19,949 research outputs found
Industrial Employment Growth in Spanish Regions - the Role Played by Size, Innovation, and Spatial Aspects
The article analyses the role played by size and innovation in the growth of industrial employment in Spain during the period 1998-2002. The analysis is conducted using two different approaches: first of all, it studies the relationship, for seventeen Spanish regions, between industrial employment growth and the share of small firms and the presence of high and medium-high technological industries, following Camagni and Capello methodology. Secondly, it uses microdata, at firm level, in order to estimate the significance of size and innovation in the employment growth of those firms, also including regional variables for better explain the changes in employment. The results at regional level for the Spanish case show that there is no a relationship between industrial employment growth and the share of small firms and technological development of the regions. Similar results are obtained estimating a Heckman model using microdata. Independently of the definition of employment used, Gibrat´s law is accepted, showing that size is not a significant variable in order to explain the growth of firms. On the contrary, innovation, specially process innovation, plays an important role in the survival and growth behaviour. The regional variables are not significant, exception made of the share of industry in total employment of regions: the bigger the industrial sector, the higher the growth of employment at firm level.
Dynamical phase transition in vibrational surface modes
We consider the dynamical properties of a simple model of vibrational surface
modes. We obtain the exact spectrum of surface excitations and discuss their
dynamical features. In addition to the usually discussed localized and
oscillatory regimes we also find a second phase transition where surface mode
frequency becomes purely imaginary and describes an overdamped regime.
Noticeably, this transition has an exact correspondence to the oscillatory -
overdamped transition of the standard oscillator with a frictional force
proportional to velocity.Comment: 4 pages, 3 figures. To appear in Braz. J. Phy
Pair Partitioning in time reversal acoustics
Time reversal of acoustic waves can be achieved efficiently by the persistent
control of excitations in a finite region of the system. The procedure, called
Time Reversal Mirror, is stable against the inhomogeneities of the medium and
it has numerous applications in medical physics, oceanography and
communications. As a first step in the study of this robustness, we apply the
Perfect Inverse Filter procedure that accounts for the memory effects of the
system. In the numerical evaluation of such procedures we developed the Pair
Partitioning method for a system of coupled oscillators. The algorithm,
inspired in the Trotter strategy for quantum dynamics, obtains the dynamic for
a chain of coupled harmonic oscillators by the separation of the system in
pairs and applying a stroboscopic sequence that alternates the evolution of
each pair. We analyze here the formal basis of the method and discuss his
extension for including energy dissipation inside the medium.Comment: 6 pages, 4 figure
Antiferromagnetic spin chain behavior and a transition to 3D magnetic order in Cu(D,L-alanine)2: Roles of H-bonds
We study the spin chain behavior, a transition to 3D magnetic order and the
magnitudes of the exchange interactions for the metal-amino acid complex
Cu(D,L-alanine)2.H2O, a model compound to investigate exchange couplings
supported by chemical paths characteristic of biomolecules. Thermal and
magnetic data were obtained as a function of temperature (T) and magnetic field
(B0). The magnetic contribution to the specific heat, measured between 0.48 and
30 K, displays above 1.8 K a 1D spin-chain behavior that can be fitted with an
intrachain antiferromagnetic (AFM) exchange coupling constant 2J0 = (-2.12
0.08) cm, between neighbor coppers at 4.49 {\AA} along chains
connected by non-covalent and H-bonds. We also observe a narrow specific heat
peak at 0.89 K indicating a phase transition to a 3D magnetically ordered
phase. Magnetization curves at fixed T = 2, 4 and 7 K with B0 between 0 and 9
T, and at T between 2 and 300 K with several fixed values of B0 were globally
fitted by an intrachain AFM exchange coupling constant 2J0 = (-2.27 0.02)
cm and g = 2.091 0.005. Interchain interactions J1 between coppers
in neighbor chains connected through long chemical paths with total length of
9.51 {\AA} are estimated within the range 0.1 < |2J1| < 0.4 cm, covering
the predictions of various approximations. We analyze the magnitudes of 2J0 and
2J1 in terms of the structure of the corresponding chemical paths. The main
contribution in supporting the intrachain interaction is assigned to H-bonds
while the interchain interactions are supported by paths containing H-bonds and
carboxylate bridges, with the role of the H-bonds being predominant. We compare
the obtained intrachain coupling with studies of compounds showing similar
behavior and discuss the validity of the approximations allowing to calculate
the interchain interactions.Comment: 10 pages, 4 figure
Effects of constraints in general branched molecules: A quantitative ab initio study in HCO-L-Ala-NH2
A general approach to the design of accurate classical potentials for protein
folding is described. It includes the introduction of a meaningful statistical
measure of the differences between approximations of the same potential energy,
the definition of a set of Systematic and Approximately Separable and Modular
Internal Coordinates (SASMIC), much convenient for the simulation of general
branched molecules, and the imposition of constraints on the most rapidly
oscillating degrees of freedom. All these tools are used to study the effects
of constraints in the Conformational Equilibrium Distribution (CED) of the
model dipeptide HCO-L-Ala-NH2. We use ab initio Quantum Mechanics calculations
including electron correlation at the MP2 level to describe the system, and we
measure the conformational dependence of the correcting terms to the naive CED
based in the Potential Energy Surface (PES) without any simplifying assumption.
These terms are related to mass-metric tensors determinants and also occur in
the Fixman's compensating potential. We show that some of the corrections are
non-negligible if one is interested in the whole Ramachandran space. On the
other hand, if only the energetically lower region, containing the principal
secondary structure elements, is assumed to be relevant, then, all correcting
terms may be neglected up to peptides of considerable length. This is the first
time, as far as we know, that the analysis of the conformational dependence of
these correcting terms is performed in a relevant biomolecule with a realistic
potential energy function.Comment: 8 pages, 1 figure, LaTeX, aipproc style (included
Exponentially fitted fifth-order two-step peer explicit methods
The so called peer methods for the numerical solution of Initial Value Problems (IVP) in ordinary differential systems were introduced by R. Weiner et al [6, 7, 11, 12, 13] for solving different types of problems either in sequential or parallel computers. In this work, we study exponentially fitted three-stage peer schemes that are able to fit functional spaces with dimension six. Finally, some numerical experiments are presented to show the behaviour of the new peer schemes for some periodic problems
A Statistical Study of Photospheric Magnetic Field Changes During 75 Solar Flares
Abrupt and permanent changes of photospheric magnetic fields have been
observed during solar flares. The changes seem to be linked to the
reconfiguration of magnetic fields, but their origin is still unclear. We
carried out a statistical analysis of permanent line-of-sight magnetic field
() changes during 18 X-, 37 M-, 19 C- and 1 B-class flares using
data from Solar Dynamics Observatory/Helioseismic and Magnetic Imager. We
investigated the properties of permanent changes, such as frequency, areas, and
locations. We detected changes of in 59/75 flares. We find that
strong flares are more likely to show changes, with all flares M1.6
exhibiting them. For weaker flares, permanent changes are observed in 6/17
C-flares. 34.3\% of the permanent changes occurred in the penumbra and 18.9\%
in the umbra. Parts of the penumbra appeared or disappeared in 23/75 flares.
The area where permanent changes occur is larger for stronger flares. Strong
flares also show a larger change of flux, but there is no dependence of the
magnetic flux change on the heliocentric angle. The mean rate of change of
flare-related magnetic field changes is 20.7 Mx cm min. The
number of permanent changes decays exponentially with distance from the
polarity inversion line. The frequency of the strength of permanent changes
decreases exponentially, and permanent changes up to 750 Mx cm were
observed. We conclude that permanent magnetic field changes are a common
phenomenon during flares, and future studies will clarify their relation to
accelerated electrons, white light emission, and sunquakes to further
investigate their origin.Comment: Piblished in Ap
Enhancing single-parameter quantum charge pumping in carbon-based devices
We present a theoretical study of quantum charge pumping with a single ac
gate applied to graphene nanoribbons and carbon nanotubes operating with low
resistance contacts. By combining Floquet theory with Green's function
formalism, we show that the pumped current can be tuned and enhanced by up to
two orders of magnitude by an appropriate choice of device length, gate voltage
intensity and driving frequency and amplitude. These results offer a promising
alternative for enhancing the pumped currents in these carbon-based devices.Comment: 3.5 pages, 2 figure
Interaction-induced charge and spin pumping through a quantum dot at finite bias
We investigate charge and spin transport through an adiabatically driven,
strongly interacting quantum dot weakly coupled to two metallic contacts with
finite bias voltage. Within a kinetic equation approach, we identify
coefficients of response to the time-dependent external driving and relate
these to the concepts of charge and spin emissivities previously discussed
within the time-dependent scattering matrix approach. Expressed in terms of
auxiliary vector fields, the response coefficients allow for a straightforward
analysis of recently predicted interaction-induced pumping under periodic
modulation of the gate and bias voltage [Phys. Rev. Lett. 104, 226803 (2010)].
We perform a detailed study of this effect and the related adiabatic Coulomb
blockade spectroscopy, and, in particular, extend it to spin pumping. Analytic
formulas for the pumped charge and spin in the regimes of small and large
driving amplitude are provided for arbitrary bias. In the absence of a magnetic
field, we obtain a striking, simple relation between the pumped charge at zero
bias and at bias equal to the Coulomb charging energy. At finite magnetic
field, there is a possibility to have interaction-induced pure spin pumping at
this finite bias value, and generally, additional features appear in the pumped
charge. For large-amplitude adiabatic driving, the magnitude of both the pumped
charge and spin at the various resonances saturate at values which are
independent of the specific shape of the pumping cycle. Each of these values
provide an independent, quantitative measurement of the junction asymmetry.Comment: 17 pages, 8 figure
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