34,803 research outputs found
Studying Intermediate pT Hadron Production with Fluctuations
Mechanisms for particle production at intermediate pT in nuclear collisions
at RHIC are discussed, emphasizing the differences in associated jet-like
correlations between color-neutral and colored production. An alternative
production mechanism involving both recombination and fragmentation is
suggested, which might simultaneously lead to an enhancement of baryons and to
jet-like correlations. To gain more insight into the relative importance of
different mechanisms a study of constrained distributions of associated
multiplicity is proposed. In a simple model it is shown that these multiplicity
distributions may change significantly, if the nature of the production
mechanism fluctuates from event to event.Comment: 7 pages, 4 figures, talk at Hot Quarks 2004 conferenc
Classical and Quantum Plasmonics in Graphene Nanodisks: the Role of Edge States
Edge states are ubiquitous for many condensed matter systems with
multicomponent wave functions. For example, edge states play a crucial role in
transport in zigzag graphene nanoribbons. Here, we report microscopic
calculations of quantum plasmonics in doped graphene nanodisks with zigzag
edges. We express the nanodisk conductivity as a sum of the
conventional bulk conductivity ,
and a novel term , corresponding
to a coupling between the edge and bulk states. We show that the edge states
give rise to a red-shift and broadening of the plasmon resonance, and that they
often significantly impact the absorption efficiency. We further develop
simplified models, incorporating nonlocal response within a hydrodynamical
approach, which allow a semiquantitative description of plasmonics in the
ultrasmall size regime. However, the polarization dependence is only given by
fully microscopic models. The approach developed here should have many
applications in other systems supporting edge states.Comment: 5 pages, 4 figure
Plasmonic eigenmodes in individual and bow-tie graphene nanotriangles
Serving as a new two-dimensional plasmonic material, graphene has stimulated
an intensive study of its optical properties which benefit from the unique
electronic band structure of the underlying honeycomb lattice of carbon atoms.
In classical electrodynamics, nanostructured graphene is commonly modeled by
the computationally demanding problem of a three-dimensional conducting film of
atomic-scale thickness. Here, we propose an efficient alternative
two-dimensional electrostatic approach where all the calculation procedures are
restricted to the plane of the graphene sheet. To explore possible quantum
effects, we perform tight-binding calculations, adopting a random-phase
approximation. We investigate the multiple plasmon modes in triangles of
graphene, treating the optical response classically as well as quantum
mechanically in the case of both armchair and zigzag edge termination of the
underlying atomic lattice. Compared to the classical plasmonic spectrum which
is "blind" to the edge termination, we find that the quantum plasmon
frequencies exhibit blueshifts in the case of armchair edge termination, while
redshifts are found for zigzag edges. Furthermore, we find spectral features in
the zigzag case which are associated with electronic edge states not present
for armchair termination. Merging pairs of such triangles into dimers, the
plasmon hybridization leads to energy splitting in accordance with
plasmon-hybridization theory, with a lower energy for the antisymmetric modes
and a smaller splitting for modes with less confinement to the gap region. The
hybridization appears strongest in classical calculations while the splitting
is lower for armchair edges and even more reduced for zigzag edges. Our various
results illustrate a surprising phenomenon: Even 20 nm large graphene
structures clearly exhibit quantum plasmonic features due to atomic-scale
details in the edge termination.Comment: 27 pages including 7 figures. Supplementary information available
upon request to author
Using the Discrete Dipole Approximation and Holographic Microscopy to Measure Rotational Dynamics of Non-spherical Colloidal Particles
We present a new, high-speed technique to track the three-dimensional
translation and rotation of non-spherical colloidal particles. We capture
digital holograms of micrometer-scale silica rods and sub-micrometer-scale
Janus particles freely diffusing in water, and then fit numerical scattering
models based on the discrete dipole approximation to the measured holograms.
This inverse-scattering approach allows us to extract the the position and
orientation of the particles as a function of time, along with static
parameters including the size, shape, and refractive index. The best-fit sizes
and refractive indices of both particles agree well with expected values. The
technique is able to track the center of mass of the rod to a precision of 35
nm and its orientation to a precision of 1.5, comparable to or better
than the precision of other 3D diffusion measurements on non-spherical
particles. Furthermore, the measured translational and rotational diffusion
coefficients for the silica rods agree with hydrodynamic predictions for a
spherocylinder to within 0.3%. We also show that although the Janus particles
have only weak optical asymmetry, the technique can track their 2D translation
and azimuthal rotation over a depth of field of several micrometers, yielding
independent measurements of the effective hydrodynamic radius that agree to
within 0.2%. The internal and external consistency of these measurements
validate the technique. Because the discrete dipole approximation can model
scattering from arbitrarily shaped particles, our technique could be used in a
range of applications, including particle tracking, microrheology, and
fundamental studies of colloidal self-assembly or microbial motion.Comment: 11 pages, 9 figures, 2 table
Neutron stars and strange stars in the chiral SU(3) quark mean field model
We investigate the equations of state for pure neutron matter and strange
hadronic matter in -equilibrium, including , and
hyperons. The masses and radii of pure neutron stars and strange hadronic stars
are obtained. For a pure neutron star, the maximum mass is about , while for a strange hadronic star, the maximum mass is
around . The typical radii of pure neutron stars and
strange hadronic stars are about 11.0-12.3 km and 10.7-11.7 km, respectively.Comment: 18 pages, 7 figure
Frustrated spin order and stripe fluctuations in FeSe
The charge and spin dynamics of the structurally simplest iron-based
superconductor, FeSe, may hold the key to understanding the physics of high
temperature superconductors in general. Unlike the iron pnictides, FeSe lacks
long range magnetic order in spite of a similar structural transition around
90\,K. Here, we report results of Raman scattering experiments as a function of
temperature and polarization and simulations based on exact diagonalization of
a frustrated spin model. Both experiment and theory find a persistent low
energy peak close to 500cm in symmetry, which softens slightly
around 100\,K, that we assign to spin excitations. By comparing with results
from neutron scattering, this study provides evidence for nearly frustrated
stripe order in FeSe.Comment: 12 pages, 12 figure
Sulfonated sporopollenin as an efficient and recyclable heterogeneous catalyst for dehydration of D-xylose and xylan into furfural
The natural acidity of sporopollenin, the biopolymer coating the outer walls of pollen grains, was enhanced by the sulfonation of its surface. Modified sporopollenin displaying sulfonic acid groups has been prepared, characterized by elemental analysis, SEM, EDX, FTIR and XPS and tested as a heterogeneous catalyst in the dehydration of D-xylose and xylan to produce furfural. The optimal reaction conditions involve 10 wt % of sulfonated sporopollenin in the presence of 1.5 mmol of NaCl in a biphasic water-CPME system. When heated at 190 °C, the reaction affords furfural in a yield of 69% after 40 min under microwave irradiation. The time dependence of the dehydration and influence of temperature, pentose loading and positive effect of chloride ions on the reaction rate are reported. It was found that the catalytic system, recharged with the pentose and solvent, could be recycled ten times without loss of performance. The transformation of xylan into furfural at 190 °C for 50 min gave furfural in a yield of 37%
System thermal-hydraulic modelling of the phénix dissymmetric test benchmark
Phénix is a French pool-type sodium-cooled prototype reactor; before the definitive shutdown, occurred in 2009, a final set of experimental tests are carried out in order to increase the knowledge on the operation and the safety aspect of the pool-type liquid metal-cooled reactors. One of the experiments was the Dissymmetric End-of-Life Test which was selected for the validation benchmark activity in the frame of SESAME project. The computer code validation plays a key role in the safety assessment of the innovative nuclear reactors and the Phénix dissymmetric test provides useful experimental data to verify the computer codes capability in the asymmetric thermal-hydraulic behaviour into a pool-type liquid metal-cooled reactor. This paper shows the comparison of the outcomes obtained with six different System Thermal-Hydraulic (STH) codes: RELAP5-3D©, SPECTRA, ATHLET, SAS4A/SASSYS-1, ASTEC-Na and CATHARE. The nodalization scheme of the reactor was individually achieved by the participants; during the development of the thermal-hydraulic model, the pool nodalization methodology had a special attention in order to investigate the capability of the STH codes to reproduce the dissymmetric effects which occur in each loop and into pools, caused by the azimuthal asymmetry of the boundary conditions. The modelling methodology of the participants is discussed and the main results are compared in this paper to obtain useful guide lines for the future modelling of innovative liquid metal pool-type reactors
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