241 research outputs found
Suppressing Roughness of Virtual Times in Parallel Discrete-Event Simulations
In a parallel discrete-event simulation (PDES) scheme, tasks are distributed
among processing elements (PEs), whose progress is controlled by a
synchronization scheme. For lattice systems with short-range interactions, the
progress of the conservative PDES scheme is governed by the Kardar-Parisi-Zhang
equation from the theory of non-equilibrium surface growth. Although the
simulated (virtual) times of the PEs progress at a nonzero rate, their standard
deviation (spread) diverges with the number of PEs, hindering efficient data
collection. We show that weak random interactions among the PEs can make this
spread nondivergent. The PEs then progress at a nonzero, near-uniform rate
without requiring global synchronizations
Going through Rough Times: from Non-Equilibrium Surface Growth to Algorithmic Scalability
Efficient and faithful parallel simulation of large asynchronous systems is a
challenging computational problem. It requires using the concept of local
simulated times and a synchronization scheme. We study the scalability of
massively parallel algorithms for discrete-event simulations which employ
conservative synchronization to enforce causality. We do this by looking at the
simulated time horizon as a complex evolving system, and we identify its
universal characteristics. We find that the time horizon for the conservative
parallel discrete-event simulation scheme exhibits Kardar-Parisi-Zhang-like
kinetic roughening. This implies that the algorithm is asymptotically scalable
in the sense that the average progress rate of the simulation approaches a
non-zero constant. It also implies, however, that there are diverging memory
requirements associated with such schemes.Comment: to appear in the Proceedings of the MRS, Fall 200
Nonequilibrium electron heating in inter-subband terahertz lasers
Inter-subband laser performance can be critically dependent on the nature of the electron distributions in each subband. In these first Monte Carlo device simulations of optically pumped inter-subband THz lasers, we can see that there are two main causes of electron heating: intersubband decay processes, and inter-subband energy transfer from the "hot" nonequilibrium tails of lower subbands. These processes mean that devices relying on low electron temperatures are disrupted by electron heating, to the extent that slightly populated subbands can have average energies far in excess of the that of either the lattice or other subbands. However, although these heating effects invalidate designs relying on low temperature electron distributions, we see that population inversion is still possible in the high-THz range at 77 K in both stepped and triple-well structures, and that our 11.7 THz triple-well structure even promises inversion at 300 K. © 2002 American Institute of Physics
Synchronization Landscapes in Small-World-Connected Computer Networks
Motivated by a synchronization problem in distributed computing we studied a
simple growth model on regular and small-world networks, embedded in one and
two-dimensions. We find that the synchronization landscape (corresponding to
the progress of the individual processors) exhibits Kardar-Parisi-Zhang-like
kinetic roughening on regular networks with short-range communication links.
Although the processors, on average, progress at a nonzero rate, their spread
(the width of the synchronization landscape) diverges with the number of nodes
(desynchronized state) hindering efficient data management. When random
communication links are added on top of the one and two-dimensional regular
networks (resulting in a small-world network), large fluctuations in the
synchronization landscape are suppressed and the width approaches a finite
value in the large system-size limit (synchronized state). In the resulting
synchronization scheme, the processors make close-to-uniform progress with a
nonzero rate without global intervention. We obtain our results by ``simulating
the simulations", based on the exact algorithmic rules, supported by
coarse-grained arguments.Comment: 20 pages, 22 figure
Extreme fluctuations in noisy task-completion landscapes on scale-free networks
We study the statistics and scaling of extreme fluctuations in noisy
task-completion landscapes, such as those emerging in synchronized
distributed-computing networks, or generic causally-constrained queuing
networks, with scale-free topology. In these networks the average size of the
fluctuations becomes finite (synchronized state) and the extreme fluctuations
typically diverge only logarithmically in the large system-size limit ensuring
synchronization in a practical sense. Provided that local fluctuations in the
network are short-tailed, the statistics of the extremes are governed by the
Gumbel distribution. We present large-scale simulation results using the exact
algorithmic rules, supported by mean-field arguments based on a coarse-grained
description.Comment: 16 pages, 6 figures, revte
Electronic Green's functions in a T-shaped multi-quantum dot system
We developed a set of equations to calculate the electronic Green's functions
in a T-shaped multi-quantum dot system using the equation of motion method. We
model the system using a generalized Anderson Hamiltonian which accounts for
{\em finite} intradot on-site Coulomb interaction in all component dots as well
as for the interdot electron tunneling between adjacent quantum dots. Our
results are obtained within and beyond the Hartree-Fock approximation and
provide a path to evaluate all the electronic correlations in the multi-quantum
dot system in the Coulomb blockade regime. Both approximations provide
information on the physical effects related to the finite intradot on-site
Coulomb interaction. As a particular example for our generalized results, we
considered the simplest T-shaped system consisting of two dots and proved that
our approximation introduces important corrections in the detector and side
dots Green's functions, and implicitly in the evaluation of the system's
transport properties. The multi-quantum dot T-shaped setup may be of interest
for the practical realization of qubit states in quantum dots systems.Comment: 13 pages, 2 figure
Multilayer metamaterial absorbers inspired by perfectly matched layers
We derive periodic multilayer absorbers with effective uniaxial properties
similar to perfectly matched layers (PML). This approximate representation of
PML is based on the effective medium theory and we call it an effective medium
PML (EM-PML). We compare the spatial reflection spectrum of the layered
absorbers to that of a PML material and demonstrate that after neglecting gain
and magnetic properties, the absorber remains functional. This opens a route to
create electromagnetic absorbers for real and not only numerical applications
and as an example we introduce a layered absorber for the wavelength of
~m made of SiO and NaCl. We also show that similar cylindrical
core-shell nanostructures derived from flat multilayers also exhibit very good
absorptive and reflective properties despite the different geometry
Elucidation of key odorants and sensory propertiesof five different extra virgin olive oils from Turkey by GC-MS-Olfactometry
The present study investigates the aroma, key odorants and sensory profile of extra virgin olive oils from five well-known Turkish cultivars. The aromatic extract obtained by the purge and trap extraction system, according to a sensory analysis, resembled the odor of olive oil. A total of 22, 21, 18, 22 and 21 aroma-active compounds were detected in the extracts of Ayvalık, Memecik, Gemlik, Sarı Ulak and Beylik olive oils, respectively. The results show that Ayvalık has the highest flavor dilution (FD) value of 1024 with hexanal, (E)-2-hexenal and α-farnesene. Memecik has the highest FD value at 2048 with (E)-2-hexenal. Gemlik has the highest FD value of 1024 with (Z)-3-hexenyl acetate, (E)-2-hexen-1-ol and α-farnesene. Sarı Ulak has the highest FD value at 2048 with (E)-2-hexenal. Beylik has the highest FD value of 2048 with (E)-2-hexenal and hexanal. All cultivars represent the characteristic green, cut-grass, fruity odor notes
Airy Distribution Function: From the Area Under a Brownian Excursion to the Maximal Height of Fluctuating Interfaces
The Airy distribution function describes the probability distribution of the
area under a Brownian excursion over a unit interval. Surprisingly, this
function has appeared in a number of seemingly unrelated problems, mostly in
computer science and graph theory. In this paper, we show that this
distribution also appears in a rather well studied physical system, namely the
fluctuating interfaces. We present an exact solution for the distribution
P(h_m,L) of the maximal height h_m (measured with respect to the average
spatial height) in the steady state of a fluctuating interface in a one
dimensional system of size L with both periodic and free boundary conditions.
For the periodic case, we show that P(h_m,L)=L^{-1/2}f(h_m L^{-1/2}) for all L
where the function f(x) is the Airy distribution function. This result is valid
for both the Edwards-Wilkinson and the Kardar-Parisi-Zhang interfaces. For the
free boundary case, the same scaling holds P(h_m,L)=L^{-1/2}F(h_m L^{-1/2}),
but the scaling function F(x) is different from that of the periodic case. We
compute this scaling function explicitly for the Edwards-Wilkinson interface
and call it the F-Airy distribution function. Numerical simulations are in
excellent agreement with our analytical results. Our results provide a rather
rare exactly solvable case for the distribution of extremum of a set of
strongly correlated random variables. Some of these results were announced in a
recent Letter [ S.N. Majumdar and A. Comtet, Phys. Rev. Lett., 92, 225501
(2004)].Comment: 27 pages, 10 .eps figures included. Two figures improved, new
discussion and references adde
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