9,619 research outputs found
Electronic properties of disordered corner-sharing tetrahedral lattices
We have examined the behaviour of noninteracting electrons moving on a
corner-sharing tetrahedral lattice into which we introduce a uniform (box)
distribution, of width W, of random on-site energies. We have used both the
relative localization length and the spectral rigidity to analyze the nature of
the eigenstates, and have determined both the mobility edge trajectories as a
function of W, and the critical disorder, Wc, beyond which all states are
localized. We find (i) that the mobility edge trajectories (energies Ec vs.
disorder W) are qualitatively different from those found for a simple cubic
lattice, and (ii) that the spectral rigidity is scale invariant at Wc and thus
provides a reliable method of estimating this quantity -- we find Wc/t=14.5. We
discuss our results in the context of the metal-to-insulator transition
undergone by LiAlyTi{2-y}O4 in a quantum site percolation model that also
includes the above-mentioned Anderson disorder, and show that the effects
produced by Anderson disorder are far less important than those produced by
quantum site percolation, at least in the determination of the doping
concentration at which the metal-to-insulator transition is predicted to occur
Composition Conditions for Classes of Analytic Functions
We prove that for classes of analytic functions tree composition condition
and composition condition coincide.Comment: 13 page
Optimized Effective Potential Model for the Double Perovskites Sr2-xYxVMoO6 and Sr2-xYxVTcO6
In attempt to explore half-metallic properties of the double perovskites
Sr2-xYxVMoO6 and Sr2-xYxVTcO6, we construct an effective low-energy model,
which describes the behavior of the t2g-states of these compounds. All
parameters of such model are derived rigorously on the basis of
first-principles electronic structure calculations. In order to solve this
model we employ the optimized effective potential method and treat the
correlation interactions in the random phase approximation. Although
correlation interactions considerably reduce the intraatomic exchange splitting
in comparison with the Hartree-Fock method, this splitting still substantially
exceeds the typical values obtained in the local-spin-density approximation
(LSDA), which alters many predictions based on the LSDA. Our main results are
summarized as follows: (i) all ferromagnetic states are expected to be
half-metallic. However, their energies are generally higher than those of the
ferrimagnetic ordering between V- and Mo/Tc-sites (except Sr2VMoO6); (ii) all
ferrimagnetic states are metallic (except fully insulating Y2VTcO6) and no
half-metallic antiferromagnetism has been found; (iii) moreover, many of the
ferrimagnetic structures appear to be unstable with respect to the spin-spiral
alignment. Thus, the true magnetic ground state of the most of these systems is
expected to be more complex. In addition, we discuss several methodological
issues related to the nonuniqueness of the effective potential for the magnetic
half-metallic and insulating states.Comment: 15 pages, 9 figure
Fibrations of genus two on complex surfaces
We consider fibrations of genus 2 over complex surfaces. The purpose of this
paper is primarily to provide a geometric description of the possible
structures of the fibration on a neighborhood of a singular fiber. In
particular it is shown that the "geometric data" of the singular fiber
determines the fibration on its neighborhood up to a transversely holomorphic
-diffeomorphism. The method employed is quite flexible and it
applies to good extent to fibrations of arbitrary genus.Comment: This is the final version, June 201
Pseudoconvex domains spread over complex homogeneous manifolds
Using the concept of inner integral curves defined by Hirschowitz we
generalize a recent result by Kim, Levenberg and Yamaguchi concerning the
obstruction of a pseudoconvex domain spread over a complex homogeneous manifold
to be Stein. This is then applied to study the holomorphic reduction of
pseudoconvex complex homogeneous manifolds X=G/H. Under the assumption that G
is solvable or reductive we prove that X is the total space of a G-equivariant
holomorphic fiber bundle over a Stein manifold such that all holomorphic
functions on the fiber are constant.Comment: 21 page
Far-ultraviolet Spectroscopy of Venus and Mars at 4 A Resolution with the Hopkins Ultraviolet Telescope on Astro-2
Far-ultraviolet spectra of Venus and Mars in the range 820-1840 A at 4 A
resolution were obtained on 13 and 12 March 1995, respectively, by the Hopkins
Ultraviolet Telescope (HUT), which was part of the Astro-2 observatory on the
Space Shuttle Endeavour. Longward of 1250 A, the spectra of both planets are
dominated by emission of the CO Fourth Positive band system and strong OI and
CI multiplets. In addition, CO Hopfield-Birge bands, B - X (0,0) at 1151 A and
C - X (0,0) at 1088 A, are detected for the first time, and there is a weak
indication of the E - X (0,0) band at 1076 A in the spectrum of Venus. The B -
X band is blended with emission from OI 1152. Modeling the relative intensities
of these bands suggests that resonance fluorescence of CO is the dominant
source of the emission, as it is for the Fourth Positive system. Shortward of
Lyman-alpha, other emission features detected include OII 834, OI lambda 989,
HI Lyman-beta, and NI 1134 and 1200. For Venus, the derived disk brightnesses
of the OI, OII, and HI features are about one-half of those reported by Hord et
al. (1991) from Galileo EUV measurements made in February 1990. This result is
consistent with the expected variation from solar maximum to solar minimum. The
ArI 1048, 1066 doublet is detected only in the spectrum of Mars and the derived
mixing ratio of Ar is of the order of 2%, consistent with previous
determinations.Comment: 8 pages, 5 figures, accepted for publication in ApJ, July 20, 200
Novel Nonreciprocal Acoustic Effects in Antiferromagnets
The possible occurrence of nonreciprocal acoustic effects in antiferromagnets
in the absence of an external magnetic field is investigated using both (i) a
microscopic formulation of the magnetoelastic interaction between spins and
phonons and (ii) symmetry arguments. We predict for certain antiferromagnets
the existence of two new nonreciprocal (non-time invariant) effects:
A boundary-condition induced nonreciprocal effect and the occurrence of
transversal phonon modes propagating in opposite directions having different
velocities. Estimates are given and possible materials for these effects to be
observed are suggested.Comment: Euro. Phys. Lett. (in press
Effect of Semicore Orbitals on the Electronic Band Gaps of Si, Ge, and GaAs within the GW Approximation
We study the effect of semicore states on the self-energy corrections and
electronic energy gaps of silicon, germanium and GaAs. Self-energy effects are
computed within the GW approach, and electronic states are expanded in a
plane-wave basis. For these materials, we generate {\it ab initio}
pseudopotentials treating as valence states the outermost two shells of atomic
orbitals, rather than only the outermost valence shell as in traditional
pseudopotential calculations. The resulting direct and indirect energy gaps are
compared with experimental measurements and with previous calculations based on
pseudopotential and ``all-electron'' approaches. Our results show that,
contrary to recent claims, self-energy effects due to semicore states on the
band gaps can be well accounted for in the standard valence-only
pseudopotential formalism.Comment: 6 pages, 3 figures, submitted to Phys. Rev.
Photometric Properties of Kiso Ultraviolet-Excess Galaxies in the Lynx-Ursa Major Region
We have performed a systematic study of several regions in the sky where the
number of galaxies exhibiting star formation (SF) activity is greater than
average. We used Kiso ultraviolet-excess galaxies (KUGs) as our SF-enhanced
sample. By statistically comparing the KUG and non-KUG distributions, we
discovered four KUG-rich regions with a size of . One of these regions corresponds spatially to a filament of length
Mpc in the Lynx-Ursa Major region (). We call this ``the Lynx-Ursa
Major (LUM) filament''. We obtained surface photometry of 11 of
the KUGs in the LUM filament and used these to investigate the integrated
colors, distribution of SF regions, morphologies, and local environments. We
found that these KUGs consist of distorted spiral galaxies and compact galaxies
with blue colors. Their star formation occurs in the entire disk, and is not
confined to just the central regions. The colors of the SF regions imply that
active star formation in the spiral galaxies occurred yr ago,
while that of the compact objects occurred yr ago. Though the
photometric characteristics of these KUGs are similar to those of interacting
galaxies or mergers, most of these KUGs do not show direct evidence of merger
processes.Comment: 39 pages LaTeX, using aasms4.sty, 20 figures, ApJS accepted. The
Title of the previous one was truncated by the author's mistake, and is
corrected. Main body of the paper is unchange
The -genus and a regularization of an -equivariant Euler class
We show that a new multiplicative genus, in the sense of Hirzebruch, can be
obtained by generalizing a calculation due to Atiyah and Witten. We introduce
this as the -genus, compute its value for some examples and
highlight some of its interesting properties. We also indicate a connection
with the study of multiple zeta values, which gives an algebraic interpretation
for our proposed regularization procedure.Comment: 14 pages; version to appear in J. Phys.
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