5,168 research outputs found
Spatiotemporal evolution of polaronic states in finite quantum systems
We study the quantum dynamics of small polaron formation and polaron
transport through finite quantum structures in the framework of the
one-dimensional Holstein model with site-dependent potentials and interactions.
Combining Lanczos diagonalization with Chebyshev moment expansion of the time
evolution operator, we determine how different initial states, representing
stationary ground states or injected wave packets, after an electron-phonon
interaction quench, develop in real space and time. Thereby, the full quantum
nature and dynamics of electrons and phonons is preserved. We find that the
decay out of the initial state sensitively depends on the energy and momentum
of the incoming particle, the electron-phonon coupling strength, and the phonon
frequency, whereupon bound polaron-phonon excited states may emerge in the
strong-coupling regime. The tunneling of a Holstein polaron through a quantum
wall/dot is generally accompanied by strong phonon number fluctuations due to
phonon emission and re-absorption processes.Comment: 13 pages, 15 figures, final versio
A simple spatiotemporal evolution model of a transmission power grid
In this paper, we present a model for the spatial and temporal evolution of a particularly large human-made network: the 400-kV French transmission power grid. This is based on 1) an attachment procedure that diminishes the connection probability between two nodes as the network grows and 2) a coupled cost function characterizing the available budget at every time step. Two differentiated and consecutive processes can be distinguished: a first global space-filling process and a secondary local meshing process that increases connectivity at a local level. Results show that even without power system engineering design constraints (i.e., population and energy demand), the evolution of a transmission network can be remarkably explained by means of a simple attachment procedure. Given a distribution of resources and a time span, the model can also be used to generate the probability distribution of cable lengths at every time step, thus facilitating network planning. Implications for network's fragility are suggested as a starting point for new design perspectives in this kind of infrastructures.Peer ReviewedPostprint (author's final draft
Spatiotemporal evolution of runaway electrons from synchrotron images in Alcator C-Mod
In the Alcator C-Mod tokamak, relativistic runaway electron (RE) generation
can occur during the flattop current phase of low density, diverted plasma
discharges. Due to the high toroidal magnetic field (B = 5.4 T), RE synchrotron
radiation is measured by a wide-view camera in the visible wavelength range
(~400-900 nm). In this paper, a statistical analysis of over one thousand
camera images is performed to investigate the plasma conditions under which
synchrotron emission is observed in C-Mod. In addition, the spatiotemporal
evolution of REs during one particular discharge is explored in detail via a
thorough analysis of the distortion-corrected synchrotron images. To accurately
predict RE energies, the kinetic solver CODE [Landreman et al 2014 Comput.
Phys. Commun. 185 847-855] is used to evolve the electron momentum-space
distribution at six locations throughout the plasma: the magnetic axis and flux
surfaces q = 1, 4/3, 3/2, 2, and 3. These results, along with the
experimentally-measured magnetic topology and camera geometry, are input into
the synthetic diagnostic SOFT [Hoppe et al 2018 Nucl. Fusion 58 026032] to
simulate synchrotron emission and detection. Interesting spatial structure near
the surface q = 2 is found to coincide with the onset of a locked mode and
increased MHD activity. Furthermore, the RE density profile evolution is fit by
comparing experimental to synthetic images, providing important insight into RE
spatiotemporal dynamics
An Open-Source Microscopic Traffic Simulator
We present the interactive Java-based open-source traffic simulator available
at www.traffic-simulation.de. In contrast to most closed-source commercial
simulators, the focus is on investigating fundamental issues of traffic
dynamics rather than simulating specific road networks. This includes testing
theories for the spatiotemporal evolution of traffic jams, comparing and
testing different microscopic traffic models, modeling the effects of driving
styles and traffic rules on the efficiency and stability of traffic flow, and
investigating novel ITS technologies such as adaptive cruise control,
inter-vehicle and vehicle-infrastructure communication
Particles and Fields in Superfluids: Insights from the Two-dimensional Gross-Pitaevskii Equation
We carry out extensive direct numerical simulations (DNSs) to investigate the
interaction of active particles and fields in the two-dimensional (2D)
Gross-Pitaevskii (GP) superfluid, in both simple and turbulent flows. The
particles are active in the sense that they affect the superfluid even as they
are affected by it. We tune the mass of the particles, which is an important
control parameter. At the one-particle level, we show how light, neutral, and
heavy particles move in the superfluid, when a constant external force acts on
them; in particular, beyond a critical velocity, at which a vortex-antivortex
pair is emitted, particle motion can be periodic or chaotic. We demonstrate
that the interaction of a particle with vortices leads to dynamics that depends
sensitively on the particle characteristics. We also demonstrate that
assemblies of particles and vortices can have rich, and often turbulent
spatiotemporal evolution. In particular, we consider the dynamics of the
following illustrative initial configurations: (a) one particle placed in front
of a translating vortex-antivortex pair; (b) two particles placed in front of a
translating vortex-antivortex pair; (c) a single particle moving in the
presence of counter-rotating vortex clusters; and (d) four particles in the
presence of counter-rotating vortex clusters. We compare our work with earlier
studies and examine its implications for recent experimental studies in
superfluid Helium and Bose-Einstein condensates.Comment: 24 figure
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