708 research outputs found
Eccentricities of Double Neutron Star Binaries
Recent pulsar surveys have increased the number of observed double neutron
stars (DNS) in our galaxy enough so that observable trends in their properties
are starting to emerge. In particular, it has been noted that the majority of
DNS have eccentricities less than 0.3, which are surprisingly low for binaries
that survive a supernova explosion that we believe imparts a significant kick
to the neutron star. To investigate this trend, we generate many different
theoretical distributions of DNS eccentricities using Monte Carlo population
synthesis methods. We determine which eccentricity distributions are most
consistent with the observed sample of DNS binaries. In agreement with
Chaurasia & Bailes (2005), assuming all double neutron stars are equally as
probable to be discovered as binary pulsars, we find that highly eccentric,
coalescing DNS are less likely to be observed because of their accelerated
orbital evolution due to gravitational wave emission and possible early
mergers. Based on our results for coalescing DNS, we also find that models with
vanishingly or moderately small kicks (sigma < about 50 km/s) are inconsistent
with the current observed sample of such DNS. We discuss the implications of
our conclusions for DNS merger rate estimates of interest to ground-based
gravitational-wave interferometers. We find that, although orbital evolution
due to gravitational radiation affects the eccentricity distribution of the
observed sample, the associated upwards correction factor to merger rate
estimates is rather small (typically 10-40%).Comment: 9 pages, 8 figures, accepted by ApJ. Figures reduced and some content
changed, references adde
Ab initio Pseudopotential Plane-wave Calculations of the Electronic Structure of YBa_2Cu_3O_7
We present an ab initio pseudopotential local density functional calculation
for stoichiometric high-Tc cuprate YBa_2Cu_3O_7 using the plane-wave basis set.
We have overcome well-known difficulties in applying pseudopotential methods to
first-row elements, transition metals, and rare-earth materials by carefully
generating norm-conserving pseudopotentials with excellent transferability and
employing an extremely efficient iterative diagonalization scheme optimized for
our purpose. The self-consistent band structures, the total and site-projected
densities of states, the partial charges and their symmetry-decompositions, and
some characteristic charge densities near E_f are presented. We compare our
results with various existing (F)LAPW and (F)LMTO calculations and establish
that the ab initio pseudopotential method is competitive with other methods in
studying the electronic structure of such complicated materials as high-Tc
cuprates. [8 postscript files in uuencoded compressed form]Comment: 14 pages, RevTeX v3.0, 8 figures (appended in postscript file), SNUTP
94-8
Formation, Manipulation, and Elasticity Measurement of a Nanometric Column of Water Molecules
Nanometer-sized columns of condensed water molecules are created by an
atomic-resolution force microscope operated in ambient conditions. Unusual
stepwise decrease of the force gradient associated with the thin water bridge
in the tip-substrate gap is observed during its stretch, exhibiting regularity
in step heights (~0.5 N/m) and plateau lengths (~1 nm). Such "quantized"
elasticity is indicative of the atomic-scale stick-slip at the tip-water
interface. A thermodynamic-instability-induced rupture of the water meniscus
(5-nm long and 2.6-nm wide) is also found. This work opens a high-resolution
study of the structure and the interface dynamics of a nanometric aqueous
column.Comment: 4 pages, 3 figure
Exact zero-point energy shift in the , many modes dynamic Jahn-Teller systems at strong coupling
We find the exact semiclassical (strong coupling) zero-point energy shifts
applicable to the and dynamic Jahn-Teller
problems, for an arbitrary number of discrete vibrational modes
simultaneously coupled to one single electronic level. We also obtain an
analytical formula for the frequency of the resulting normal modes, which has
an attractive and apparently general Slater-Koster form. The limits of validity
of this approach are assessed by comparison with O'Brien's previous
effective-mode approach, and with accurate numerical diagonalizations.
Numerical values obtained for with and coupling
constants appropriate to C are used for this purpose, and are
discussed in the context of fullerene.Comment: 20 pages, 4 ps figure
Investigation of A1g phonons in YBa2Cu3O7 by means of LAPW atomic-force calculations
We report first-principles frozen-phonon calculations for the determination
of the force-free geometry and the dynamical matrix of the five Raman-active
A1g modes in YBa2Cu3O7. To establish the shape of the phonon potentials atomic
forces are calculated within the LAPW method. Two different schemes - the local
density approximation (LDA) and a generalized gradient approximation (GGA) -
are employed for the treatment of electronic exchange and correlation effects.
We find that in the case of LDA the resulting phonon frequencies show a
deviation from experimental values of approximately -10%. Invoking GGA the
frequency values are significantly improved and also the eigenvectors are in
very good agreement with experimental findings.Comment: 15 page
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Cytogenetic and Array-CGH Characterization of a Complex de novo Rearrangement Involving Duplication and Deletion of 9p and Clinical Findings in a 4-Month-Old Female
Approximately 15 patients with partial trisomy 9p involving de novo duplications have been previously described. Here, we present clinical, cytogenetic, FISH and aCGH findings in a patient with a de novo complex rearrangement in the short arm of chromosome 9 involving an inverted duplication at 9p24→p21.3 and a deletion at 9pter→p24.2. FISH probes generated from BACs selected from the UCSC genome browser were utilized to verify this rearrangement. It is likely that some previously described duplications of 9p may also be products of complex chromosomal aberrations. This report in which FISH and aCGH were used to more comprehensively characterize the genomic rearrangement in a patient with clinical manifestations of 9p duplication syndrome underscores the importance of further characterizing cytogenetically detected rearrangements
Effects of charge doping and constrained magnetization on the electronic structure of an FeSe monolayer
The electronic structural properties in the presence of constrained
magnetization and a charged background are studied for a monolayer of FeSe in
non-magnetic, checkerboard-, and striped-antiferromagnetic (AFM) spin
configurations. First principles techniques based on the pseudopotential
density functional approach and the local spin density approximation are
utilized. Our findings show that the experimentally observed shape of the Fermi
surface is best described by the checkerboard AFM spin pattern. To explore the
underlying pairing mechanism, we study the evolution of the non-magnetic to the
AFM-ordered structures under constrained magnetization. We estimate the
strength of electronic coupling to magnetic excitations involving an increase
in local moment and, separately, a partial moment transfer from one Fe atom to
another. We also show that the charge doping in the FeSe can lead to an
increase in the density of states at the Fermi level and possibly produce
higher superconducting transition temperatures
Supercell technique for total-energy calculations of finite charged and polar systems
We study the behavior of total-energy supercell calculations for dipolar molecules and charged clusters. Using a cutoff Coulomb interaction within the framework of a plane-wave basis set formalism, with all other aspects of the method (pseudopotentials, basis set, exchange-correlation functional) unchanged, we are able to assess directly the interaction effects present in the supercell technique. We find that the supercell method gives structures and energies in almost total agreement with the results of calculations for finite systems, even for molecules with large dipole moments. We also show that the performance of finite-grid calculations can be improved by allowing a degree of aliasing in the Hartree energy, and by using a reciprocal space definition of the cutoff Coulomb interaction
Direct observation of localized defect states in semiconductor nanotube junctions
Scanning tunneling microscopy of semiconductor-semiconductor carbon nanotube junctions with different band gaps was studied. Characteristic features of the wave functions at different energy levels were exhibited in the atomically resolved scanning tunneling microscopy. The experimental observations in terms of the pentagon-heptagon defects in the junction were interpreted.open888
Thermal and Tunneling Pair Creation of Quasiparticles in Quantum Hall Systems
We make a semiclassical analysis of thermal pair creations of quasiparticles
at various filling factors in quantum Hall systems. It is argued that the gap
energy is reduced considerably by the Coulomb potential made by impurities. It
is also shown that a tunneling process becomes important at low temperature and
at strong magnetic field. We fit typical experimental data excellently based on
our semiclassical results of the gap energy.Comment: 6 pages, 6 PS figures, to be published in Phys.Rev.
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