46,177 research outputs found
Automorphisms of abelian group extensions
Let be an abelian extension. The purpose of this
paper is to study the problem of extending automorphisms of and lifting
automorphisms of to certain automorphisms of .Comment: 11 page
X-ray spectrum of the high polarization quasar PKS 1510-089
We present results on the X-ray spectra of the radio-loud, high-polarization
quasar, PKS 1510-089, based on new data obtained using ASCA, and from archival
ROSAT data. The X-ray spectrum obtained by ASCA is unusually hard, with the
photon index=1.30+-0.06, while the (non-simultaneous) ROSAT data indicate a
steeper spectrum (1.9+-0.3). The X-ray flux at 1 keV is within 10% during both
observations. A break in the underlying continuum at about 0.7 keV is
suggested. Flat X-ray spectra seem to be the characteristic of high
polarization quasars, and their spectra also appear to be harder than that of
the other radio-loud but low-polarization quasars. The multiwavelength spectrum
of PKS 1510-089 is similar to many other gamma-ray blazars, suggesting the
emission is dominated by that from a relativistic jet. A big blue-bump is also
seen in its multiwavelength spectrum, suggesting the presence of a strong
thermal component as well.Comment: 19 pages (Latex + 5 ps figures), Accpeted for publication in the
Astrophysical Journal, December 20, 199
Frustration of tilts and A-site driven ferroelectricity in KNbO_3-LiNbO_3 alloys
Density functional calculations for K_{0.5}Li_{0.5}NbO_3 show strong A-site
driven ferroelectricity, even though the average tolerance factor is
significantly smaller than unity and there is no stereochemically active A-site
ion. This is due to the frustration of tilt instabilities by A-site disorder.
There are very large off-centerings of the Li ions, which contribute strongly
to the anisotropy between the tetragonal and rhombohedral ferroelectric states,
yielding a tetragonal ground state even without strain coupling.Comment: 4 pages, 5 figure
Electronic Structure, Local Moments and Transport in Fe_2VAl
Local spin density approximation calculations are used to elucidate
electronic and magnetic properties of Heusler structure Fe_2VAl. The compound
is found to be a low carrier density semimetal. The Fermi surface has small
hole pockets derived from a triply degenerate Fe derived state at Gamma
compensated by an V derived electron pocket at the X point. The ideal compound
is found to be stable against ferromagnetism. Fe impurities on V sites,
however, behave as local moments. Because of the separation of the hole and
electron pockets the RKKY interaction between such local moments should be
rapidly oscillating on the scale of its decay, leading to the likelihood of
spin-glass behavior for moderate concentrations of Fe on V sites. These
features are discussed in relation to experimental observations of an unusual
insulating state in this compound.Comment: 16 pages, RevTeX, 5 figure
Fermionic representation for the ferromagnetic Kondo lattice model -- diagrammatic study of spin-charge coupling effects on magnon excitations
A purely fermionic representation is introduced for the ferromagnetic Kondo
lattice model which allows conventional diagrammatic tools to be employed to
study correlation effects. Quantum 1/S corrections to magnon excitations are
investigated using a systematic inverse-degeneracy expansion scheme which
incorporates correlation effects in the form of self-energy and vertex
corrections, while explicitly preserving the continuous spin-rotation symmetry.
Magnon self-energy is studied in the full range of interaction strength, and
shown to result in strong magnon damping and anomalous softening for zone
boundary modes, which accounts for several zone-boundary anomalies observed in
recent spin-wave measurements of ferromagnetic manganites.Comment: 16 pages, 9 figure
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