2,564 research outputs found
Structure and optical properties of high light output halide scintillators
Structural and optical properties of several high light output halide
scintillators and closely related materials are presented based on first
principles calculations. The optical properties are based on the Engel-Vosko
generalized gradient approximation and the recently developed density
functional of Tran and Blaha. The materials investigated are BaBr, BaIBr,
BaCl, BaF, BaI, BiI, CaI, Cs_6_2_5_2_5_2_5_2_5_2_5_3_3_2_3_4_4$, most of these halides are highly isotropic from an
optical point of view even though in many cases the crystal structures and
other properties are not. This general result is rationalized in terms of
halide chemistry. Implications for the development of ceramic halide
scintillators are discussed
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Rice marketing in Indonesia: Methodology, results and implications of a research study
There is a continuing trend in developing countries, as elsewhere, to devolve activities which have hitherto been performed by the public sector to the private sector, and to less regulated markets. The marketing of staple foods is no exception to this trend, but it raises important issues about national food security, price stability, and the protection of poor people from price rises which could mean malnutrition or starvation. Rice Marketing in Indonesia describes a research project which explores these issues in depth for a staple food commodity in a large developing country. The research focuses on seasonal rice price formation, on storage and sales decisions by farmers and traders, and on competition and efficiency in marketing channels. The research approach, which involves tracing marketing chains from sample farmers, is likely to be of interest to all those concerned with studying the liberalization of food markets in developing countries
On the complexity of posets
AbstractThe purpose of this paper is to discuss several invariants each of which provides a measure of the intuitive notion of complexity for a finite partially ordered set. For a poset X the invariants discussed include cardinality, width, length, breadth, dimension, weak dimension, interval dimension and semiorder dimension denoted respectively X, W(X), L(X), B(X), dim(X). Wdim(X), Idim(X) and Sdim(X). Among these invariants the following inequalities hold. B(X)⩽Idim(X)⩽Sdim(X)⩽Wdim(X)⩽dim(X)⩽W(X). We prove that every poset X with three of more points contains a partly with Idim(X) Idim(X) {x,v}). If M denotes the set of maximal elements and A an arbitrary anticham of X we show that Idim(X)⩽W(X-M) and Idim(X)⩽2W(X-A). We also show that there exist functions f(n,t) and (gt) such that I(X)⩽n and Idim(X)⩽tsimply dim(X)⩽f(n,t and Sdim(X)⩽t implies dim(X)⩽g(t)
Exploring Variation Between Artificial Grammar Learning Experiments: Outlining a Meta-Analysis Approach
Artificial grammar learning (AGL) has become an important tool used to understand aspects of human language learning and whether the abilities underlying learning may be unique to humans or found in other species. Successful learning is typically assumed when human or animal participants are able to distinguish stimuli generated by the grammar from those that are not at a level better than chance. However, the question remains as to what subjects actually learn in these experiments. Previous studies of AGL have frequently introduced multiple potential contributors to performance in the training and testing stimuli, but meta‐analysis techniques now enable us to consider these multiple information sources for their contribution to learning—enabling intended and unintended structures to be assessed simultaneously. We present a blueprint for meta‐analysis approaches to appraise the effect of learning in human and other animal studies for a series of artificial grammar learning experiments, focusing on studies that examine auditory and visual modalities. We identify a series of variables that differ across these studies, focusing on both structural and surface properties of the grammar, and characteristics of training and test regimes, and provide a first step in assessing the relative contribution of these design features of artificial grammars as well as species‐specific effects for learning
On the Sign Problem in the Hirsch-Fye Algorithm for Impurity Problems
We show that there is no fermion sign problem in the Hirsch and Fye algorithm
for the single-impurity Anderson model. Beyond the particle-hole symmetric case
for which a simple proof exists, this has been known only empirically. Here we
prove the nonexistence of a sign problem for the general case by showing that
each spin trace for a given Ising configuration is separately positive. We
further use this insight to analyze under what conditions orbitally degenerate
Anderson models or the two-impurity Anderson model develop a sign.Comment: 2 pages, no figure; published versio
A multipole-Taylor expansion for the potential of gravitational lens MG J0414+0534
We employ a multipole-Taylor expansion to investigate how tightly the
gravitational potential of the quadruple-image lens MG J0414+0534 is
constrained by recent VLBI observations. These observations revealed that each
of the four images of the background radio source contains four distinct
components, thereby providing more numerous and more precise constraints on the
lens potential than were previously available. We expand the two-dimensional
lens potential using multipoles for the angular coordinate and a modified
Taylor series for the radial coordinate. After discussing the physical
significance of each term, we compute models of MG J0414+0534 using only VLBI
positions as constraints. The best-fit model has both interior and exterior
quadrupole moments as well as exterior m=3 and m=4 multipole moments. The
deflector centroid in the models matches the optical galaxy position, and the
quadrupoles are aligned with the optical isophotes. The radial distribution of
mass could not be well constrained. We discuss the implications of these models
for the deflector mass distribution and for the predicted time delays between
lensed components.Comment: 44 pages, 5 figures, 11 tables, accepted for publication in Ap
On FO2 quantifier alternation over words
We show that each level of the quantifier alternation hierarchy within
FO^2[<] -- the 2-variable fragment of the first order logic of order on words
-- is a variety of languages. We then use the notion of condensed rankers, a
refinement of the rankers defined by Weis and Immerman, to produce a decidable
hierarchy of varieties which is interwoven with the quantifier alternation
hierarchy -- and conjecturally equal to it. It follows that the latter
hierarchy is decidable within one unit: given a formula alpha in FO^2[<], one
can effectively compute an integer m such that alpha is equivalent to a formula
with at most m+1 alternating blocks of quantifiers, but not to a formula with
only m-1 blocks. This is a much more precise result than what is known about
the quantifier alternation hierarchy within FO[<], where no decidability result
is known beyond the very first levels
Transport in the 3-dimensional Anderson model: an analysis of the dynamics on scales below the localization length
Single-particle transport in disordered potentials is investigated on scales
below the localization length. The dynamics on those scales is concretely
analyzed for the 3-dimensional Anderson model with Gaussian on-site disorder.
This analysis particularly includes the dependence of characteristic transport
quantities on the amount of disorder and the energy interval, e.g., the mean
free path which separates ballistic and diffusive transport regimes. For these
regimes mean velocities, respectively diffusion constants are quantitatively
given. By the use of the Boltzmann equation in the limit of weak disorder we
reveal the known energy-dependencies of transport quantities. By an application
of the time-convolutionless (TCL) projection operator technique in the limit of
strong disorder we find evidence for much less pronounced energy dependencies.
All our results are partially confirmed by the numerically exact solution of
the time-dependent Schroedinger equation or by approximative numerical
integrators. A comparison with other findings in the literature is additionally
provided.Comment: 23 pages, 10 figure
Limitations of Quantum Simulation Examined by Simulating a Pairing Hamiltonian using Nuclear Magnetic Resonance
Quantum simulation uses a well-known quantum system to predict the behavior
of another quantum system. Certain limitations in this technique arise,
however, when applied to specific problems, as we demonstrate with a
theoretical and experimental study of an algorithm to find the low-lying
spectrum of a Hamiltonian. While the number of elementary quantum gates does
scale polynomially with the size of the system, it increases inversely to the
desired error bound . Making such simulations robust to decoherence
using fault-tolerance constructs requires an additional factor of
gates. These constraints are illustrated by using a three qubit nuclear
magnetic resonance system to simulate a pairing Hamiltonian, following the
algorithm proposed by Wu, Byrd, and Lidar.Comment: 6 pages, 2 eps figure
Robust Chauvenet Outlier Rejection
Sigma clipping is commonly used in astronomy for outlier rejection, but the
number of standard deviations beyond which one should clip data from a sample
ultimately depends on the size of the sample. Chauvenet rejection is one of the
oldest, and simplest, ways to account for this, but, like sigma clipping,
depends on the sample's mean and standard deviation, neither of which are
robust quantities: Both are easily contaminated by the very outliers they are
being used to reject. Many, more robust measures of central tendency, and of
sample deviation, exist, but each has a tradeoff with precision. Here, we
demonstrate that outlier rejection can be both very robust and very precise if
decreasingly robust but increasingly precise techniques are applied in
sequence. To this end, we present a variation on Chauvenet rejection that we
call "robust" Chauvenet rejection (RCR), which uses three decreasingly
robust/increasingly precise measures of central tendency, and four decreasingly
robust/increasingly precise measures of sample deviation. We show this
sequential approach to be very effective for a wide variety of contaminant
types, even when a significant -- even dominant -- fraction of the sample is
contaminated, and especially when the contaminants are strong. Furthermore, we
have developed a bulk-rejection variant, to significantly decrease computing
times, and RCR can be applied both to weighted data, and when fitting
parameterized models to data. We present aperture photometry in a contaminated,
crowded field as an example. RCR may be used by anyone at
https://skynet.unc.edu/rcr, and source code is available there as well.Comment: 62 pages, 48 figures, 7 tables, accepted for publication in ApJ
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