14,460 research outputs found

### McStas and Mantid integration

McStas and Mantid are two well established software frameworks within the
neutron scattering community. McStas has been primarily used for simulating the
neutron transport of instruments, while Mantid has been primarily used for data
reduction. We report here the status of our work done on the interoperability
between the instrument simulation software McStas and the data reduction
software Mantid. This provides a demonstration of how to successfully link
together two software that otherwise have been developed independently, and in
particular here show how this has been achieved for an instrument simulation
software and a data reduction software. This paper will also provide examples
of some of the expected future enhanced analysis that can be achieved from
combining accurate instrument and sample simulations with software for
correcting raw data. In the case of this work for raw data collected at large
scale neutron facilities.Comment: 17 pages, 12 figures, POSTPRINT with proofs of article submitted to
Journal of Neutron Researc

### Coulomb blockade and Non-Fermi-liquid behavior in quantum dots

The non-Fermi-liquid properties of an ultrasmall quantum dot coupled to a
lead and to a quantum box are investigated. Tuning the ratio of the tunneling
amplitudes to the lead and box, we find a line of two-channel Kondo fixed
points for arbitrary Coulomb repulsion on the dot, governing the transition
between two distinct Fermi-liquid regimes. The Fermi liquids are characterized
by different values of the conductance. For an asymmetric dot, spin and charge
degrees of freedom are entangled: a continuous transition from a spin to a
charge two-channel Kondo effect evolves. The crossover temperature to the
two-channel Kondo effect is greatly enhanced away from the local-moment regime,
making this exotic effect accessible in realistic quantum-dot devices.Comment: 5 figure

### One-Dimensional Theory of the Quantum Hall System

We consider the lowest Landau level on a torus as a function of its
circumference $L_1$. When $L_1\to 0$, the ground state at general rational
filling fraction is a crystal with a gap--a Tao-Thouless state. For filling
fractions $\nu=p/(2pm+1)$, these states are the limits of Laughlin's or Jain's
wave functions describing the gapped quantum Hall states when $L_1\to \infty$.
For the half-filled Landau level, there is a transition to a Fermi sea of
non-interacting neutral dipoles, or rather to a Luttinger liquid modification
thereof, at $L_1\sim5$ magnetic lengths. This state is a version of the
Rezayi-Read state, and develops continuously into the state that is believed to
describe the observed metallic phase as $L_1\to \infty$. Furthermore, the
effective Landau level structure that emerges within the lowest Landau level
follows from the magnetic symmetries.Comment: 4 pages, 1 figur

### Half-Filled Lowest Landau Level on a Thin Torus

We solve a model that describes an interacting electron gas in the
half-filled lowest Landau level on a thin torus, with radius of the order of
the magnetic length. The low energy sector consists of non-interacting,
one-dimensional, neutral fermions. The ground state, which is homogeneous, is
the Fermi sea obtained by filling the negative energy states and the excited
states are gapless neutral excitations out of this one-dimensional sea.
Although the limit considered is extreme, the solution has a striking
resemblance to the composite fermion description of the bulk $\nu=1/2$
state--the ground state is homogeneous and the excitations are neutral and
gapless. This suggests a one-dimensional Luttinger liquid description, with
possible observable effects in transport experiments, of the bulk state where
it develops continuously from the state on a thin torus as the radius
increases.Comment: 4 pages, 1 figur

### Constant net-time headway as key mechanism behind pedestrian flow dynamics

We show that keeping a constant lower limit on the net-time headway is the
key mechanism behind the dynamics of pedestrian streams. There is a large
variety in flow and speed as functions of density for empirical data of
pedestrian streams, obtained from studies in different countries. The net-time
headway however, stays approximately constant over all these different data
sets. By using this fact, we demonstrate how the underlying dynamics of
pedestrian crowds, naturally follows from local interactions. This means that
there is no need to come up with an arbitrary fit function (with arbitrary fit
parameters) as has traditionally been done. Further, by using not only the
average density values, but the variance as well, we show how the recently
reported stop-and-go waves [Helbing et al., Physical Review E, 75, 046109]
emerge when local density variations take values exceeding a certain maximum
global (average) density, which makes pedestrians stop.Comment: 7 pages, 7 figure

### Multiadaptive Galerkin Methods for ODEs III: A Priori Error Estimates

The multiadaptive continuous/discontinuous Galerkin methods mcG(q) and mdG(q)
for the numerical solution of initial value problems for ordinary differential
equations are based on piecewise polynomial approximation of degree q on
partitions in time with time steps which may vary for different components of
the computed solution. In this paper, we prove general order a priori error
estimates for the mcG(q) and mdG(q) methods. To prove the error estimates, we
represent the error in terms of a discrete dual solution and the residual of an
interpolant of the exact solution. The estimates then follow from interpolation
estimates, together with stability estimates for the discrete dual solution

### Ground state phases of the Half-Filled One-Dimensional Extended Hubbard Model

Using quantum Monte Carlo simulations, results of a strong-coupling
expansion, and Luttinger liquid theory, we determine quantitatively the ground
state phase diagram of the one-dimensional extended Hubbard model with on-site
and nearest-neighbor repulsions U and V. We show that spin frustration
stabilizes a bond-ordered (dimerized) state for U appr. V/2 up to U/t appr. 9,
where t is the nearest-neighbor hopping. The transition from the dimerized
state to the staggered charge-density-wave state for large V/U is continuous
for U up to appr. 5.5 and first-order for higher U.Comment: 4 pages, 4 figure

### Dynamical mean field solution of the Bose-Hubbard model

We present the effective action and self-consistency equations for the
bosonic dynamical mean field (B-DMFT) approximation to the bosonic Hubbard
model and show that it provides remarkably accurate phase diagrams and
correlation functions. To solve the bosonic dynamical mean field equations we
use a continuous-time Monte Carlo method for bosonic impurity models based on a
diagrammatic expansion in the hybridization and condensate coupling. This
method is readily generalized to bosonic mixtures, spinful bosons, and
Bose-Fermi mixtures.Comment: 10 pages, 3 figures. includes supplementary materia

### X-ray Halos and Large Grains in the Diffuse Interstellar Medium

Recent observations with dust detectors on board the interplanetary
spacecraft Ulysses and Galileo have recorded a substantial flux of large
interstellar grains with radii between 0.25 and 2.0 mu entering the solar
system from the local interstellar cloud. The most commonly used interstellar
grain size distribution is characterized by a a^-3.5 power law in grain radii
a, and extends to a maximum grain radius of 0.25 mu. The extension of the
interstellar grain size distribution to such large radii will have a major
effect on the median grain size, and on the amount of mass needed to be tied up
in dust for a given visual optical depth. It is therefore important to
investigate whether this population of larger dust particles prevails in the
general interstellar medium, or if it is merely a local phenomenon. The
presence of large interstellar grains can be mainly inferred from their effect
on the intensity and radial profiles of scattering halos around X-ray sources.
In this paper we examine the grain size distribution that gives rise to the
X-ray halo around Nova Cygni 1992. The results of our study confirm the need to
extend the interstellar grain size distribution in the direction of this source
to and possibly beyond 2.0 mu. The model that gives the best fit to the halo
data is characterized by: (1) a grain size distribution that follows an a^-3.5
power law up to 0.50 mu, followed by an a^-4.0 extension from 0.50 mu to 2.0
mu; and (2) silicate and graphite (carbon) dust-to-gas mass ratios of 0.0044
and 0.0022, respectively, consistent with solar abundances constraints.
Additional observations of X-ray halos probing other spatial directions are
badly needed to test the general validity of this result.Comment: 17 pages, incl. 1 figure, accepted for publ. by ApJ Letter

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