observed. We sought to develop a novel model which could incorporate the effects of mutations on distance travelled and velocity. A mechanical model for the dynein mediated transport of endosomes is derived from first principles and solved numerically. The effects of variations in model parameter values are analysed to find those that have a significant impact on velocity and distance travelled. The model successfully describes the processivity of dynein and matches qualitatively the velocity profiles observed in experiments
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