17,941 research outputs found
The geostationary orbit and developing countries
The geostationary orbit is becoming congested due to use by several countries throughout the world, and the request for use of this orbit is increasing. There are 188 geostationary stations in operation. An equitable distribution of stations on this orbit is requested
A thermodynamic counterpart of the Axelrod model of social influence: The one-dimensional case
We propose a thermodynamic version of the Axelrod model of social influence.
In one-dimensional (1D) lattices, the thermodynamic model becomes a coupled
Potts model with a bonding interaction that increases with the site matching
traits. We analytically calculate thermodynamic and critical properties for a
1D system and show that an order-disorder phase transition only occurs at T = 0
independent of the number of cultural traits q and features F. The 1D
thermodynamic Axelrod model belongs to the same universality class of the Ising
and Potts models, notwithstanding the increase of the internal dimension of the
local degree of freedom and the state-dependent bonding interaction. We suggest
a unifying proposal to compare exponents across different discrete 1D models.
The comparison with our Hamiltonian description reveals that in the
thermodynamic limit the original out-of-equilibrium 1D Axelrod model with noise
behaves like an ordinary thermodynamic 1D interacting particle system.Comment: 19 pages, 5 figure
Rashba spin-orbit interaction enhanced by graphene in-plane deformations
Graphene consists in a single-layer carbon crystal where 2 electrons
display a linear dispersion relation in the vicinity of the Fermi level,
conveniently described by a massless Dirac equation in spacetime.
Spin-orbit effects open a gap in the band structure and offer perspectives for
the manipulation of the conducting electrons spin. Ways to manipulate
spin-orbit couplings in graphene have been generally assessed by proximity
effects to metals that do not compromise the mobility of the unperturbed system
and are likely to induce strain in the graphene layer. In this work we explore
the gauge fields that result from the uniform
stretching of a graphene sheet under a perpendicular electric field.
Considering such deformations is particularly relevant due to the
counter-intuitive enhancement of the Rashba coupling between 30-50% for small
bond deformations well known from tight-binding and DFT calculations. We report
the accessible changes that can be operated in the band structure in the
vicinity of the K points as a function of the deformation strength and
direction.Comment: 10 pages, 7 figure
Quasi purchasing power parity: Structural change in the Mexican peso/us dollar real exchange rate
This paper analyzes whether the real exchange-rate of the Mexican peso/US dollar revert to a long-run equilibrium value, and whether this value is unique. We use a method for testing stationarity, that allows for an unknown number of structural breaks in the level of the series. Using a long span of annual data covering the period 1925-1994, our results provide evidence favouring long-run Quasi-Purchasing Power Parity. In particular, we find that the real peso/dollar exchange rate has fluctuated stationarily around a 70 year long-run level, perturbed by a series of events, both domestic and external, in or around 1981.
Antiresonances as precursors of decoherence
We show that, in presence of a complex spectrum, antiresonances act as a
precursor for dephasing enabling the crossover to a fully decoherent transport
even within a unitary Hamiltonian description. This general scenario is
illustrated here by focusing on a quantum dot coupled to a chaotic cavity
containing a finite, but large, number of states using a Hamiltonian
formulation. For weak coupling to a chaotic cavity with a sufficiently dense
spectrum, the ensuing complex structure of resonances and antiresonances leads
to phase randomization under coarse graining in energy. Such phase
instabilities and coarse graining are the ingredients for a mechanism producing
decoherence and thus irreversibility. For the present simple model one finds a
conductance that coincides with the one obtained by adding a ficticious voltage
probe within the Landauer-Buettiker picture. This sheds new light on how the
microscopic mechanisms that produce phase fluctuations induce decoherence.Comment: 7 pages, 2 figures, to appear in Europhys. Let
Dynamic Renormalization Group Approach to Self-Organized Critical Phenomena
Two different models exhibiting self-organized criticality are analyzed by
means of the dynamic renormalization group. Although the two models differ by
their behavior under a parity transformation of the order parameter, it is
shown that they both belong to the same universality class, in agreement with
computer simulations. The asymptotic values of the critical exponents are
estimated up to one loop order from a systematic expansion of a nonlinear
equation in the number of coupling constants.Comment: 8 pages, RevTeX 3.0, 1 PostScript figure available upon reques
Gauge field theory approach to spin transport in a 2D electron gas
We discuss the Pauli Hamiltonian including the spin-orbit interaction within
an U(1) x SU(2) gauge theory interpretation, where the gauge symmetry appears
to be broken. This interpretation offers new insight into the problem of spin
currents in the condensed matter environment, and can be extended to Rashba and
Dresselhaus spin-orbit interactions. We present a few outcomes of the present
formulation: i) it automatically leads to zero spin conductivity, in contrast
to predictions of Gauge symmetric treatments, ii) a topological quantization
condition leading to voltage quantization follows, and iii) spin
interferometers can be conceived in which, starting from a arbitrary incoming
unpolarized spinor, it is always possible to construct a perfect spin filtering
condition.Comment: Invited contribution to Statphys conference, June 2009, Lviv
(Ukraine
Magneto-Conductance Anisotropy and Interference Effects in Variable Range Hopping
We investigate the magneto-conductance (MC) anisotropy in the variable range
hopping regime, caused by quantum interference effects in three dimensions.
When no spin-orbit scattering is included, there is an increase in the
localization length (as in two dimensions), producing a large positive MC. By
contrast, with spin-orbit scattering present, there is no change in the
localization length, and only a small increase in the overall tunneling
amplitude. The numerical data for small magnetic fields , and hopping
lengths , can be collapsed by using scaling variables , and
in the perpendicular and parallel field orientations
respectively. This is in agreement with the flux through a `cigar'--shaped
region with a diffusive transverse dimension proportional to . If a
single hop dominates the conductivity of the sample, this leads to a
characteristic orientational `finger print' for the MC anisotropy. However, we
estimate that many hops contribute to conductivity of typical samples, and thus
averaging over critical hop orientations renders the bulk sample isotropic, as
seen experimentally. Anisotropy appears for thin films, when the length of the
hop is comparable to the thickness. The hops are then restricted to align with
the sample plane, leading to different MC behaviors parallel and perpendicular
to it, even after averaging over many hops. We predict the variations of such
anisotropy with both the hop size and the magnetic field strength. An
orientational bias produced by strong electric fields will also lead to MC
anisotropy.Comment: 24 pages, RevTex, 9 postscript figures uuencoded Submitted to PR
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