4,268 research outputs found
The importance of the electronic contribution to linear magnetoelectricity
We demonstrate that the electronic contribution to the linear magnetoelectric
response, usually omitted in first-principles studies, can be comparable in
magnitude to that mediated by lattice distortions, even for materials in which
responses are strong. Using a self-consistent Zeeman response to an applied
magnetic field for noncollinear electron spins, we show how electric
polarization emerges in linear magnetoelectrics through both electronic- and
lattice-mediated components -- in analogy with the high- and low-frequency
dielectric response to an electric field. The approach we use is conceptually
and computationally simple, and can be applied to study both linear and
non-linear responses to magnetic fields.Comment: 5 pages, 3 figure
Freshly Formed Dust in the Cassiopeia A Supernova Remnant as Revealed by the Spitzer Space Telescope
We performed Spitzer Infrared Spectrograph mapping observations covering
nearly the entire extent of the Cassiopeia A supernova remnant (SNR), producing
mid-infrared (5.5-35 micron) spectra every 5-10". Gas lines of Ar, Ne, O, Si, S
and Fe, and dust continua were strong for most positions. We identify three
distinct ejecta dust populations based on their continuum shapes. The dominant
dust continuum shape exhibits a strong peak at 21 micron. A line-free map of 21
micron-peak dust made from the 19-23 micron range closely resembles the [Ar
II], [O IV], and [Ne II] ejecta-line maps implying that dust is freshly formed
in the ejecta. Spectral fitting implies the presence of SiO2, Mg
protosilicates, and FeO grains in these regions. The second dust type exhibits
a rising continuum up to 21 micron and then flattens thereafter. This ``weak 21
micron'' dust is likely composed of Al2O3 and C grains. The third dust
continuum shape is featureless with a gently rising spectrum and is likely
composed of MgSiO3 and either Al2O3 or Fe grains. Using the least massive
composition for each of the three dust classes yields a total mass of 0.02
Msun. Using the most-massive composition yields a total mass of 0.054 Msun. The
primary uncertainty in the total dust mass stems from the selection of the dust
composition necessary for fitting the featureless dust as well as 70 micron
flux. The freshly formed dust mass derived from Cas A is sufficient from SNe to
explain the lower limit on the dust masses in high redshift galaxies.Comment: 8 figures: Accepted for the publication in Ap
Kinematics of X‐Ray–Emitting Components in Cassiopeia A
We present high-resolution X-ray proper-motion measurements of Cassiopeia A using Chandra X-Ray Observatory observations from 2000 and 2002. We separate the emission into four spectrally distinct classes: Si-dominated, Fe-dominated, low-energy-enhanced, and continuum-dominated. These classes also represent distinct spatial and kinematic components. The Si- and Fe-dominated classes are ejecta and have a mean expansion rate of 0.2% yr-1. This is the same as for the forward shock filaments but less than the 0.3% yr-1 characteristic of optical ejecta. The low-energy-enhanced spectral class possibly illuminates a clumpy circumstellar component and has a mean expansion rate of 0.05% yr-1. The continuum-dominated emission likely represents the forward shock and consists of diffuse circumstellar material, which is seen as a circular ring around the periphery of the remnant as well as projected across the center
Temperature-Dependent Magnetoelectric Effect from First Principles
We show that nonrelativistic exchange interactions and spin fluctuations can give rise to a linear magnetoelectric effect in collinear antiferromagnets at elevated temperatures that can exceed relativistic magnetoelectric responses by more than 1 order of magnitude. We show how symmetry arguments, ab initio methods, and Monte Carlo simulations can be combined to calculate temperature-dependent magnetoelectric susceptibilities entirely from first principles. The application of our method to Cr2O3 gives quantitative agreement with experiment.
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