105 research outputs found
Yang-Lee Edge Singularity on a Class of Treelike Lattices
The density of zeros of the partition function of the Ising model on a class
of treelike lattices is studied. An exact closed-form expression for the
pertinent critical exponents is derived by using a couple of recursion
relations which have a singular behavior near the Yang-Lee edge.Comment: 9 pages AmsTex, 2 eps figures, to appear in J.Phys.
Surface Instability in Windblown Sand
We investigate the formation of ripples on the surface of windblown sand
based on the one-dimensional model of Nishimori and Ouchi [Phys. Rev. Lett. 71,
197 (1993)], which contains the processes of saltation and grain relaxation. We
carry out a nonlinear analysis to determine the propagation speed of the
restabilized ripple patterns, and the amplitudes and phases of their first,
second, and third harmonics. The agreement between the theory and our numerical
simulations is excellent near the onset of instability. We also determine the
Eckhaus boundary, outside which the steady ripple patterns are unstable.Comment: 23 pages, 8 figure
Hyperfine interaction mediated exciton spin relaxation in (In,Ga)As quantum dots
The population dynamics of dark and bright excitons in (In,Ga)As/GaAs quantum
dots is studied by two-color pump-probe spectroscopy in an external magnetic
field. With the field applied in Faraday geometry and at T<20 K, the dark
excitons decay on a ten nanoseconds time scale unless the magnetic field
induces a resonance with a bright exciton state. At these crossings their
effective lifetime is drastically shortened due to spin flips of either
electron or hole by which the dark excitons are converted into bright ones. Due
to the quasielastic character we attribute the origin of these flips to the
hyperfine interaction with the lattice nuclei. We compare the exciton spin
relaxation times in the two resonances and find that the spin flip involving an
electron is approximately 25 times faster than the one of the hole. A
temperature increase leads to a considerable, nonmonotonic decrease of the dark
exciton lifetime. Here phonon-mediated spin flips due to the spin-orbit
interaction gradually become more important.Comment: expanded version (cf. sections III.C and III.D
Structural design studies of a supersonic cruise arrow wing configuration
Structural member cross sections were sized with a system of integrated computer programs to satisfy strength and flutter design requirements for several variants of the arrow wing supersonic cruise vehicle. The resulting structural weights provide a measure of the structural efficiency of the planform geometry, structural layout, type of construction, and type of material including composites. The material distribution was determined for a baseline metallic structure and the results indicate that an approximate fatigue constraint has an important effect on the structural weight required for strength but, in all cases, additional material had to be added to satisfy flutter requirements with lighter mass engines with minimum fuel onboard. The use of composite materials on the baseline configuration was explored and indicated increased structural efficiency. In the strength sizing, the all-composite construction provided a lower weight design than the hybrid construction which contained composites only in the wing cover skins. Subsequent flutter analyses indicated a corresponding lower flutter speed
Systematic study of carrier correlations in the electron-hole recombination dynamics of quantum dots
The ground state carrier dynamics in self-assembled (In,Ga)As/GaAs quantum
dots has been studied using time-resolved photoluminescence and transmission.
By varying the dot design with respect to confinement and doping, the dynamics
is shown to follow in general a non-exponential decay. Only for specific
conditions in regard to optical excitation and carrier population, for example,
the decay can be well described by a mono-exponential form. For resonant
excitation of the ground state transition a strong shortening of the
luminescence decay time is observed as compared to the non-resonant case. The
results are consistent with a microscopic theory that accounts for deviations
from a simple two-level picture.Comment: 8 pages, 7 figure
Correlated Photon-Pair Emission from a Charged Single Quantum Dot
The optical creation and recombination of charged biexciton and trion
complexes in an (In,Ga)As/GaAs quantum dot is investigated by
micro-photoluminescence spectroscopy. Photon cross-correlation measurements
demonstrate the temporally correlated decay of charged biexciton and trion
states. Our calculations provide strong evidence for radiative decay from the
excited trion state which allows for a deeper insight into the spin
configurations and their dynamics in these systems.Comment: 5 pages, 3 figures, submitted for publicatio
Kink Solution in a Fluid Model of Traffic Flows
Traffic jam in a fluid model of traffic flows proposed by Kerner and
Konh\"auser (B. S. Kerner and P. Konh\"auser, Phys. Rev. E 52 (1995), 5574.) is
analyzed. An analytic scaling solution is presented near the critical point of
the hetero-clinic bifurcation. The validity of the solution has been confirmed
from the comparison with the simulation of the model.Comment: RevTeX v3.1, 6 pages, and 2 figure
Scaling and Density of Lee-Yang Zeroes in the Four Dimensional Ising Model
The scaling behaviour of the edge of the Lee--Yang zeroes in the four
dimensional Ising model is analyzed. This model is believed to belong to the
same universality class as the model which plays a central role in
relativistic quantum field theory. While in the thermodynamic limit the scaling
of the Yang--Lee edge is not modified by multiplicative logarithmic
corrections, such corrections are manifest in the corresponding finite--size
formulae. The asymptotic form for the density of zeroes which recovers the
scaling behaviour of the susceptibility and the specific heat in the
thermodynamic limit is found to exhibit logarithmic corrections too. The
density of zeroes for a finite--size system is examined both analytically and
numerically.Comment: 17 pages (4 figures), LaTeX + POSTSCRIPT-file, preprint UNIGRAZ-UTP
20-11-9
Generalized Force Model of Traffic Dynamics
Floating car data of car-following behavior in cities were compared to
existing microsimulation models, after their parameters had been calibrated to
the experimental data. With these parameter values, additional simulations have
been carried out, e.g. of a moving car which approaches a stopped car. It
turned out that, in order to manage such kinds of situations without producing
accidents, improved traffic models are needed. Good results have been obtained
with the proposed generalized force model.Comment: For related work see
http://www.theo2.physik.uni-stuttgart.de/helbing.htm
Identity of the universal repulsive-core singularity with Yang-Lee edge criticality
Lattice and continuum fluid models with repulsive-core interactions typically
display a dominant, critical-type singularity on the real, negative activity
axis. Lai and Fisher recently suggested, mainly on numerical grounds, that this
repulsive-core singularity is universal and in the same class as the Yang-Lee
edge singularities, which arise above criticality at complex activities with
positive real part. A general analytic demonstration of this identification is
presented here using a field-theory approach with separate representation of
the repulsive and attractive parts of the pair interactions.Comment: 6 pages, 3 figure
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