9,646 research outputs found
X-ray powder diffraction of high-absorption materials at the XRD1 beamline off the best conditions: Application to (Gd,Nd)5Si4 compounds
Representative compounds of the new family of magnetic materials Gd5-xNdxSi4
were analyzed by X-ray diffraction at the XRD1 beamline at LNLS. To reduce
X-ray absorption, thin layers of the powder samples were mounted outside the
capillaries and measured in Debye-Scherrer geometry as usual. The X-ray
diffraction analyses and the magnetometry results indicate that the behavior of
the magnetic transition temperature as a function of Nd content may be directly
related to the average of the four smallest interatomic distances between
different rare earth sites of the majority phase of each compound. The quality
and consistency of the results show that the XRD1 beamline is able to perform
satisfactory X-ray diffraction experiments on high-absorption materials even
off the best conditions.Comment: 12 pages, 3 figures, 3 table
Coupling and higher-order effects in the 12C(d,p)13C and 13C(p,d)12C reactions
Coupled channels calculations are performed for the 12C(d,p)13C and
13C(p,d)12C reactions between 7 and 60 MeV to study the effect of inelastic
couplings in transfer reactions. The effect of treating transfer beyond Born
approximation is also addressed. The coupling to the 12C 2+ state is found to
change the peak cross-section by up to 15 %. Effects beyond Born approximation
lead to a significant renormalization of the cross-sections, between 5 and 10 %
for deuteron energies above 10 MeV, and larger than 10 % for lower energies. We
also performed calculations including the remnant term in the transfer
operator, which has a small impact on the 12C(d,p)13C(g.s.) and
13C(p,d)12C(g.s.) reactions. Above 30 MeV deuteron energy, the effect of the
remnant term is larger than 10 % for the 12C(d,p)13C(3.09 MeV) reaction and is
found to increase with decreasing neutron separation energy for the 3.09 MeV
state of 13C. This is of importance for transfer reactions with weakly bound
nuclei.Comment: 7 pages, 7 figures, submitted to Phys. Rev.
Stability of the Period-Doubled Core of the 90-degree Partial in Silicon
In a recent Letter [N. Lehto and S. Oberg, Phys. Rev. Lett. 80, 5568 (1998)],
Lehto and Oberg investigated the effects of strain fields on the core structure
of the 90-degree partial dislocation in silicon, especially the influence of
the choice of supercell periodic boundary conditions in theoretical
simulations. We show that their results for the relative stability between the
two structures are in disagreement with cell-size converged tight-binding total
energy (TBTE) calculations, which suggest the DP core to be more stable,
regardless of the choice of boundary condition. Moreover, we argue that this
disagreement is due to their use of a Keating potential.Comment: 1 page. Submitted to Comments section of PRL. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/rn_dcom/index.htm
Stochastic oscillations in models of epidemics on a network of cities
We carry out an analytic investigation of stochastic oscillations in a
susceptible-infected-recovered model of disease spread on a network of
cities. In the model a fraction of individuals from city commute
to city , where they may infect, or be infected by, others. Starting from a
continuous time Markov description of the model the deterministic equations,
which are valid in the limit when the population of each city is infinite, are
recovered. The stochastic fluctuations about the fixed point of these equations
are derived by use of the van Kampen system-size expansion. The fixed point
structure of the deterministic equations is remarkably simple: a unique
non-trivial fixed point always exists and has the feature that the fraction of
susceptible, infected and recovered individuals is the same for each city
irrespective of its size. We find that the stochastic fluctuations have an
analogously simple dynamics: all oscillations have a single frequency, equal to
that found in the one city case. We interpret this phenomenon in terms of the
properties of the spectrum of the matrix of the linear approximation of the
deterministic equations at the fixed point.Comment: 13 pages, 7 figure
Modeling the long term dynamics of pre-vaccination pertussis
The dynamics of strongly immunizing childhood infections is still not well
understood. Although reports of successful modeling of several incidence data
records can be found in the literature, the key determinants of the observed
temporal patterns have not been clearly identified. In particular, different
models of immunity waning and degree of protection applied to disease and
vaccine induced immunity have been debated in the literature on pertussis. Here
we study the effect of disease acquired immunity on the long term patterns of
pertussis prevalence. We compare five minimal models, all of which are
stochastic, seasonally forced, well-mixed models of infection based on
susceptible-infective-recovered dynamics in a closed population. These models
reflect different assumptions about the immune response of naive hosts, namely
total permanent immunity, immunity waning, immunity waning together with
immunity boosting, reinfection of recovered, and repeat infection after partial
immunity waning. The power spectra of the output prevalence time series
characterize the long term dynamics of the models. For epidemiological
parameters consistent with published data for pertussis, the power spectra show
quantitative and even qualitative differences that can be used to test their
assumptions by comparison with ensembles of several decades long
pre-vaccination data records. We illustrate this strategy on two publicly
available historical data sets.Comment: paper (31 pages, 11 figures, 1 table) and supplementary material (19
pages, 5 figures, 2 tables
Firm Performance and Modes of Innovation
It has been shown that firms tend to develop different modes of innovation, based on the relationship between the different types of underlying knowledge and learning processes. This paper seeks to identify different modes of business innovation, relating them to firms’ innovative and economic performance, and to analyze the relationship between the different innovation modes and the economic impact of the crisis on firms’ performance. These hypotheses are tested by regression and latent class models for the Portuguese population of firms, using a sample of 397 firms, classified according to technological intensity, firm size and region. Our results show three different modes of innovation in terms of the relationship between economic and innovative performance, with significant differences in terms of resilience in the face of the economic crisis. These findings lead to a reflection on innovation, competitiveness and regional policy.FC
Making sense of the sharing economy: a category formation approach
The sharing economy (SE) has drawn significant attention from several society stakeholders in the last five years. While business actors are interested in financial opportunities to meet consumer needs, new business models, academia and governmental organisations are concerned with potential unintended effects on society and the environment. Despite its notable global growth, there is still a lack of more solid ground in understanding its origins and respective mechanisms through which it has been evolving as a category. This research addresses the problematics of the origins and ascendency of the SE by examining the process by which it is arising as a new category, searching for conceptual clarification, and pinpointing the legitimacy granted by stakeholders. Our guiding research questions are: how the SE was formed and evolved as a category, and as a category, is the SE legitimate? Additionally, we attempt to identify the nature of the SE as a category. Making a historical analysis of the expression SE and its equivalents, this paper deepens the discussion about the SE’s nature by providing evidence that it has predominantly been formed by emergence processes, comprising social movement, similarity clustering, and truce components, which render the SE a particular case of category formation and allow communication, entrepreneurship, regulation, and research about what it is. Moreover, the findings reveal a generalised legitimacy granted to the SE by a vast number of stakeholders, although still lacking the consolidation of socio-political legitimation. The SE’s nature seems to fall into a metaphorical approach, notably, the notion of radial categories.info:eu-repo/semantics/publishedVersio
Temperature effects on dislocation core energies in silicon and germanium
Temperature effects on the energetics of the 90-degree partial dislocation in
silicon and germanium are investigated, using non-equilibrium methods to
estimate free energies, coupled with Monte Carlo simulations. Atomic
interactions are described by Tersoff and EDIP interatomic potentials. Our
results indicate that the vibrational entropy has the effect of increasing the
difference in free energy between the two possible reconstructions of the
90-degree partial, namely, the single-period and the double-period geometries.
This effect further increases the energetic stability of the double-period
reconstruction at high temperatures. The results also indicate that anharmonic
effects may play an important role in determining the structural properties of
these defects in the high-temperature regime.Comment: 8 pages in two-column physical-review format with six figure
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