130 research outputs found
Order-by-disorder in classical oscillator systems
We consider classical nonlinear oscillators on hexagonal lattices. When the
coupling between the elements is repulsive, we observe coexisting states, each
one with its own basin of attraction. These states differ by their degree of
synchronization and by patterns of phase-locked motion. When disorder is
introduced into the system by additive or multiplicative Gaussian noise, we
observe a non-monotonic dependence of the degree of order in the system as a
function of the noise intensity: intervals of noise intensity with low
synchronization between the oscillators alternate with intervals where more
oscillators are synchronized. In the latter case, noise induces a higher degree
of order in the sense of a larger number of nearly coinciding phases. This
order-by-disorder effect is reminiscent to the analogous phenomenon known from
spin systems. Surprisingly, this non-monotonic evolution of the degree of order
is found not only for a single interval of intermediate noise strength, but
repeatedly as a function of increasing noise intensity. We observe noise-driven
migration of oscillator phases in a rough potential landscape.Comment: 12 pages, 13 figures; comments are welcom
Pair-factorized steady states on arbitrary graphs
Stochastic mass transport models are usually described by specifying hopping
rates of particles between sites of a given lattice, and the goal is to predict
the existence and properties of the steady state. Here we ask the reverse
question: given a stationary state that factorizes over links (pairs of sites)
of an arbitrary connected graph, what are possible hopping rates that converge
to this state? We define a class of hopping functions which lead to the same
steady state and guarantee current conservation but may differ by the induced
current strength. For the special case of anisotropic hopping in two dimensions
we discuss some aspects of the phase structure. We also show how this case can
be traced back to an effective zero-range process in one dimension which is
solvable for a large class of hopping functions.Comment: IOP style, 9 pages, 1 figur
Eine verlustleistungsoptimierte Dezimator-Architektur für kaskadierte Sigma-Delta Analog-Digital Umsetzer
Dieser Beitrag stellt einen neuartigen Ansatz einer leistungsfähigen Dezimator-Architektur f¨ur kaskadierte Sigma-Delta Modulatoren vor. Die Besonderheit der dargestellten Struktur ist die Integration der Rekombinationslogik kaskadierter Modulatoren und der Korrektur des Verstärkungsfehlers zeitkontinuierlicher (continuous time, CT) Modulatoren in die erste Stufe des Dezimators. Der Entwurf einer passenden Filtertopologie wird abgeleitet, analysiert und durch Simulationen verifiziert. Die vorgeschlagene Struktur wird mit einer herk¨ommlichen Implementierung verglichen. Das Ergebnis dieses Vergleiches ist eine Verbesserung der Effizienz um Dekaden
Towards the Deconfinement Phase Transition in Hot Gauge Theories
The phase structure of hot gauge theories with dynamical matter fields is
reexamined in the canonical ensemble with respect to triality. We discuss
properties of chromoelectric and chromomagnetic sectors of the theory and show
whereas electric charges carrying a unit of Z(N) charge are screened at high
temperatures via dynamical matter loops, this is not the case for the Z(N)
magnetic flux. An order parameter is constructed to probe the realization of
local Z(N) symmetry in the magnetic sector. We argue this order parameter may
be used to detect the deconfinement phase transition which is defined in terms
of the screening mechanism.Comment: poster presented at LATTICE97; 3 pages, late
State scaling of continuous-time sigma-delta modulators
In this paper, the common method of scaling the feedback coefficients of
continuous time sigma delta modulators in order to stabilize the system is
enhanced. The presented approach scales the different states of the system
instead of the coefficients. The new corresponding coefficients are then
calculated from the solution of the state space description. Therewith, it is
possible to tune the maximum out-of-band gain directly in continuous time. In
addition, the input amplitude distribution between each quantization level of
multi bit sigma-delta modulator can be adapted
Implementierung eines verlustleistungsoptimierten Dezimators für kaskadierte Sigma-Delta Analog-Digital Umsetzer
Dieser Beitrag stellt die Implementierung eines neuartigen Ansatzes einer effizienten Dezimator-Architektur für kaskadierte Sigma-Delta Modulatoren vor. Die Rekombinationslogik kaskadierter Modulatoren und die Korrektur des Verstärkungsfehlers zeitkontinuierlicher (CT) Modulatoren werden in die erste Stufe des Dezimators integriert. Eine entsprechende Filtertopologie wird hergeleitet und auf einem Hardware-Emulator der Firma Mentor Graphics implementiert. Der Vergleich der vorgeschlagenen Struktur mit einer herkömmlichen Implementierung zeigt eine nennenswerte Verbesserung der Effizienz
Critical Phenomena with Linked Cluster Expansions in a Finite Volume
Linked cluster expansions are generalized from an infinite to a finite
volume. They are performed to 20th order in the expansion parameter to approach
the critical region from the symmetric phase. A new criterion is proposed to
distinguish 1st from 2nd order transitions within a finite size scaling
analysis. The criterion applies also to other methods for investigating the
phase structure such as Monte Carlo simulations. Our computational tools are
illustrated at the example of scalar O(N) models with four and six-point
couplings for and in three dimensions. It is shown how to localize
the tricritical line in these models. We indicate some further applications of
our methods to the electroweak transition as well as to models for
superconductivity.Comment: 36 pages, latex2e, 7 eps figures included, uuencoded, gzipped and
tarred tex file hdth9607.te
STF Optimierung von single-bit CT ΣΔ Modulatoren basierend auf skalierten Filterkoeffizienten
Die vorliegende Arbeit beschäftigt sich mit dem Signalübertragungsverhalten von single-bit continuous-time (CT) ΣΔ Modulatoren. Dabei liegt der Fokus der Untersuchung auf dem Peaking der Signaltransferfunktion (STF). Dieser Effekt kann die Performance und die Stabilität des Gesamtsystems negativ beeinflussen, da bei auftretendem STF-Peaking Signale außerhalb des Signalbands verstärkt werden. In dieser Arbeit wird ein neuer Ansatz zur Reduktion des Peakings vorgestellt, der auf der Optimierung der Systemdynamik basiert. Dabei werden die Filterkoeffizienten des Modulators systematisch angepasst. Anhand eines Beispielsystems wird gezeigt, dass der Ansatz genutzt werden kann, um das Übertragungsverhalten des Modulators abhängig vom Ausgangssystem zu verändern. So kann entweder die Systemsperformance verbessert werden, ohne Peaking in der STF zu erzeugen, oder das STF-Peaking reduziert werden, ohne die Systemperformance stark zu beeinflussen.DF
Magnon dispersion in bilayers of two-dimensional ferromagnets
We determine magnon spectra of an atomic bilayer magnet with ferromagnetic
intra- and both ferro- and anti- ferromagnetic interlayer coupling. Analytic
expressions for the full magnon band of the latter case reveal that both
exchange interactions govern the fundamental magnon gap. The inter and
intralayer magnetic ordering are not independent: the intralayer ferromagnetism
stabilizes antiferromagnetic inter-layer order. The topology of these
exchange-anisotropy spin models without spin-orbit interaction turns out to be
trivial
Islanding the power grid on the transmission level: less connections for more security
Islanding is known as a management procedure of the power system that is implemented at the distribution level to preserve sensible loads from outages and to guarantee the continuity in electricity supply, when a high amount of distributed generation occurs. In this paper we study islanding on the level of the transmission grid and shall show that it is a suitable measure to enhance energy security and grid resilience. We consider the German and Italian transmission grids. We remove links either randomly to mimic random failure events, or according to a topological characteristic, their so-called betweenness centrality, to mimic an intentional attack and test whether the resulting fragments are self-sustainable. We test this option via the tool of optimized DC power flow equations. When transmission lines are removed according to their betweenness centrality, the resulting islands have a higher chance of being dynamically self-sustainable than for a random removal. Less connections may even increase the grid’s stability. These facts should be taken into account in the design of future power grids
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