225 research outputs found
Photoluminescence Properties of the Zn1-x Y (x) O Tubes Prepared by Polycarbonate Templates
We have prepared Zn1-x Y (x) O (x=0 and 0.01) tubes to study its structural and photoluminescent properties. A pore wetting process of porous polycarbonate templates with the liquid precursor and following thermal treatment were utilized for preparing the Zn1-x Y (x) O tube structure. Using the polycarbonate template with pore size of about 2 mu m diameter, the Zn1-x Y (x) O tubes were obtained. Photoluminescence (PL) spectroscopy was used to measure optical emissions from 350 to 650 nm with a He-Cd laser. The results of the PL spectra show that the Zn1-x Y (x) O tubes have evident emission peaks at the UV (about 380 nm) and visible (around 500 to 650 nm) region. The emission peak at the UV region was slightly shifted to higher wavelengths with increasing Y content. Meanwhile, the green and yellow emission peaks intensity increases as Y content increases. These results are explained by the structure tuning and oxygen deficiency with the introduction of Y
SU(N) Antiferromagnets and Strongly Coupled QED: Effective Field Theory for Josephson Junctions Arrays
We review our analysis of the strong coupling of compact QED on a lattice
with staggered Fermions. We show that, for infinite coupling, compact QED is
exactly mapped in a quantum antiferromagnet. We discuss some aspects of this
correspondence relevant for effective field theories of Josephson junctions
arrays.Comment: 33 pages,latex,Proceedings of "Common Trends in Condensed Matter and
High Energy Physics",DFUPG 1/9
Reversible Random Sequential Adsorption of Dimers on a Triangular Lattice
We report on simulations of reversible random sequential adsorption of dimers
on three different lattices: a one-dimensional lattice, a two-dimensional
triangular lattice, and a two-dimensional triangular lattice with the nearest
neighbors excluded. In addition to the adsorption of particles at a rate K+, we
allow particles to leave the surface at a rate K-. The results from the
one-dimensional lattice model agree with previous results for the continuous
parking lot model. In particular, the long-time behavior is dominated by
collective events involving two particles. We were able to directly confirm the
importance of two-particle events in the simple two-dimensional triangular
lattice. For the two-dimensional triangular lattice with the nearest neighbors
excluded, the observed dynamics are consistent with this picture. The
two-dimensional simulations were motivated by measurements of Ca++ binding to
Langmuir monolayers. The two cases were chosen to model the effects of changing
pH in the experimental system.Comment: 9 pages, 10 figure
The breakdown of the Nagaoka phase in the 2D t-J model
In the limit of weak exchange, J, at low hole concentration, the ground state
of the 2D t-J model is believed to be ferromagnetic. We study the leading
instability of this Nagaoka state, which emerges with increasing J. Both exact
diagonalization of small clusters, and a semiclassical analytical calculation
of larger systems show that above a certain critical value of the exchange,
Nagaoka's state is unstable to phase separation. In a finite-size system a
bubble of antiferromagnetic Mott insulator appears in the ground state above
this threshold. The size of this bubble depends on the hole concentration and
scales as a power of the system size, N
Dynamic structure selection and instabilities of driven Josephson lattice in high-temperature superconductors
We investigate the dynamics of the Josephson vortex lattice in layered
high-T superconductors at high magnetic fields. Starting from coupled
equations for superconducting phases and magnetic field we derive equations for
the relative displacements [phase shifts] between the planar Josephson arrays
in the layers. These equations reveal two families of steady-state solutions:
lattices with constant phase shifts between neighboring layers, starting from
zero for a rectangular configuration to for a triangular configuration,
and double-periodic lattices. We find that the excess Josephson current is
resonantly enhanced when the Josephson frequency matches the frequency of the
plasma mode at the wave vector selected by the lattice structure. The regular
lattices exhibit several kinds of instabilities. We find stability regions of
the moving lattice in the plane lattice structure - Josephson frequency. A
specific lattice structure at given velocity is selected uniquely by boundary
conditions, which are determined by the reflection properties of
electromagnetic waves generated by the moving lattice. With increase of
velocity the moving configuration experiences several qualitative
transformations. At small velocities the regular lattice is stable and the
phase shift between neighboring layers smoothly decreases with increase of
velocity, starting from for a static lattice. At the critical velocity
the lattice becomes unstable. At even higher velocity a regular lattice is
restored again with the phase shift smaller than . With increase of
velocity, the structure evolves towards a rectangular configuration.Comment: 28 pages, 12 figures, submitted to Phys. Rev.
Spatially homogeneous ground state of the two-dimensional Hubbard model
We investigate the stability with respect to phase separation or charge
density-wave formation of the two-dimensional Hubbard model for various values
of the local Coulomb repulsion and electron densities using Green-function
Monte Carlo techniques. The well known sign problem is particularly serious in
the relevant region of small hole doping. We show that the difference in
accuracy for different doping makes it very difficult to probe the phase
separation instability using only energy calculations, even in the
weak-coupling limit () where reliable results are available. By contrast,
the knowledge of the charge correlation functions allows us to provide clear
evidence of a spatially homogeneous ground state up to .Comment: 7 pages and 5 figures. Phys. Rev. B, to appear 200
Identification and phylogenetic analysis of orf virus from goats in Taiwan
An outbreak of contagious ecthyma in goats in central Taiwan was investigated. The disease was diagnosed by physical and histopathologic examinations, and the etiology of the disease was identified as orf virus by electron microscopy and polymerase chain reaction (PCR) and sequence of major envelope protein (B2L) gene. The entire protein-coding region of B2L gene were cloned and sequenced. Phylogenetic analysis of B2L amino acid sequences showed that the orf virus identified in this outbreak was closer to the Indian ORFV-Mukteswar 59/05 isolate. This is the first report on the molecular characterization of orf virus in Taiwan
Photodissociation of small carbonaceous molecules of astrophysical interest
Astronomical observations have shown that small carbonaceous molecules can
persist in interstellar clouds exposed to intense ultraviolet radiation.
Current astrochemical models lack quantitative information on photodissociation
rates in order to interpret these data. We here present ab initio
multi-reference configuration-interaction calculations of the vertical
excitation energies, transition dipole moments and oscillator strengths for a
number of astrophysically relevant molecules: C3, C4, C2H, l- and c-C3H, l- and
c-C3H2, HC3H, l-C4H and l-C5H. Highly excited states up to the 9'th root of
each symmetry are computed, and several new states with large oscillator
strengths are found below the ionization potentials. These data are used to
calculate upper limits on photodissociation rates in the unattenuated
interstellar radiation field by assuming that all absorptions above the
dissociation limit lead to dissociation.Comment: Full tables, rates and cross sections are posted at
http://www.strw.leidenuniv.nl/~ewine/phot
Measurement of Cosmic-ray Muon-induced Spallation Neutrons in the Aberdeen Tunnel Underground Laboratory
AbstractMuon-induced neutrons are one of the major backgrounds to various underground experiments, such as dark matter searches, low-energy neutrino oscillation experiments and neutrino-less double beta-decay experiments. Previous experiments on the underground production rate of muon-induced neutrons were mostly carried out either at shallow sites or at very deep sites. The Aberdeen Tunnel experiment aims to measure the neutron production rate at a moderate depth of 611 meters water equivalent. Our apparatus comprises of six layers of plastic-scintillator hodoscopes for tracking the incident cosmic-ray muons, and 760 L of gadolinium-doped liquid-scintillator for both neutron production and detection targets. In this paper, we describe the design and the performance of the apparatus. The preliminary result on the measurement of neutron production rate is also presented
Active Galactic Nuclei at the Crossroads of Astrophysics
Over the last five decades, AGN studies have produced a number of spectacular
examples of synergies and multifaceted approaches in astrophysics. The field of
AGN research now spans the entire spectral range and covers more than twelve
orders of magnitude in the spatial and temporal domains. The next generation of
astrophysical facilities will open up new possibilities for AGN studies,
especially in the areas of high-resolution and high-fidelity imaging and
spectroscopy of nuclear regions in the X-ray, optical, and radio bands. These
studies will address in detail a number of critical issues in AGN research such
as processes in the immediate vicinity of supermassive black holes, physical
conditions of broad-line and narrow-line regions, formation and evolution of
accretion disks and relativistic outflows, and the connection between nuclear
activity and galaxy evolution.Comment: 16 pages, 5 figures; review contribution; "Exploring the Cosmic
Frontier: Astrophysical Instruments for the 21st Century", ESO Astrophysical
Symposia Serie
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