645 research outputs found
Comparison of ion sites and diffusion paths in glasses obtained by molecular dynamics simulations and bond valence analysis
Based on molecular dynamics simulations of a lithium metasilicate glass we
study the potential of bond valence sum calculations to identify sites and
diffusion pathways of mobile Li ions in a glassy silicate network. We find that
the bond valence method is not well suitable to locate the sites, but allows
one to estimate the number of sites. Spatial regions of the glass determined as
accessible for the Li ions by the bond valence method can capture up to 90% of
the diffusion path. These regions however entail a significant fraction that
does not belong to the diffusion path. Because of this low specificity, care
must be taken to determine the diffusive motion of particles in amorphous
systems based on the bond valence method. The best identification of the
diffusion path is achieved by using a modified valence mismatch in the BV
analysis that takes into account that a Li ion favors equal partial valences to
the neighboring oxygen ions. Using this modified valence mismatch it is
possible to replace hard geometric constraints formerly applied in the BV
method. Further investigations are necessary to better understand the relation
between the complex structure of the host network and the ionic diffusion
paths.Comment: 16 pages, 10 figure
Statistically interacting vacancy particles
The equilibrium statistical mechanics of one-dimensional lattice gases with
interactions of arbitrary range and shape between first-neighbor atoms is
solved exactly on the basis of statistically interacting vacancy particles. Two
sets of vacancy particles are considered. In one set all vacancies are of
one-cell size. In the other set the sizes of vacancy particles match the
separation between atoms. Explicit expressions are obtained for the Gibbs free
energy and the distribution of spaces between atoms at thermal equilibrium.
Applications to various types of interaction potentials are discussed,
including long-range potentials that give rise to phase transitions. Extensions
to hard rod systems are straightforward and are shown to agree with existing
results for lattice models and their continuum limits.Comment: 10 pages, 5 figure
Providing Advice to Job Seekers at Low Cost:An Experimental Study on On-Line Advice
Helping job seekers to identify suitable jobs is a key challenge for policy makers. We develop
and evaluate experimentally a novel tool that provides tailored advice at low cost and thereby
redesigns the process through which job seekers search for jobs. We invited 300 job seekers to
our computer facilities for 12 consecutive weekly sessions. They searched for real jobs using our
web interface. After 3 weeks, we introduced a manipulation of the interface for half of the sample: instead of relying on their own search criteria, we displayed relevant other occupations to them and the jobs that were available in these occupations. These suggestions were based on background information and readily available labor market data. We recorded search behavior on our site but also surveyed participants every week on their other search activities, applications and job interviews. We find that these suggestions broaden the set of jobs considered by the average
participant. More importantly, we find that they are invited to significantly more job interviews.
These effects are predominantly driven by job seekers who searched relatively narrowly initially
and who have been unemployed for a few months.JEL: D83, J62, C9
Providing advice to jobseekers at low cost:An experimental study on online advice
Published: 05 October 2018We develop and evaluate experimentally a novel tool that redesigns the job search process by providing tailored advice at low cost. We invited job seekers to our computer facilities for twelve consecutive weekly sessions to search for real jobs on our web interface. For one-half, instead of relying on their own search criteria, we use readily available labour market data to display relevant alternative occupations and associated jobs. The data indicate that this broadens the set of jobs they consider and increases their job interviews especially for participants who otherwise search narrowly and have been unemployed for a few months
Analysis of aggregated tick returns: evidence for anomalous diffusion
In order to investigate the origin of large price fluctuations, we analyze
stock price changes of ten frequently traded NASDAQ stocks in the year 2002.
Though the influence of the trading frequency on the aggregate return in a
certain time interval is important, it cannot alone explain the heavy tailed
distribution of stock price changes. For this reason, we analyze intervals with
a fixed number of trades in order to eliminate the influence of the trading
frequency and investigate the relevance of other factors for the aggregate
return. We show that in tick time the price follows a discrete diffusion
process with a variable step width while the difference between the number of
steps in positive and negative direction in an interval is Gaussian
distributed. The step width is given by the return due to a single trade and is
long-term correlated in tick time. Hence, its mean value can well characterize
an interval of many trades and turns out to be an important determinant for
large aggregate returns. We also present a statistical model reproducing the
cumulative distribution of aggregate returns. For an accurate agreement with
the empirical distribution, we also take into account asymmetries of the step
widths in different directions together with crosscorrelations between these
asymmetries and the mean step width as well as the signs of the steps.Comment: 9 pages, 10 figures, typos correcte
Copolymerization Studies of Vinyl Chloride and Vinyl Acetate with Ethylene Using a Transition-Metal Catalyst
Since the advent of Ziegler−Natta polymerization of ethylene, attempts have been made to extend coordination polymerization to commercially important monomers with polar functionality. In this study we examined the copolymerization of perdeuterated vinyl chloride (VC) and perdeuterated vinyl acetate (VA) with ethylene using a tridentate Fe(II) dichloride pyridine diimine metal catalyst. The resulting ethylene oligomers were examined by GC/MS and ^2H NMR spectroscopy. It was shown that VC was inserted once for every ∼180 ethylene monomers and VA was inserted once for every ∼350 ethylene monomers. VC and VA behave as comonomers for coordination/insertion polymerizations with ethylene. However, we find that insertion with either monomer leads to termination of the growing chain via β-elimination processes. The deuterium atoms are exclusively located at the olefin terminus for each of the monomers
Control of Directed Cell Migration In Vivo by Membrane-to-Cortex Attachment
Analysis of cell migration in vivo combined with biophysical measurements reveals how membrane-to-cortex attachment fine-tunes the type of protrusions formed by cells and, as a consequence, controls directed migration during zebrafish gastrulation
Generative replay underlies compositional inference in the hippocampal-prefrontal circuit
Human reasoning depends on reusing pieces of information by putting them together in new ways. However, very little is known about how compositional computation is implemented in the brain. Here, we ask participants to solve a series of problems that each require constructing a whole from a set of elements. With fMRI, we find that representations of novel constructed objects in the frontal cortex and hippocampus are relational and compositional. With MEG, we find that replay assembles elements into compounds, with each replay sequence constituting a hypothesis about a possible configuration of elements. The content of sequences evolves as participants solve each puzzle, progressing from predictable to uncertain elements and gradually converging on the correct configuration. Together, these results suggest a computational bridge between apparently distinct functions of hippocampal-prefrontal circuitry and a role for generative replay in compositional inference and hypothesis testing
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