18,105 research outputs found
Probability distribution of the order parameter
The probability distribution of the order parameter is exploited in order to
obtain the criticality of magnetic systems. Monte Carlo simulations have been
employed by using single spin flip Metropolis algorithm aided by finite-size
scaling and histogram reweighting techniques. A method is proposed to obtain
this probability distribution even when the transition temperature of the model
is unknown. A test is performed on the two-dimensional spin-1/2 and spin-1
Ising model and the results show that the present procedure can be quite
efficient and accurate to describe the criticality of the system.Comment: 5 pages, 7 figures, to appear in Braz. J. Phys. 34, June 200
Probability distribution of the order parameter in the directed percolation universality class
The probability distributions of the order parameter for two models in the
directed percolation universality class were evaluated. Monte Carlo simulations
have been performed for the one-dimensional generalized contact process and the
Domany-Kinzel cellular automaton. In both cases, the density of active sites
was chosen as the order parameter. The criticality of those models was obtained
by solely using the corresponding probability distribution function. It has
been shown that the present method, which has been successfully employed in
treating equilibrium systems, is indeed also useful in the study of
nonequilibrium phase transitions.Comment: 6 pages, 4 figure
The influence of the Al stabilizer layer thickness on the normal zone propagation velocity in high current superconductors
The stability of high-current superconductors is challenging in the design of
superconducting magnets. When the stability requirements are fulfilled, the
protection against a quench must still be considered. A main factor in the
design of quench protection systems is the resistance growth rate in the magnet
following a quench. The usual method for determining the resistance growth in
impregnated coils is to calculate the longitudinal velocity with which the
normal zone propagates in the conductor along the coil windings.
Here, we present a 2D numerical model for predicting the normal zone
propagation velocity in Al stabilized Rutherford NbTi cables with large cross
section. By solving two coupled differential equations under adiabatic
conditions, the model takes into account the thermal diffusion and the current
redistribution process following a quench. Both the temperature and magnetic
field dependencies of the superconductor and the metal cladding materials
properties are included. Unlike common normal zone propagation analyses, we
study the influence of the thickness of the cladding on the propagation
velocity for varying operating current and magnetic field.
To assist in the comprehension of the numerical results, we also introduce an
analytical formula for the longitudinal normal zone propagation. The analysis
distinguishes between low-current and high-current regimes of normal zone
propagation, depending on the ratio between the characteristic times of thermal
and magnetic diffusion. We show that above a certain thickness, the cladding
acts as a heat sink with a limited contribution to the acceleration of the
propagation velocity with respect to the cladding geometry. Both numerical and
analytical results show good agreement with experimental data.Comment: To be published in Physics Procedia (ICEC 25 conference special
issue
Vorton Formation
In this paper we present the first analytic model for vorton formation. We
start by deriving the microscopic string equations of motion in Witten's
superconducting model, and show that in the relevant chiral limit these
coincide with the ones obtained from the supersonic elastic models of Carter
and Peter. We then numerically study a number of solutions of these equations
of motion and thereby suggest criteria for deciding whether a given
superconducting loop configuration can form a vorton. Finally, using a recently
developed model for the evolution of currents in superconducting strings we
conjecture, by comparison with these criteria, that string networks formed at
the GUT phase transition should produce no vortons. On the other hand, a
network formed at the electroweak scale can produce vortons accounting for up
to 6% of the critical density. Some consequences of our results are discussed.Comment: 41 pages; color figures 3-6 not included, but available from authors.
To appear in Phys. Rev.
Utilização de vermicomposto em plantios de freijó-louro em solos de baixa fertilidade.
Este trabalho avaliou o efeito de sete dosagens de vermicomposto (húmus de minhoca produzido a partir do esterco bovino curtido) variando de zero a seis quilos por cova incorporado ao solo antes do plantio da muda no crescimento de freijó-louro em solos de baixa fertilidade, 36 meses após o plantio.bitstream/item/100453/1/Cot206-freijo-louro.pd
A new investigation of the binary HD 48099
With an orbital period of about 3.078 days, the double-lined spectroscopic
binary HD 48099 is, still now, the only short-period O+O system known in the
Mon OB2 association. Even though an orbital solution has already been derived
for this system, few information are available about the individual stars. We
present, in this paper, the results of a long-term spectroscopic campaign. We
derive a new orbital solution and apply a disentangling method to recover the
mean spectrum of each star. To improve our knowledge concerning both
components, we determine their spectral classifications and their projected
rotational velocities. We also constrain the main stellar parameters of both
stars by using the CMFGEN atmosphere code and provide the wind properties for
the primary star through the study of IUE spectra. This investigation reveals
that HD 48099 is an O5.5 V((f))+O9 V binary with M_1 sin^3 i = 0.70 M_{\sun}
and M_2 sin^3 i = 0.39 M_{\sun}, implying a rather low orbital inclination.
This result, combined with both a large effective temperature and log g,
suggests that the primary star (v sini ~ 91 km s^-1) is actually a fast rotator
with a strongly clumped wind and a nitrogen abundance of about 8 times the
solar value.Comment: 12 pages, 7 figures, accepted by Ap
Caracterização térmica da borracha natural de clones IAC da série 400 da cidade de Jaú em São Paulo.
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