671 research outputs found
Empirical Traffic Data and Their Implications for Traffic Modeling
From single vehicle data a number of new empirical results about the temporal
evolution, correlation, and density-dependence of macroscopic traffic
quantities have been determined. These have relevant implications for traffic
modeling and allow to test existing traffic models.Comment: For related work see
http://www.theo2.physik.uni-stuttgart.de/helbing.htm
An Efficient and Accurate Car-Parrinello-like Approach to Born-Oppenheimer Molecular Dynamics
We present a new method which combines Car-Parrinello and Born-Oppenheimer
molecular dynamics in order to accelerate density functional theory based
ab-initio simulations. Depending on the system a gain in efficiency of one to
two orders of magnitude has been observed, which allows ab-initio molecular
dynamics of much larger time and length scales than previously thought
feasible. It will be demonstrated that the dynamics is correctly reproduced and
that high accuracy can be maintained throughout for systems ranging from
insulators to semiconductors and even to metals in condensed phases. This
development considerably extends the scope of ab-initio simulations.Comment: 4 pages, 3 figures; Accepted by Phys. Rev. Lett. for publicatio
Modeling and Simulation of Multi-Lane Traffic Flow
A most important aspect in the field of traffic modeling is the simulation of
bottleneck situations. For their realistic description a macroscopic multi-lane
model for uni-directional freeways including acceleration, deceleration,
velocity fluctuations, overtaking and lane-changing maneuvers is systematically
deduced from a gas-kinetic (Boltzmann-like) approach. The resulting equations
contain corrections with respect to previous models. For efficient computer
simulations, a reduced model delineating the coarse-grained temporal behavior
is derived and applied to bottleneck situations.Comment: For related work see
http://www.theo2.physik.uni-stuttgart.de/helbing.htm
Polarization selection rules for inter-Landau level transitions in epitaxial graphene revealed by infrared optical Hall effect
We report on polarization selection rules of inter-Landau level transitions
using reflection-type optical Hall effect measurements from 600 to 4000 cm-1 on
epitaxial graphene grown by thermal decomposition of silicon carbide. We
observe symmetric and anti-symmetric signatures in our data due to polarization
preserving and polarization mixing inter-Landau level transitions,
respectively. From field-dependent measurements we identify that transitions in
decoupled graphene mono-layers are governed by polarization mixing selection
rules, whereas transitions in coupled graphene mono-layers are governed by
polarization preserving selection rules. The selection rules may find
explanation by different coupling mechanisms of inter-Landau level transitions
with free charge carrier magneto-optic plasma oscillations
Fundamentals of Traffic Flow
From single vehicle data a number of new empirical results concerning the
density-dependence of the velocity distribution and its moments as well as the
characteristics of their temporal fluctuations have been determined. These are
utilized for the specification of some fundamental relations of traffic flow
and compared with existing traffic theories.Comment: For related work see
http://www.theo2.physik.uni-stuttgart.de/helbing.htm
Probabilistic Description of Traffic Breakdowns
We analyze the characteristic features of traffic breakdown. To describe this
phenomenon we apply to the probabilistic model regarding the jam emergence as
the formation of a large car cluster on highway. In these terms the breakdown
occurs through the formation of a certain critical nucleus in the metastable
vehicle flow, which enables us to confine ourselves to one cluster model. We
assume that, first, the growth of the car cluster is governed by attachment of
cars to the cluster whose rate is mainly determined by the mean headway
distance between the car in the vehicle flow and, may be, also by the headway
distance in the cluster. Second, the cluster dissolution is determined by the
car escape from the cluster whose rate depends on the cluster size directly.
The latter is justified using the available experimental data for the
correlation properties of the synchronized mode. We write the appropriate
master equation converted then into the Fokker-Plank equation for the cluster
distribution function and analyze the formation of the critical car cluster due
to the climb over a certain potential barrier. The further cluster growth
irreversibly gives rise to the jam formation. Numerical estimates of the
obtained characteristics and the experimental data of the traffic breakdown are
compared. In particular, we draw a conclusion that the characteristic intrinsic
time scale of the breakdown phenomenon should be about one minute and explain
the case why the traffic volume interval inside which traffic breakdown is
observed is sufficiently wide.Comment: RevTeX 4, 14 pages, 10 figure
Single-vehicle data of highway traffic - a statistical analysis
In the present paper single-vehicle data of highway traffic are analyzed in
great detail. By using the single-vehicle data directly empirical time-headway
distributions and speed-distance relations can be established. Both quantities
yield relevant information about the microscopic states. Several fundamental
diagrams are also presented, which are based on time-averaged quantities and
compared with earlier empirical investigations. In the remaining part
time-series analyses of the averaged as well as the single-vehicle data are
carried out. The results will be used in order to propose objective criteria
for an identification of the different traffic states, e.g. synchronized
traffic.Comment: 12 pages, 19 figures, RevTe
Thermodynamical Properties of a Spin 1/2 Heisenberg Chain Coupled to Phonons
We performed a finite-temperature quantum Monte Carlo simulation of the
one-dimensional spin-1/2 Heisenberg model with nearest-neighbor interaction
coupled to Einstein phonons. Our method allows to treat easily up to 100
phonons per site and the results presented are practically free from truncation
errors. We studied in detail the magnetic susceptibility, the specific heat,
the phonon occupation, the dimerization, and the spin-correlation function for
various spin-phonon couplings and phonon frequencies. In particular we give
evidence for the transition from a gapless to a massive phase by studying the
finite-size behavior of the susceptibility. We also show that the dimerization
is proportional to for .Comment: 10 pages, 17 Postscript Figure
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