897 research outputs found
Resummation of Large Logarithms in
In the collinear factorization of the form factor for the transition
the hard part contains double log terms as with as the momentum fraction of partons from 0 to 1. A simple
exponentiation for resummation leads to divergent results. We study the
resummation of these terms. We show that the terms come
partly from the light-cone wave function(LCWF) and partly from the form factor.
We introduce a jet factor to factorize the term in the form factor.
To handel the terms from the LCWF we introduce a nonstandard
light-cone wave function(NLCWF) with the gauge links off the light-cone
direction. An interesting relation between two wave function is found. With the
introduced NLCWF and the jet factor we can re-factorize the form factor and
obtain a new hard part which does not contain terms with . Beside the
renormalization scale the introduce NLCWF and jet factor have extra
scales to characterize their -behaviors. Using the evolutions of the extra
scales and the relation we can do the resummation perturbatively in sense that
the LCWF is the only nonpertubative object in the resumed formula. Our results
with some models of LCWF show that there is a significant difference between
numerical predictions with the resummation and that without the resummation,
and the resummed predictions can describe the experimental data.Comment: one reference adde
Identifying topological edge states in 2D optical lattices using light scattering
We recently proposed in a Letter [Physical Review Letters 108 255303] a novel
scheme to detect topological edge states in an optical lattice, based on a
generalization of Bragg spectroscopy. The scope of the present article is to
provide a more detailed and pedagogical description of the system - the
Hofstadter optical lattice - and probing method. We first show the existence of
topological edge states, in an ultra-cold gas trapped in a 2D optical lattice
and subjected to a synthetic magnetic field. The remarkable robustness of the
edge states is verified for a variety of external confining potentials. Then,
we describe a specific laser probe, made from two lasers in Laguerre-Gaussian
modes, which captures unambiguous signatures of these edge states. In
particular, the resulting Bragg spectra provide the dispersion relation of the
edge states, establishing their chiral nature. In order to make the Bragg
signal experimentally detectable, we introduce a "shelving method", which
simultaneously transfers angular momentum and changes the internal atomic
state. This scheme allows to directly visualize the selected edge states on a
dark background, offering an instructive view on topological insulating phases,
not accessible in solid-state experiments.Comment: 17 pages, 10 figures. Revised and extended version, to appear in EJP
Special Topic for the special issue on "Novel Quantum Phases and Mesoscopic
Physics in Quantum Gases". Extended version of arXiv:1203.124
Gauss-Bonnet Black Holes in dS Spaces
We study the thermodynamic properties associated with black hole horizon and
cosmological horizon for the Gauss-Bonnet solution in de Sitter space. When the
Gauss-Bonnet coefficient is positive, a locally stable small black hole appears
in the case of spacetime dimension , the stable small black hole
disappears and the Gauss-Bonnet black hole is always unstable quantum
mechanically when . On the other hand, the cosmological horizon is
found always locally stable independent of the spacetime dimension. But the
solution is not globally preferred, instead the pure de Sitter space is
globally preferred. When the Gauss-Bonnet coefficient is negative, there is a
constraint on the value of the coefficient, beyond which the gravity theory is
not well defined. As a result, there is not only an upper bound on the size of
black hole horizon radius at which the black hole horizon and cosmological
horizon coincide with each other, but also a lower bound depending on the
Gauss-Bonnet coefficient and spacetime dimension. Within the physical phase
space, the black hole horizon is always thermodynamically unstable and the
cosmological horizon is always stable, further, as the case of the positive
coefficient, the pure de Sitter space is still globally preferred. This result
is consistent with the argument that the pure de Sitter space corresponds to an
UV fixed point of dual field theory.Comment: Rextex, 17 pages including 8 eps figures, v2: minor changes, to
appear in PRD, v3: references adde
Electronic structure in underdoped cuprates due to the emergence of a pseudogap
The phenomenological Green's function developed in the works of Yang, Rice
and Zhang has been very successful in understanding many of the anomalous
superconducting properties of the deeply underdoped cuprates. It is based on
considerations of the resonating valence bond spin liquid approximation and is
designed to describe the underdoped regime of the cuprates. Here we emphasize
the region of doping, , just below the quantum critical point at which the
pseudogap develops. In addition to Luttinger hole pockets centered around the
nodal direction, there are electron pockets near the antinodes which are
connected to the hole pockets by gapped bridging contours. We determine the
contours of nearest approach as would be measured in angular resolved
photoemission experiments and emphasize signatures of the Fermi surface
reconstruction from the large Fermi contour of Fermi liquid theory (which
contains hole states) to the Luttinger pocket (which contains hole
states). We find that the quasiparticle effective mass renormalization
increases strongly towards the edge of the Luttinger pockets beyond which it
diverges.Comment: 11 pages, 9 figure
Magnetic enhancement of CoZnFeO spinel oxide by mechanical milling
We report the magnetic properties of mechanically milled
CoZnFeO spinel oxide. After 24 hours milling of the
bulk sample, the XRD spectra show nanostructure with average particle size
20 nm. The as milled sample shows an enhancement in magnetization and
ordering temperature compared to the bulk sample. If the as milled sample is
annealed at different temperatures for the same duration, recrystallization
process occurs and approaches to the bulk structure on increasing the annealing
temperatures. The magnetization of the annealed samples first increases and
then decreases. At higher annealing temperature ( 1000C) the system
shows two coexisting magnetic phases {\it i.e.}, spin glass state and
ferrimagnetic state, similar to the as prepared bulk sample. The room
temperature M\"{o}ssbauer spectra of the as milled sample, annealed at
300C for different durations (upto 575 hours), suggest that the observed
change in magnetic behaviour is strongly related with cations redistribution
between tetrahedral (A) and octahedral (O) sites in the spinel structure. Apart
from the cation redistribution, we suggest that the enhancement of
magnetization and ordering temperature is related with the reduction of B site
spin canting and increase of strain induced anisotropic energy during
mechanical milling.Comment: 14 pages LaTeX, 10 ps figure
Confront Holographic QCD with Regge Trajectories of vectors and axial-vectors
We derive the general 5-dimension metric structure of the system in
type II superstring theory, and demonstrate the physical meaning of the
parameters characterizing the 5-dimension metric structure of the
\textit{holographic} QCD model by relating them to the parameters describing
Regge trajectories. By matching the spectra of vector mesons with
deformed soft-wall model, we find that the spectra of vector mesons
can be described very well in the soft-wall model, i.e,
soft-wall model. We then investigate how well the soft-wall
model can describe the Regge trajectory of axial-vector mesons . We find
that the constant component of the 5-dimension mass square of axial-vector
mesons plays an efficient role to realize the chiral symmetry breaking in the
vacuum, and a small negative correction in the 5-dimension mass square is
helpful to realize the chiral symmetry restoration in high excitation states.Comment: 9 pages, 3 figure and 3 tables, one section adde
Scaling Behavior of Anomalous Hall Effect and Longitudinal Nonlinear Response in High-Tc Superconductors
Based on existing theoretical model and by considering our longitudinal
nonlinear response function, we derive a nonliear equation in which the mixed
state Hall resistivity can be expressed as an analytical function of magnetic
field, temperature and applied current. This equation enables one to compare
quantitatively the experimental data with theoretical model. We also find some
new scaling relations of the temperature and field dependency of Hall
resistivity. The comparison between our theoretical curves and experimental
data shows a fair agreement.Comment: 4 pages, 3 figure
Low-temperature SCR of NO with NH 3 over noble metal promoted Fe-ZSM-5 catalysts
We have reported previously the excellent performance of Fe-exchanged ZSM-5 for selective catalytic reduction (SCR) of NO with ammonia at high temperatures (300–400 °C). In this work, we found that the reaction temperature could be decreased to 200–300 °C when a small amount of noble metal (Pt, Rh, or Pd) was added to the Fe-ZSM-5. The SCR activity follows the order Pt/Fe-ZSM-5 > Rh/Fe-ZSM-5 > Pd/Fe-ZSM-5 at 250 °C. On the Pt promoted Fe-ZSM-5, 90% NO conversion was obtained at 250 °C at GHSV = 1.1 × 10 5 h −1 . Moreover, the noble metal improved the resistance to H 2 O and SO 2 . The presence of H 2 O and SO 2 decreased the SCR performance only very slightly.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44250/1/10562_2004_Article_3462.pd
Star and Planet Formation with ALMA: an Overview
Submillimeter observations with ALMA will be the essential next step in our
understanding of how stars and planets form. Key projects range from detailed
imaging of the collapse of pre-stellar cores and measuring the accretion rate
of matter onto deeply embedded protostars, to unravelling the chemistry and
dynamics of high-mass star-forming clusters and high-spatial resolution studies
of protoplanetary disks down to the 1 AU scale.Comment: Invited review, 8 pages, 5 figures; to appear in the proceedings of
"Science with ALMA: a New Era for Astrophysics". Astrophysics & Space
Science, in pres
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