20,704 research outputs found
Understanding co-operative R&D activity: evidence from four European countries
This paper investigates co-operative research activity by firms using data from
the 3rd Community Innovation Survey for four countries, France, Germany, Spain and the
UK. We build on the Cassiman and Veugelers (CV) (2002) study of Belgian manufacturing
firms, by incorporating information on the service sector, and considering the role of public
support in affecting firms’ decisions to co-operate. Our results support those in CV, in that
we find a positive relationship between the likelihood of undertaking co-operative R&D
and both incoming knowledge spillovers and the extent to which firms find strategic
methods important in appropriating the returns to innovative activity. We find that public
support is positively related to the probability of undertaking co-operative agreements
particularly with regard to the likelihood of co-operation with the research base. We find
some evidence, in particular for Spain, that firms carry out co-operative R&D to overcome
excessive perceived risks and financial constraints
Universal behavior of optimal paths in weighted networks with general disorder
We study the statistics of the optimal path in both random and scale free
networks, where weights are taken from a general distribution . We
find that different types of disorder lead to the same universal behavior.
Specifically, we find that a single parameter ( for
-dimensional lattices, and for random networks)
determines the distributions of the optimal path length, including both strong
and weak disorder regimes. Here is the percolation connectivity exponent,
and depends on the percolation threshold and . For uniform,
Poisson or Gaussian the crossover from weak to strong does not occur, and only
weak disorder exists.Comment: Accepted by PR
Testing strong line metallicity diagnostics at z~2
High-z galaxy gas-phase metallicities are usually determined through
observations of strong optical emission lines with calibrations tied to the
local universe. Recent debate has questioned if these calibrations are valid in
the high-z universe. We investigate this by analysing a sample of 16 galaxies
at z~2 available in the literature, and for which the metallicity can be
robustly determined using oxygen auroral lines. The sample spans a redshift
range of 1.4 < z < 3.6, has metallicities of 7.4-8.4 in 12+log(O/H) and stellar
masses 10^7.5-10^11 Msun. We test commonly used strong line diagnostics (R23,
O3, O2, O32, N2, O3N2 and Ne3O2 ) as prescribed by four different sets of
empirical calibrations, as well as one fully theoretical calibration. We find
that none of the strong line diagnostics (or calibration set) tested perform
consistently better than the others. Amongst the line ratios tested, R23 and O3
deliver the best results, with accuracies as good as 0.01-0.04 dex and
dispersions of ~0.2 dex in two of the calibrations tested. Generally, line
ratios involving nitrogen predict higher values of metallicity, while results
with O32 and Ne3O2 show large dispersions. The theoretical calibration yields
an accuracy of 0.06 dex, comparable to the best strong line methods. We
conclude that, within the metallicity range tested in this work, the locally
calibrated diagnostics can still be reliably applied at z~2.Comment: 12 pages, 8 Figures, accepted for publication in MNRA
Magnetoasymmetric transport in a mesoscopic interferometer: From the weak to the strong coupling regime
The microreversibility principle implies that the conductance of a
two-terminal Aharonov-Bohm interferometer is an even function of the applied
magnetic flux. Away from linear response, however, this symmetry is not
fulfilled and the conductance phase of the interferometer when a quantum dot is
inserted in one of its arms can be a continuous function of the bias voltage.
Such magnetoasymmetries have been investigated in related mesoscopic systems
and arise as a consequence of the asymetric response of the internal potential
of the conductor out of equilibrium. Here we discuss magnetoasymmetries in
quantum-dot Aharonov-Bohm interferometers when strong electron-electron
interactions are taken into account beyond the mean-field approach. We find
that at very low temperatures the asymmetric element of the differential
conductance shows an abrupt change for voltages around the Fermi level. At
higher temperatures we recover a smooth variation of the magnetoasymmetry as a
function of the bias. We illustrate our results with the aid of the electron
occupation at the dot, demonstrating that its nonequilibrium component is an
asymmetric function of the flux even to lowest order in voltage. We also
calculate the magnetoasymmetry of the current-current correlations (the noise)
and find that it is given, to a good extent, by the magnetoasymmetry of the
weakly nonlinear conductance term. Therefore, both magnetoasymmetries (noise
and conductance) are related to each other via a higher-order
fluctuation-dissipation relation. This result appears to be true even in the
low temperature regime, where Kondo physics and many-body effects dominate the
transport properties.Comment: 17 pages, 9 figure
Critical behavior of self-assembled rigid rods on triangular and honeycomb lattices
Using Monte Carlo simulations and finite-size scaling analysis, the critical
behavior of self-assembled rigid rods on triangular and honeycomb lattices at
intermediate density has been studied. The system is composed of monomers with
two attractive (sticky) poles that, by decreasing temperature or increasing
density, polymerize reversibly into chains with three allowed directions and,
at the same time, undergo a continuous isotropic-nematic (IN) transition. The
determination of the critical exponents, along with the behavior of Binder
cumulants, indicate that the IN transition belongs to the q=1 Potts
universality class.Comment: 6 pages, 5 figure
Velocity quantization approach of the one-dimensional dissipative harmonic oscillator
Given a constant of motion for the one-dimensional harmonic oscillator with
linear dissipation in the velocity, the problem to get the Hamiltonian for this
system is pointed out, and the quantization up to second order in the
perturbation approach is used to determine the modification on the eigenvalues
when dissipation is taken into consideration. This quantization is realized
using the constant of motion instead of the Hamiltonian.Comment: 10 pages, 2 figure
Four dimensional Lie symmetry algebras and fourth order ordinary differential equations
Realizations of four dimensional Lie algebras as vector fields in the plane
are explicitly constructed. Fourth order ordinary differential equations which
admit such Lie symmetry algebras are derived. The route to their integration is
described.Comment: 12 page
Monomial integrals on the classical groups
This paper presents a powerfull method to integrate general monomials on the
classical groups with respect to their invariant (Haar) measure. The method has
first been applied to the orthogonal group in [J. Math. Phys. 43, 3342 (2002)],
and is here used to obtain similar integration formulas for the unitary and the
unitary symplectic group. The integration formulas turn out to be of similar
form. They are all recursive, where the recursion parameter is the number of
column (row) vectors from which the elements in the monomial are taken. This is
an important difference to other integration methods. The integration formulas
are easily implemented in a computer algebra environment, which allows to
obtain analytical expressions very efficiently. Those expressions contain the
matrix dimension as a free parameter.Comment: 16 page
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