1,990 research outputs found
Destructive Impact of Molecular Noise on Nanoscale Electrochemical Oscillators
We study the loss of coherence of electrochemical oscillations on meso- and
nanosized electrodes with numeric simulations of the electrochemical master
equation for a prototypical electrochemical oscillator, the hydrogen peroxide
reduction on Pt electrodes in the presence of halides. On nanoelectrodes, the
electrode potential changes whenever a stochastic electron-transfer event takes
place. Electrochemical reaction rate coefficients depend exponentially on the
electrode potential and become thus fluctuating quantities as well. Therefore,
also the transition rates between system states become time-dependent which
constitutes a fundamental difference to purely chemical nanoscale oscillators.
Three implications are demonstrated: (a) oscillations and steady states shift
in phase space with decreasing system size, thereby also decreasing
considerably the oscillating parameter regions; (b) the minimal number of
molecules necessary to support correlated oscillations is more than 10 times as
large as for nanoscale chemical oscillators; (c) the relation between
correlation time and variance of the period of the oscillations predicted for
chemical oscillators in the weak noise limit is only fulfilled in a very
restricted parameter range for the electrochemical nano-oscillator.Comment: 18 pages, 9 figure
Unusual synchronization phenomena during electrodissolution of silicon: the role of nonlinear global coupling
The photoelectrodissolution of n-type silicon constitutes a convenient model
system to study the nonlinear dynamics of oscillatory media. On the silicon
surface, a silicon oxide layer forms. In the lateral direction, the thickness
of this layer is not uniform. Rather, several spatio-temporal patterns in the
oxide layer emerge spontaneously, ranging from cluster patterns and turbulence
to quite peculiar dynamics like chimera states. Introducing a nonlinear global
coupling in the complex Ginzburg-Landau equation allows us to identify this
nonlinear coupling as the essential ingredient to describe the patterns found
in the experiments. The nonlinear global coupling is designed in such a way, as
to capture an important, experimentally observed feature: the spatially
averaged oxide-layer thickness shows nearly harmonic oscillations. Simulations
of the modified complex Ginzburg-Landau equation capture the experimental
dynamics very well.Comment: To appear as a chapter in "Engineering of Chemical Complexity II"
(eds. A.S. Mikhailov and G.Ertl) at World Scientific in Singapor
Scaling full seismic waveform inversions
The main goal of this research study is to scale full seismic waveform inversions using the adjoint-state method to the data volumes that are nowadays available in seismology. Practical issues hinder the routine application of this, to a certain extent theoretically well understood, method. To a large part this comes down to outdated or flat out missing tools and ways to automate the highly iterative procedure in a reliable way.
This thesis tackles these issues in three successive stages. It first introduces a modern and properly designed data processing framework sitting at the very core of all the consecutive developments. The ObsPy toolkit is a Python library providing a bridge for seismology into the scientific Python ecosystem and bestowing seismologists with effortless I/O and a powerful signal processing library, amongst other things.
The following chapter deals with a framework designed to handle the specific data management and organization issues arising in full seismic waveform inversions, the Large-scale Seismic Inversion Framework. It has been created to orchestrate the various pieces of data accruing in the course of an iterative waveform inversion.
Then, the Adaptable Seismic Data Format, a new, self-describing, and scalable data format for seismology is introduced along with the rationale why it is needed for full waveform inversions in particular and seismology in general.
Finally, these developments are put into service to construct a novel full seismic waveform inversion model for elastic subsurface structure beneath the North American continent and the Northern Atlantic well into Europe. The spectral element method is used for the forward and adjoint simulations coupled with windowed time-frequency phase misfit measurements. Later iterations use 72 events, all happening after the USArray project has commenced, resulting in approximately 150`000 three components recordings that are inverted for. 20 L-BFGS iterations yield a model that can produce complete seismograms at a period range between 30 and 120 seconds while comparing favorably to observed data
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