21,169 research outputs found
Quantum Monte Carlo calculations of neutron-alpha scattering
We describe a new method to treat low-energy scattering problems in
few-nucleon systems, and we apply it to the five-body case of neutron-alpha
scattering. The method allows precise calculations of low-lying resonances and
their widths. We find that a good three-nucleon interaction is crucial to
obtain an accurate description of neutron-alpha scattering.Comment: 4 pages, 2 figures, submitted to Physical Review Letter
Rate dependent shear bands in a shear transformation zone model of amorphous solids
We use Shear Transformation Zone (STZ) theory to develop a deformation map
for amorphous solids as a function of the imposed shear rate and initial
material preparation. The STZ formulation incorporates recent simulation
results [Haxton and Liu, PRL 99 195701 (2007)] showing that the steady state
effective temperature is rate dependent. The resulting model predicts a wide
range of deformation behavior as a function of the initial conditions,
including homogeneous deformation, broad shear bands, extremely thin shear
bands, and the onset of material failure. In particular, the STZ model predicts
homogeneous deformation for shorter quench times and lower strain rates, and
inhomogeneous deformation for longer quench times and higher strain rates. The
location of the transition between homogeneous and inhomogeneous flow on the
deformation map is determined in part by the steady state effective
temperature, which is likely material dependent. This model also suggests that
material failure occurs due to a runaway feedback between shear heating and the
local disorder, and provides an explanation for the thickness of shear bands
near the onset of material failure. We find that this model, which resolves
dynamics within a sheared material interface, predicts that the stress weakens
with strain much more rapidly than a similar model which uses a single state
variable to specify internal dynamics on the interface.Comment: 10 pages, 13 figures, corrected typos, added section on rate
strengthening vs. rate weakening material
Incompatibility of modulated checkerboard patterns with the neutron scattering resonance peak in cuprate superconductors
Checkerboard patterns have been proposed in order to explain STM experiments
on the cuprates BSCCO and Na-CCOC. However the presence of these patterns has
not been confirmed by a bulk probe such as neutron scattering. In particular,
simple checkerboard patterns are inconsistent with neutron scattering data, in
that they have low energy incommsensurate (IC) spin peaks rotated 45 degrees
from the direction of the charge IC peaks. However, it is unclear whether other
checkerboard patterns can solve the problem. In this paper, we have studied
more complicated checkerboard patterns ("modulated checkerboards") by using
spin wave theory and analyzed noncollinear checkerboards as well. We find that
the high energy response of the modulated checkerboards is inconsistent with
neutron scattering results, since they fail to exhibit a resonance peak at
(pi,pi), which has recently been shown to be a universal feature of cuprate
superconductors. We further argue that the newly proposed noncollinear
checkerboard also lacks a resonance peak. We thus conclude that to date no
checkerboard pattern has been proposed which satisfies both the low energy
constraints and the high energy constraints imposed by the current body of
experimental data in cuprate superconductors.Comment: 5 pages, 5 figures, Fig.2 update
A pure S-wave covariant model for the nucleon
Using the manifestly covariant spectator theory, and modeling the nucleon as
a system of three constituent quarks with their own electromagnetic structure,
we show that all four nucleon electromagnetic form factors can be very well
described by a manifestly covariant nucleon wave function with zero orbital
angular momentum. Since the concept of wave function depends on the formalism,
the conclusions of light-cone theory requiring nonzero angular momentum
components are not inconsistent with our results. We also show that our model
gives a qualitatively correct description of deep inelastic scattering,
unifying the phenomenology at high and low momentum transfer. Finally we review
two different definitions of nuclear shape and show that the nucleon is
spherical in this model, regardless of how shape is defined.Comment: 20 pages and 10 figures; greatly expanded version with new fits and
discussion of DIS; similar to published versio
Proton structure corrections to hyperfine splitting in muonic hydrogen
We present the derivation of the formulas for the proton structure-dependent
terms in the hyperfine splitting of muonic hydrogen. We use compatible
conventions throughout the calculations to derive a consistent set of formulas
that reconcile differences between our results and some specific terms in
earlier work. Convention conversion corrections are explicitly presented, which
reduce the calculated hyperfine splitting by about 46 ppm. We also note that
using only modern fits to the proton elastic form factors gives a smaller than
historical spread of Zemach radii and leads to a reduced uncertainty in the
hyperfine splitting. Additionally, hyperfine splittings have an impact on the
muonic hydrogen Lamb shift/proton radius measurement, however the correction we
advocate has a small effect there.Comment: 6 pages, 3 figure
Effective Widths and Effective Number of Phonons of Multiphonon Giant Resonances
We discuss the origin of the difference between the harmonic value of the
width of the multiphonon giant resonances and the smaller observed value.
Analytical expressions are derived for both the effective width and the average
cross-section. The contribution of the Brink-Axel mechanism in resolving the
discrepancy is pointed out.Comment: 9 pages, 4 figure
The Breakdown of Kinetic Theory in Granular Shear Flows
We examine two basic assumptions of kinetic theory-- binary collisions and
molecular chaos-- using numerical simulations of sheared granular materials. We
investigate a wide range of densities and restitution coefficients and
demonstrate that kinetic theory breaks down at large density and small
restitution coefficients. In the regimes where kinetic theory fails, there is
an associated emergence of clusters of spatially correlated grains
Statistical multifragmentation model with discretized energy and the generalized Fermi breakup. I. Formulation of the model
The Generalized Fermi Breakup recently demonstrated to be formally equivalent
to the Statistical Multifragmentation Model, if the contribution of excited
states are included in the state densities of the former, is implemented. Since
this treatment requires the application of the Statistical Multifragmentation
Model repeatedly on the hot fragments until they have decayed to their ground
states, it becomes extremely computational demanding, making its application to
the systems of interest extremely difficult. Based on exact recursion formulae
previously developed by Chase and Mekjian to calculate the statistical weights
very efficiently, we present an implementation which is efficient enough to
allow it to be applied to large systems at high excitation energies. Comparison
with the GEMINI++ sequential decay code shows that the predictions obtained
with our treatment are fairly similar to those obtained with this more
traditional model.Comment: 8 pages, 6 figure
Weak phase separation and the pseudogap in the electron-doped cuprates
We study the quantum transition from an antiferromagnet to a superconductor
in a model for electron- and hole-doped cuprates by means of a variational
cluster perturbation theory approach. In both cases, our results suggest a
tendency towards phase separation between a mixed
antiferromagnetic-superconducting phase at low doping and a pure
superconducting phase at larger doping. However, in the electron-doped case the
energy scale for phase separation is an order of magnitude smaller than for
hole doping. We argue that this can explain the different pseudogap and
superconducting transition scales in hole- and electron-doped materials.Comment: Final version, accepted for publication in Europhysics Letter
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