616 research outputs found
Process algebraic non-product-forms
AbstractA generalization of the Reversed Compound Agent Theorem of Markovian process algebra is derived that yields separable, but non-product-form solutions for collections of interacting processes such as arise in multi-class queueing networks with Processor Sharing servers. It is based on an analysis of the minimal cycles in the state space of a multi-agent cooperation, which can be simply identified. The extended methodology leads to what we believe are new separable solutions and, more generally, the results represent a viable practical application of the theory of Markovian process algebras in stochastic modelling
Sobre a Faixa de Pedestres
The interference colors resulting from thin films of Al 2O 3 deposited by atomic layer deposition (ALD) on silicon have been rigorously analyzed using a recently developed robotic gonioreflectometer. A series of eleven increasingly thick films was deposited, up to 1613 Å, and their reflectance values obtained for the visible spectrum. A comparison of these values with the predictions of computer simulations employing Fresnel equations has revealed that while there was generally good agreement between predicted and measured spectra, there are some spectral regions that exhibit large deviations from predicted reflectances, typically at near-normal measurement angles and shorter wavelengths. The effect of these discrepancies on color appearance was investigated in the CIE L*a*b* color space for the daylight illuminant D65. Large iridescence is both predicted and measured for most film thicknesses. Chroma and hue differences as large as 20 CIELAB units between the predicted and the measured color centers were obtained. Simulation also predicts larger iridescence than what is actually measured. A likely cause for the observed discrepancies is that the dielectric constants of the ALD films deviate from the literature values for the bulk material
Transport on percolation clusters with power-law distributed bond strengths: when do blobs matter?
The simplest transport problem, namely maxflow, is investigated on critical
percolation clusters in two and three dimensions, using a combination of
extremal statistics arguments and exact numerical computations, for power-law
distributed bond strengths of the type .
Assuming that only cutting bonds determine the flow, the maxflow critical
exponent \ve is found to be \ve(\alpha)=(d-1) \nu + 1/(1-\alpha). This
prediction is confirmed with excellent accuracy using large-scale numerical
simulation in two and three dimensions. However, in the region of anomalous
bond capacity distributions () we demonstrate that, due to
cluster-structure fluctuations, it is not the cutting bonds but the blobs that
set the transport properties of the backbone. This ``blob-dominance'' avoids a
cross-over to a regime where structural details, the distribution of the number
of red or cutting bonds, would set the scaling. The restored scaling exponents
however still follow the simplistic red bond estimate. This is argued to be due
to the existence of a hierarchy of so-called minimum cut-configurations, for
which cutting bonds form the lowest level, and whose transport properties scale
all in the same way. We point out the relevance of our findings to other scalar
transport problems (i.e. conductivity).Comment: 9 pages + Postscript figures. Revtex4+psfig. Submitted to PR
Water-like anomalies for core-softened models of fluids: One dimension
We use a one-dimensional (1d) core-softened potential to develop a physical
picture for some of the anomalies present in liquid water. The core-softened
potential mimics the effect of hydrogen bonding. The interest in the 1d system
stems from the facts that closed-form results are possible and that the
qualitative behavior in 1d is reproduced in the liquid phase for higher
dimensions. We discuss the relation between the shape of the potential and the
density anomaly, and we study the entropy anomaly resulting from the density
anomaly. We find that certain forms of the two-step square well potential lead
to the existence at T=0 of a low-density phase favored at low pressures and of
a high-density phase favored at high pressures, and to the appearance of a
point at a positive pressure, which is the analog of the T=0 ``critical
point'' in the Ising model. The existence of point leads to anomalous
behavior of the isothermal compressibility and the isobaric specific heat
.Comment: 22 pages, 7 figure
Specific heat and magnetic measurements in Nd0.5Sr0.5MnO3, Nd0.5Ca0.5MnO3 and Ho0.5Ca0.5MnO3 samples
We studied the magnetization as a function of temperature and magnetic field
in the compounds Nd0.5Sr0.5MnO3, Nd0.5Ca0.5MnO3 and Ho0.5Ca0.5MnO3. It allowed
us to identify the ferromagnetic, antiferromagnetic and charge ordering phases
in each case. The intrinsic magnetic moments of Nd3+ and Ho3+ ions experienced
a short range order at low temperatures. We also did specific heat measurements
with applied magnetic fields between 0 and 9 T and temperatures between 2 and
300 K in all three samples. Close to the charge ordering and ferromagnetic
transition temperatures the specific heat curves showed peaks superposed to the
characteristic response of the lattice oscillations. Below 10 K the specific
heat measurements evidenced a Schottky-like anomaly for all samples. However,
we could not successfully fit the curves to either a two level nor a
distribution of two-level Schottky anomaly. Our results indicated that the peak
temperature of the Schottky anomaly was higher in the compounds with narrower
conduction band.Comment: submitted to PR
Unusual magnetic relaxation behavior in La0.5Ca0.5MnO3 and Nd0.5Sr0.5MnO3
We have carried out a systematic magnetic relaxation study, measured after
applying and switching off a 5 T magnetic field to polycrystalline samples of
La0.5Ca0.5MnO3 and Nd0.5Sr0.5MnO3. The long time logarithmic relaxation rate
(LTLRR), decreased from 10 K to 150 K and increased from 150 K to 195 K in
La0.5Ca0.5MnO3. This change in behavior was found to be related to the complete
suppression of the antiferromagnetic phase above 150 K and in the presence of a
5 T magnetic field. At 195 K, the magnetization first decreased, and after a
few minutes increased slowly as a function of time. Moreover, between 200 K and
245 K, the magnetization increased throughout the measured time span. The
change in the slope of the curves, from negative to positive at about 200 K was
found to be related to the suppression of antiferromagnetic fluctuations in
small magnetic fields. A similar temperature dependence of the LTLRR was found
for the Nd0.5Sr0.5MnO3 sample. However, the temperature where the LTLRR reached
the minimum in Nd0.5Sr0.5MnO3 was lower than that of La0.5Ca0.5MnO3. This
result agrees with the stronger ferromagnetic interactions that exist in
Nd0.5Sr0.5MnO3 in comparison to La0.5Ca0.5MnO3. The above measurements
suggested that the general temperature dependence of the LTLRR and the
underlying physics were mainly independent of the particular charge ordering
system considered. All relaxation curves could be fitted using a logarithmic
law at long times. This slow relaxation was attributed to the coexistence of
ferromagnetic and antiferromagnetic interactions between Mn ions, which
produced a distribution of energy barriers.Comment: Accepted to PRB as a regular article, 10 figures, Scheduled Issue: 01
June 200
Aharonov-Bohm Interferometry with Interacting Quantum Dots: Spin Configurations, Asymmetric Interference Patterns, Bias-Voltage-Induced Aharonov-Bohm Oscillations, and Symmetries of Transport Coefficients
We study electron transport through multiply-connected mesoscopic geometries
containing interacting quantum dots. Our formulation covers both equilibrium
and non-equilibrium physics. We discuss the relation of coherent transport
channels through the quantum dot to flux-sensitive Aharonov-Bohm oscillations
in the total conductance of the device. Contributions to transport in first and
second order in the intrinsic line width of the dot levels are addressed in
detail. We predict an interaction-induced asymmetry in the amplitude of the
interference signal around resonance peaks as a consequence of incoherence
associated with spin-flip processes. This asymmetry can be used to probe the
total spin of the quantum dot. Such a probe requires less stringent
experimental conditions than the Kondo effect, which provides the same
information. We show that first-order contributions can be partially or even
fully coherent. This contrasts with the sequential-tunneling picture, which
describes first-order transport as a sequence of incoherent tunneling
processes. We predict bias-voltage induced Aharonov-Bohm oscillations of
physical quantities which are independent of flux in the linear-response
regime. Going beyond the Onsager relations we analyze the relations between the
space symmetry group of the setup and the flux-dependent non-linear
conductance.Comment: 22 pages, 11 figure
Suppressing CMB Quadrupole with a Bounce from Contracting Phase to Inflation
Recent released WMAP data show a low value of quadrupole in the CMB
temperature fluctuations, which confirms the early observations by COBE. In
this paper, a scenario, in which a contracting phase is followed by an
inflationary phase, is constructed. We calculate the perturbation spectrum and
show that this scenario can provide a reasonable explanation for lower CMB
anisotropies on large angular scales.Comment: 5 pages, 3 figure
Quantum Interference in Superconducting Wire Networks and Josephson Junction Arrays: Analytical Approach based on Multiple-Loop Aharonov-Bohm Feynman Path-Integrals
We investigate analytically and numerically the mean-field
superconducting-normal phase boundaries of two-dimensional superconducting wire
networks and Josephson junction arrays immersed in a transverse magnetic field.
The geometries we consider include square, honeycomb, triangular, and kagome'
lattices. Our approach is based on an analytical study of multiple-loop
Aharonov-Bohm effects: the quantum interference between different electron
closed paths where each one of them encloses a net magnetic flux. Specifically,
we compute exactly the sums of magnetic phase factors, i.e., the lattice path
integrals, on all closed lattice paths of different lengths. A very large
number, e.g., up to for the square lattice, exact lattice path
integrals are obtained. Analytic results of these lattice path integrals then
enable us to obtain the resistive transition temperature as a continuous
function of the field. In particular, we can analyze measurable effects on the
superconducting transition temperature, , as a function of the magnetic
filed , originating from electron trajectories over loops of various
lengths. In addition to systematically deriving previously observed features,
and understanding the physical origin of the dips in as a result of
multiple-loop quantum interference effects, we also find novel results. In
particular, we explicitly derive the self-similarity in the phase diagram of
square networks. Our approach allows us to analyze the complex structure
present in the phase boundaries from the viewpoint of quantum interference
effects due to the electron motion on the underlying lattices.Comment: 18 PRB-type pages, plus 8 large figure
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