1,087 research outputs found
Assessing the potential of social media for estimating recreational use of urban and peri-urban forests
Acknowledgements The research for this paper was financially supported through the Swiss Federal Office for the Environment (FOEN). The views and opinions expressed in this paper are those of the authors, and do not necessarily represent the policies or official positions of the FOEN or the institutions they work for. We thank Rahul Deb Das for his assistance in data collection and processing. We gratefully acknowledge the comments and feedback of two anonymous reviewers.Peer reviewedPublisher PD
Quantum critical point in the spin glass-antiferromagnetism competition for fermionic Ising Models
The competition between spin glass () and antiferromagnetic order ()
is analyzed in two sublattice fermionic Ising models in the presence of a
transverse and a parallel magnetic fields. The exchange
interaction follows a Gaussian probability distribution with mean and
standard deviation , but only spins in different sublattices can
interact. The problem is formulated in a path integral formalism, where the
spin operators have been expressed as bilinear combinations of Grassmann
fields. The results of two fermionic models are compared. In the first one, the
diagonal operator has four states, where two eigenvalues vanish (4S
model), which are suppressed by a restriction in the two states 2S model. The
replica symmetry ansatz and the static approximation have been used to obtain
the free energy. The results are showing in phase diagrams ( is the
temperature) {\it versus} , , and . When is
increased, (transition temperature to a nonergodic phase) reduces and
the Neel temperature decreases towards a quantum critical point. The field
always destroys ; however, within a certain range, it favors the
frustration. Therefore, the presence of both fields, and , produces
effects that are in competition. The critical temperatures are lower for the 4S
model and it is less sensitive to the magnetic couplings than the 2S model.Comment: 15 pages, 6 figures, accepted in Physica
Computation of protein geometry and its applications: Packing and function prediction
This chapter discusses geometric models of biomolecules and geometric
constructs, including the union of ball model, the weigthed Voronoi diagram,
the weighted Delaunay triangulation, and the alpha shapes. These geometric
constructs enable fast and analytical computaton of shapes of biomoleculres
(including features such as voids and pockets) and metric properties (such as
area and volume). The algorithms of Delaunay triangulation, computation of
voids and pockets, as well volume/area computation are also described. In
addition, applications in packing analysis of protein structures and protein
function prediction are also discussed.Comment: 32 pages, 9 figure
Risico-, versterkende en beschermende factoren voor crimineel gedrag
This research report, commissioned by the municipality of Rotterdam, summarizes the current scientific knowledge about the most important factors that increase or decrease the probability to become involved in criminal behaviour. Based on information from existing meta-analyses, systematic reviews and overview studies, we identify which risk-, promotive and protective factors have been found in robust empirical studies, and how strong they are associated with four different types of criminal behaviour: general offending, juvenile delinquency crime, cybercrime and involvement in organized crime. We distinguish factors connected to five domains: personal characteristics, the family, peers and groups, school/work and contextual/neighborhood factors
Stability conditions for fermionic Ising spin-glass models in the presence of a transverse field
The stability of spin-glass (SG) phase is analyzed in detail for a fermionic
Ising SG (FISG) model in the presence of a magnetic transverse field .
The fermionic path integral formalism, replica method and static approach have
been used to obtain the thermodynamic potential within one step replica
symmetry breaking ansatz. The replica symmetry (RS) results show that the SG
phase is always unstable against the replicon. Moreover, the two other
eigenvalues of the Hessian matrix (related to the diagonal
elements of the replica matrix) can indicate an additional instability to the
SG phase, which enhances when is increased. Therefore, this result
suggests that the study of the replicon can not be enough to guarantee the RS
stability in the present quantum FISG model, especially near the quantum
critical point. In particular, the FISG model allows changing the occupation
number of sites, so one can get a first order transition when the chemical
potential exceeds a certain value. In this region, the replicon and the
indicate instability problems for the SG solution close to all
range of first order boundary.Comment: 15 pages, 4 figures, accepted in Physica
The Longitudinal Polarimeter at HERA
The design, construction and operation of a Compton back-scattering laser
polarimeter at the HERA storage ring at DESY are described. The device measures
the longitudinal polarization of the electron beam between the spin rotators at
the HERMES experiment with a fractional systematic uncertainty of 1.6%. A
measurement of the beam polarization to an absolute statistical precision of
0.01 requires typically one minute when the device is operated in the
multi-photon mode. The polarimeter also measures the polarization of each
individual electron bunch to an absolute statistical precision of 0.06 in
approximately five minutes. It was found that colliding and non-colliding
bunches can have substantially different polarizations. This information is
important to the collider experiments H1 and ZEUS for their future
longitudinally polarized electron program because those experiments use the
colliding bunches only.Comment: 21 pages (Latex), 14 figures (EPS
Multidimensional rasch models for partial credit scoring
Rasch models for partial-credit scoring are discussed
and a multidimensional version of the model is formulated.
A model may be specified in which consecutive
item responses depend on an underlying latent trait. In
the multidimensional partial-credit model, different responses
may be explained by different latent traits. Data
from van Kuyk’s (1988) size concept test and the Raven
Progressive Matrices test were analyzed. Maximum
likelihood estimation and goodness-of-fit testing are discussed
and applied to these datasets. Goodness-of-fit
statistics show that for both tests, multidimensional partial-credit models were more appropriate than the unidimensional
partial-credit model. Index terms: X2 testing,
exponential family model, multidimensional item response
theory, multidimensional Rasch model, partial-credit
models, Progressive Matrices test, Rasch model
A 750 mW, continuous-wave, solid-state laser source at 313 nm for cooling and manipulating trapped 9Be+ ions
We present a solid-state laser system that generates 750 mW of
continuous-wave single-frequency output at 313 nm. Sum-frequency generation
with fiber lasers at 1550 nm and 1051 nm produces up to 2 W at 626 nm. This
visible light is then converted to UV by cavity-enhanced second-harmonic
generation. The laser output can be tuned over a 495 GHz range, which includes
the 9Be+ laser cooling and repumping transitions. This is the first report of a
narrow-linewidth laser system with sufficient power to perform fault-tolerant
quantum-gate operations with trapped 9Be+ ions by use of stimulated Raman
transitions.Comment: 9 pages, 4 figure
Combined CI+MBPT calculations of energy levels and transition amplitudes in Be, Mg, Ca, and Sr
Configuration interaction (CI) calculations in atoms with two valence
electrons, carried out in the V(N-2) Hartree-Fock potential of the core, are
corrected for core-valence interactions using many-body perturbation theory
(MBPT). Two variants of the mixed CI+MBPT theory are described and applied to
obtain energy levels and transition amplitudes for Be, Mg, Ca, and Sr
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