22,461 research outputs found
Grazing-angle scattering of electromagnetic waves in gratings with varying mean parameters: grating eigenmodes
A highly unusual pattern of strong multiple resonances for bulk
electromagnetic waves is predicted and analysed numerically in thick periodic
holographic gratings in a slab with the mean permittivity that is larger than
that of the surrounding media. This pattern is shown to exist in the geometry
of grazing-angle scattering (GAS), that is when the scattered wave (+1
diffracted order) in the slab propagates almost parallel to the slab (grating)
boundaries. The predicted resonances are demonstrated to be unrelated to
resonant generation of the conventional guided modes of the slab. Their
physical explanation is associated with resonant generation of a completely new
type of eigenmodes in a thick slab with a periodic grating. These new slab
eigenmodes are generically related to the grating; they do not exist if the
grating amplitude is zero. The field structure of these eigenmodes and their
dependence on structural and wave parameters is analysed. The results are
extended to the case of GAS of guided modes in a slab with a periodic groove
array of small corrugation amplitude and small variations in the mean thickness
of the slab at the array boundaries.Comment: 16 pages, 6 figure
Construction of localized wave functions for a disordered optical lattice and analysis of the resulting Hubbard model parameters
We propose a method to construct localized single particle wave functions
using imaginary time projection and thereby determine lattice Hamiltonian
parameters. We apply the method to a specific disordered potential generated by
an optical lattice experiment and calculate for each instance of disorder, the
equivalent lattice model parameters. The probability distributions of the
Hubbard parameters are then determined. Tests of localization and eigen-energy
convergence are examined.Comment: 10 pages, 16 figure
Political and Media Discourses about Integrating Refugees in the UK
The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.This article addresses political and media discourses about integrating refugees in the UK in the context of the “refugee crisis”. A discursive psychological approach is presented as the best way to understand what talk about the concept is used to accomplish in these debates. A large corpus of political discussions (13 hours of debate featuring 146 politicians) and 960 newspaper articles from the UK were discourse analysed. The analysis identified five dilemmas about integration: Integration is positive and necessary, but challenging; Host communities are presented as welcoming, but there are limits to their capacity; Refugees are responsible for integration, but host communities need to provide support; Good refugees integrate, bad ones don't; Refugees are vulnerable and are skilled. All are used to warrant the inclusion or exclusion of refugees. The responsibility of western nations to support refugees is therefore contingent on the refugees behaving in specific ways
Model-independent measurements of the sodium magneto-optical trap's excited-state population
We present model-independent measurements of the excited-state population of
atoms in a sodium (Na) magneto-optical trap (MOT) using a hybrid ion-neutral
trap composed of a MOT and a linear Paul trap (LPT). We photoionize excited Na
atoms trapped in the MOT and use two independent methods to measure the
resulting ions: directly by trapping them in our LPT, and indirectly by
monitoring changes in MOT fluorescence. By measuring the ionization rate via
these two independent methods, we have enough information to directly determine
the population of MOT atoms in the excited-state. The resulting measurement
reveals that there is a range of trapping-laser intensities where the
excited-state population of atoms in our MOT follows the standard two-level
model intensity-dependence. However, an experimentally determined effective
saturation intensity must be used instead of the theoretically predicted value
from the two-level model. We measured the effective saturation intensity to be
for the type-I Na MOT and
for the type-II Na MOT,
approximately 1.7 and 3.6 times the theoretical estimate, respectively. Lastly,
at large trapping-laser intensities, our experiment reveals a clear departure
from the two-level model at a critical intensity that we believe is due to a
state-mixing effect, whose critical intensity can be determined by a simple
power broadening model.Comment: 10 pages, 8 figure
Two-Component Fokker-Planck Models for the Evolution of Isolated Globular Clusters
Two-component (normal and degenerate stars) models are the simplest
realization of clusters with a mass spectrum because high mass stars evolve
quickly into degenerates, while low mass stars remain on the main-sequence for
the age of the universe. Here we examine the evolution of isolated globular
clusters using two-component Fokker-Planck (FP) models that include heating by
binaries formed in tidal capture and in three-body encounters. Three-body
binary heating dominates and the postcollapse expansion is self-similar, at
least in models with total mass M <= 3 x 10^5 M_\odot, initial half-mass radius
r_{h,i} >= 5 pc, component mass ratio m_2/m_1 <= 2, and number ratio N_1/N_2 <=
300 when m_2=1.4 M_\odot. We derive scaling laws for \rho_c, v_c, r_c, and r_h
as functions of m_1/m_2, N, M, and time t from simple energy-balance arguments,
and these agree well with the FP simulations. We have studied the conditions
under which gravothermal oscillations (GTOs) occur. If E_{tot} and E_c are the
energies of the cluster and of the core, respectively, and t_{rh} and t_c are
their relaxation times, then \epsilon \equiv (E_{tot}/t_{rh})/(E_c/t_{rc}) is a
good predictor of GTOs: all models with \epsilon>0.01 are stable, and all but
one with \epsilon < 0.01 oscillate. We derive a scaling law for \epsilon
against N and m_1/m_2 and compared with our numerical results. Clusters with
larger m_2/m_1 or smaller N are stabler.Comment: 15 pages (LaTeX) with 8 figures. To appear in ApJ March 10, 1998
issu
Study of muons near shower cores at sea level using the E594 neutrino detector
The E594 neutrino detector has been used to study the lateral distribution of muons of energy 3 GeV near shower cores. The detector consists of a 340 ton fine grain calorimeter with 400,000 cells of flash chamber and dimensions of 3.7 m x 20 m x 3.7 m (height). The average density in the calorimeter is 1.4 gm/sq cm, and the average Z is 21. The detector was triggered by four 0.6 sq m scintillators placed immediately on the top of the calorimeter. The trigger required at least two of these four counters. The accompanying extensive air showers (EAS) was sampled by 14 scintillation counters located up to 15 m from the calorimeter. Several off line cuts have been applied to the data. Demanding five particles in at least two of the trigger detectors, a total of 20 particles in all of them together, and an arrival angle for the shower 450 deg reduced the data sample to 11053 events. Of these in 4869 cases, a computer algorithm found at least three muons in the calorimeter
- …