434 research outputs found
Modularity revisited: A novel dynamics-based concept for decomposing complex networks
Finding modules (or clusters) in large, complex networks is a challenging task, in particular if one is not interested in a full decomposition of the whole network into modules. We consider modular networks that also contain nodes that do not belong to one of modules but to several or to none at all. A new method for analyzing such networks is presented. It is based on spectral analysis of random walks on modular networks. In contrast to other spectral clustering approaches, we use different transition rules of the random walk. This leads to much more prominent gaps in the spectrum of the adapted random walk and allows for easy identification of the network's modular structure, and also identifying the nodes belonging to these modules. We also give a characterization of that set of nodes that do not belong to any module, which we call transition region. Finally, by analyzing the transition region, we describe an algorithm that identifies so called hub-nodes inside the transition region that are important connections between modules or between a module and the rest of the network. The resulting algorithms scale linearly with network size (if the network connectivity is sparse) and thus can also be applied to very large networks
Inferring Proteolytic Processes from Mass Spectrometry Time Series Data Using Degradation Graphs
Background: Proteases play an essential part in a variety of biological
processes. Besides their importance under healthy conditions they are also
known to have a crucial role in complex diseases like cancer. In recent years,
it has been shown that not only the fragments produced by proteases but also
their dynamics, especially ex vivo, can serve as biomarkers. But so far, only
a few approaches were taken to explicitly model the dynamics of proteolysis in
the context of mass spectrometry. Results: We introduce a new concept to model
proteolytic processes, the degradation graph. The degradation graph is an
extension of the cleavage graph, a data structure to reconstruct and visualize
the proteolytic process. In contrast to previous approaches we extended the
model to incorporate endoproteolytic processes and present a method to
construct a degradation graph from mass spectrometry time series data. Based
on a degradation graph and the intensities extracted from the mass spectra it
is possible to estimate reaction rates of the underlying processes. We further
suggest a score to rate different degradation graphs in their ability to
explain the observed data. This score is used in an iterative heuristic to
improve the structure of the initially constructed degradation graph.
Conclusion: We show that the proposed method is able to recover all degraded
and generated peptides, the underlying reactions, and the reaction rates of
proteolytic processes based on mass spectrometry time series data. We use
simulated and real data to demonstrate that a given process can be
reconstructed even in the presence of extensive noise, isobaric signals and
false identifications. While the model is currently only validated on peptide
data it is also applicable to proteins, as long as the necessary time series
data can be produced
Covalency effects on the magnetism of EuRh2P2
In experiments, the ternary Eu pnictide EuRh2P2 shows an unusual coexistence
of a non-integral Eu valence of about 2.2 and a rather high Neel temperature of
50 K. In this paper, we present a model which explains the non-integral Eu
valence via covalent bonding of the Eu 4f-orbitals to P2 molecular orbitals. In
contrast to intermediate valence models where the hybridization with
delocalized conduction band electrons is known to suppress magnetic ordering
temperatures to at most a few Kelvin, covalent hybridization to the localized
P2 orbitals avoids this suppression. Using perturbation theory we calculate the
valence, the high temperature susceptibility, the Eu single-ion anisotropy and
the superexchange couplings of nearest and next-nearest neighbouring Eu ions.
The model predicts a tetragonal anisotropy of the Curie constants. We suggest
an experimental investigation of this anisotropy using single crystals. From
experimental values of the valence and the two Curie constants, the three free
parameters of our model can be determined.Comment: 9 pages, 5 figures, submitted to J. Phys.: Condens. Matte
Compact intense extreme-ultraviolet source
High-intensity laser pulses covering the ultraviolet to terahertz spectral regions are nowadays routinely generated in a large number of laboratories. In contrast, intense extreme-ultraviolet (XUV) pulses have only been demonstrated using a small number of sources including free-electron laser facilities and long high-harmonic generation (HHG) beamlines. Here, we demonstrate a concept for a compact intense XUV source based on HHG that is focused to an intensity of 2Ă—1014W/cm2, with a potential increase up to 1017W/cm2 in the future. Our approach uses tight focusing of the near-infrared (NIR) driving laser and minimizes the XUV virtual source size by generating harmonics several Rayleigh lengths away from the NIR focus. Accordingly, the XUV pulses can be refocused to a small beam waist radius of 600 nm, enabling the absorption of up to four XUV photons by a single Ar atom in a setup that fits on a modest (2 m) laser table. Our concept represents a straightforward approach for the generation of intense XUV pulses in many laboratories, providing exciting opportunities for XUV strong-field and nonlinear optics experiments, for XUV-pump XUV-probe spectroscopy and for the coherent diffractive imaging of nanoscale structures
Low cost microfluidic device for partial cell separation: micromilling approach
Several studies have already demonstrated that it
is possible to perform blood flow studies in microfluidic systems
fabricated by using low-cost techniques. However, most of these
techniques do not produce microchannels smaller than 100
microns and as a result they have several limitations related to
blood cell separation. Recently, manufacturers have been able to
produce milling tools smaller than 100 microns, which
consequently have promoted the ability of micromilling machines
to fabricate microfluidic devices able to perform separation of
red blood cells (RBCs) from plasma. In this work, we show the
ability of a micromilling machine to manufacture microchannels
with dimensions down to 30 microns. Additionally, we show for
the first time the ability of the proposed microfluidic device to
enhance the cell-free layer close to the walls, leading to perform
partial separation of RBCs from plasma.The authors acknowledge the financial support provided by
PTDC/SAU-ENB/116929/2010 and EXPL/EMSSIS/2215/2013
from FCT (Science and Technology
Foundation), COMPETE, QREN and European Union
(FEDER). RR and DP acknowledge, respectively, the PhD
scholarships SFRH/BD/97658/2013 and
SFRH/BD/89077/2012 attributed by FCT.info:eu-repo/semantics/publishedVersio
A schematic model for QCD at finite temperature
The simplest version of a class of toy models for QCD is presented. It is a
Lipkin-type model, for the quark-antiquark sector, and, for the gluon sector,
gluon pairs with spin zero are treated as elementary bosons. The model
restricts to mesons with spin zero and to few baryonic states. The
corresponding energy spectrum is discussed. We show that ground state
correlations are essential to describe physical properties of the spectrum at
low energies. Phase transitions are described in an effective manner, by using
coherent states. The appearance of a Goldstone boson for large values of the
interaction strength is discussed, as related to a collective state. The
formalism is extended to consider finite temperatures. The partition function
is calculated, in an approximate way, showing the convenience of the use of
coherent states. The energy density, heat capacity and transitions from the
hadronic phase to the quark-gluon plasma are calculated.Comment: 33 pages, 11 figure
Single- and double-beta decay Fermi-transitions in an exactly solvable model
An exactly solvable model suitable for the description of single and
double-beta decay processes of the Fermi-type is introduced. The model is
equivalent to the exact shell-model treatment of protons and neutrons in a
single j-shell. Exact eigenvalues and eigenvectors are compared to those
corresponding to the hamiltonian in the quasiparticle basis (qp) and with the
results of both the standard quasiparticle random phase approximation (QRPA)
and the renormalized one (RQRPA). The role of the scattering term of the
quasiparticle hamiltonian is analyzed. The presence of an exact eigenstate with
zero energy is shown to be related to the collapse of the QRPA. The RQRPA and
the qp solutions do not include this zero-energy eigenvalue in their spectra,
probably due to spurious correlations. The meaning of this result in terms of
symmetries is presented.Comment: 29 pages, 9 figures included in a Postsript file. Submitted to
Physcal Review
Fermion-Boson Interactions and Quantum Algebras
Quantum Algebras (q-algebras) are used to describe interactions between
fermions and bosons. Particularly, the concept of a su_q(2) dynamical symmetry
is invoked in order to reproduce the ground state properties of systems of
fermions and bosons interacting via schematic forces. The structure of the
proposed su_q(2) Hamiltonians, and the meaning of the corresponding deformation
parameters, are discussed.Comment: 20 pages, 10 figures. Physical Review C (in press
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