48,998 research outputs found
Evidence for charged critical behavior in the pyrochlore superconductor RbOs2O6
We analyze magnetic penetration depth data of the recently discovered
superconducting pyrochlore oxide RbOs2O6. Our results strongly suggest that in
RbOs2O6 charged critical fuctuations dominate the temperature dependence of the
magnetic penetration depth near Tc. This is in contrast to the mean-field
behavior observed in conventional superconductors and the uncharged critical
behavior found in nearly optimally doped cuprate superconductors. However, this
finding agrees with the theoretical predictions for charged criticality and the
charged criticality observed in underdoped YBa2Cu3O6.59.Comment: 5 pages, 4 figure
Structural, magnetic, and transport properties of CoFeSi Heusler films
We report the deposition of thin CoFeSi films by RF magnetron sputtering.
Epitaxial (100)-oriented and L2 ordered growth is observed for films grown
on MgO(100) substrates. (110)-oriented films on AlO(110) show several
epitaxial domains in the film plane. Investigation of the magnetic properties
reveals a saturation magnetization of 5.0 at low temperatures. The
temperature dependence of the resistivity exhibits a crossover
from a T^3.5 law at T<50K to a T^1.65 behaviour at elevated temperatures.
shows a small anisotropic magnetoresistive effect. A weak
dependence of the normal Hall effect on the external magnetic field indicates
the compensation of electron and hole like contributions at the Fermi surface.Comment: 10 pages, 9 figures to be published in J. Phys. D: Appl. Phy
Hall effect in laser ablated Co_2(Mn,Fe)Si thin films
Pulsed laser deposition was employed to grow thin films of the Heusler
compounds Co_2MnSi and Co_2FeSi. Epitaxial growth was realized both directly on
MgO (100) and on a Cr or Fe buffer layer. Structural analysis by x-ray and
electron diffraction shows for both materials the ordered L2_1 structure. Bulk
magnetization was determined with a SQUID magnetometer. The values agree with
the Slater-Pauling rule for half-metallic Heusler compounds. On the films grown
directly on the substrate measurements of the Hall effect have been performed.
The normal Hall effect is nearly temperature independent and points towards a
compensated Fermi surface. The anomalous contribution is found to be dominated
by skew scattering. A remarkable sign change of both normal and anomalous Hall
coefficients is observed on changing the valence electron count from 29 (Mn) to
30 (Fe).Comment: 9 pages, 6 figures submitted to J Phys
Reconstruction of Cluster Masses using Particle Based Lensing I: Application to Weak Lensing
We present Particle-Based Lensing (PBL), a new technique for gravitational
lensing mass reconstructions of galaxy clusters. Traditionally, most methods
have employed either a finite inversion or gridding to turn observational
lensed galaxy ellipticities into an estimate of the surface mass density of a
galaxy cluster. We approach the problem from a different perspective, motivated
by the success of multi-scale analysis in smoothed particle hydrodynamics. In
PBL, we treat each of the lensed galaxies as a particle and then reconstruct
the potential by smoothing over a local kernel with variable smoothing scale.
In this way, we can tune a reconstruction to produce constant signal-noise
throughout, and maximally exploit regions of high information density.
PBL is designed to include all lensing observables, including multiple image
positions and fluxes from strong lensing, as well as weak lensing signals
including shear and flexion. In this paper, however, we describe a shear-only
reconstruction, and apply the method to several test cases, including simulated
lensing clusters, as well as the well-studied ``Bullet Cluster'' (1E0657-56).
In the former cases, we show that PBL is better able to identify cusps and
substructures than are grid-based reconstructions, and in the latter case, we
show that PBL is able to identify substructure in the Bullet Cluster without
even exploiting strong lensing measurements. We also make our codes publicly
available.Comment: Accepted for publication in ApJ; Codes available at
http://www.physics.drexel.edu/~deb/PBL.htm ; 12 pages,9 figures, section 3
shortene
Analysis of Algorithms for Velocity Estimation from Discrete Position Versus Time Data
Algorithms for constructing velocity approximations from discrete position versus time data are investigated. The study is limited to algorithms suitable to provide velocity information in discrete-time feedback control systems such as microprocessor-based systems with a discrete position encoder. Velocity estimators based on lines per period, reciprocal-time, Taylor series expansion, backward difference expansions, and least-square curve fits are presented. Based on computer simulations, comparisons of relative accuracies of the different algorithms are made. The least-squares velocity estimators filtered the effect of imperfect measurements best, whereas the Taylor series expansions and backward difference equation estimators respond better to velocity transients
A general interpolation scheme for thermal fluctuations in superconductors
We present a general interpolation theory for the phenomenological effects of
thermal fluctuations in superconductors. Fluctuations are described by a simple
gauge invariant extension of the gaussian effective potential for the
Ginzburg-Landau static model. The approach is shown to be a genuine variational
method, and to be stationary for infinitesimal gauge variations around the
Landau gauge. Correlation and penetration lengths are shown to depart from the
mean field behaviour in a more or less wide range of temperature below the
critical regime, depending on the class of material considered. The method is
quite general and yields a very good interpolation of the experimental data for
very different materials.Comment: some misprints have been corrected in Eq.(15),(19); more references
and comments have been adde
Numerical Tests of Rotational Mixing in Massive Stars with the new Population Synthesis Code BONNFIRES
We use our new population synthesis code BONNFIRES to test how surface
abundances predicted by rotating stellar models depend on the numerical
treatment of rotational mixing, such as spatial resolution, temporal resolution
and computation of mean molecular weight gradients. We find that even with
identical numerical prescriptions for calculating the rotational mixing
coefficients in the diffusion equation, different timesteps lead to a deviation
of the coefficients and hence surface abundances. We find the surface
abundances vary by 10-100% between the model sequences with short timestep of
0.001Myr to model sequences with longer timesteps. Model sequences with
stronger surface nitrogen enrichment also have longer main-sequence lifetimes
because more hydrogen is mixed to the burning cores. The deviations in
main-sequence lifetimes can be as large as 20%. Mathematically speaking, no
numerical scheme can give a perfect solution unless infinitesimally small
timesteps are used. However, we find that the surface abundances eventually
converge within 10% between modelling sequences with sufficiently small
timesteps below 0.1Myr. The efficiency of rotational mixing depends on the
implemented numerical scheme and critically on the computation of the mean
molecular weight gradient. A smoothing function for the mean molecular weight
gradient results in stronger rotational mixing. If the discretization scheme or
the computational recipe for calculating the mean molecular weight gradient is
altered, re-calibration of mixing parameters may be required to fit
observations. If we are to properly understand the fundamental physics of
rotation in stars, it is crucial that we minimize the uncertainty introduced
into stellar evolution models when numerically approximating rotational mixing
processes.Comment: 8 pages, 6 figures, accepted by A&
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