622 research outputs found
Nearly-logarithmic decay in the colloidal hard-sphere system
Nearly-logarithmic decay is identified in the data for the mean-squared
displacement of the colloidal hard-sphere system at the liquid-glass transition
[v. Megen et. al, Phys. Rev. E 58, 6073(1998)]. The solutions of mode-coupling
theory for the microscopic equations of motion fit the experimental data well.
Based on these equations, the nearly-logarithmic decay is explained as the
equivalent of a beta-peak phenomenon, a manifestation of the critical
relaxation when the coupling between of the probe variable and the density
fluctuations is strong. In an asymptotic expansion, a Cole-Cole formula
including corrections is derived from the microscopic equations of motion,
which describes the experimental data for three decades in time.Comment: 4 pages, 3 figure
Bond formation and slow heterogeneous dynamics in adhesive spheres with long--ranged repulsion: Quantitative test of Mode Coupling Theory
A colloidal system of spheres interacting with both a deep and narrow
attractive potential and a shallow long-ranged barrier exhibits a prepeak in
the static structure factor. This peak can be related to an additional
mesoscopic length scale of clusters and/or voids in the system. Simulation
studies of this system have revealed that it vitrifies upon increasing the
attraction into a gel-like solid at intermediate densities. The dynamics at the
mesoscopic length scale corresponding to the prepeak represents the slowest
mode in the system. Using mode coupling theory with all input directly taken
from simulations, we reveal the mechanism for glassy arrest in the system at
40% packing fraction. The effects of the low-q peak and of polydispersity are
considered in detail. We demonstrate that the local formation of physical bonds
is the process whose slowing down causes arrest.
It remains largely unaffected by the large-scale heterogeneities, and sets
the clock for the slow cluster mode. Results from mode-coupling theory without
adjustable parameters agree semi-quantitatively with the local density
correlators but overestimate the lifetime of the mesoscopic structure (voids).Comment: 10 pages, 8 figure
Electrophysiological Signatures of Fear Conditioning: From Methodological Considerations to Catecholaminergic Mechanisms and Translational Perspectives
Fear conditioning describes a learning mechanism during which a specific stimulus gets associated with an aversive event (i.e., an unconditioned stimulus; US). Thereby, this initially neutral or arbitrary stimulus becomes a so-called “conditioned” stimulus (CS), which elicits a conditioned threat response. Fear extinction refers to the decrease in conditioned threat responses as soon as the CS is repeatedly presented in the absence of the US. While fear conditioning is an important learning model for understanding the etiology and maintenance of anxiety and fear-related disorders, extinction learning is considered to reflect the most important learning process of exposure therapy. Neurophysiological signatures of fear conditioning have been widely studied in rodents, leading to the development of groundbreaking neurobiological models, including brain regions such as the amygdala, insula, and prefrontal areas. These models aim to explain neural mechanisms of threat processing, with the ultimate goal to improve treatment strategies for pathological fear. Recording intracranial electrical activity of single units in animals offers the opportunity to uncover neural processes involved in threat processing with excellent spatial and temporal resolution. A large body of functional magnetic resonance imaging (fMRI) studies have helped to translate this knowledge about the anatomy of fear conditioning into the human realm. fMRI is an imaging technique with a high spatial resolution that is well suited to study slower brain processes. However, the temporal resolution of fMRI is relatively poor. By contrast, electroencephalography (EEG) is a neuroscientific method to capture fast and transient cortical processes. While EEG offers promising opportunities to unravel the speed of neural threat processing, it also provides the possibility to study oscillatory brain activity (e.g., prefrontal theta oscillations). The present thesis contains six research manuscripts, describing fear conditioning studies that mainly applied EEG methods in combination with other central (fMRI) and peripheral (skin conductance, heart rate, and fear-potentiated startle) measures. A special focus of this thesis lies in methodological considerations for EEG fear conditioning research. In addition, catecholaminergic mechanisms are studied, with the ultimate goal of opening up new translational perspectives. Taken together, the present thesis addresses several methodological challenges for neuroscientific (in particular, EEG) fear conditioning research (e.g., appropriate US types and experimental designs, signal-to-noise ratio, simultaneous EEG-fMRI). Furthermore, this thesis gives critical insight into catecholaminergic (noradrenaline and dopamine) mechanisms. A variety of neuroscientific methods (e.g., EEG, fMRI, peripheral physiology, pharmacological manipulation, genetic associations) have been combined, an approach that allowed us (a) to translate knowledge from animal studies to human research, and (b) to stimulate novel clinical directions
Critical Decay at Higher-Order Glass-Transition Singularities
Within the mode-coupling theory for the evolution of structural relaxation in
glass-forming systems, it is shown that the correlation functions for density
fluctuations for states at A_3- and A_4-glass-transition singularities can be
presented as an asymptotic series in increasing inverse powers of the logarithm
of the time t: , where
with p_n denoting some polynomial and x=ln (t/t_0). The results are
demonstrated for schematic models describing the system by solely one or two
correlators and also for a colloid model with a square-well-interaction
potential.Comment: 26 pages, 7 figures, Proceedings of "Structural Arrest Transitions in
Colloidal Systems with Short-Range Attractions", Messina, Italy, December
2003 (submitted
Pressure and Motion of Dry Sand -- Translation of Hagen's Paper from 1852
In a remarkable paper from 1852, Gotthilf Heinrich Ludwig Hagen measured and
explained two fundamental aspects of granular matter: The first effect is the
saturation of pressure with depth in a static granular system confined by silo
walls -- generally known as the Janssen effect. The second part of his paper
describes the dynamics observed during the flow out of the container -- today
often called the Beverloo law -- and forms the foundation of the hourglass
theory. The following is a translation of the original German paper from 1852.Comment: 4 pages, accepted for publication in Granular Matter, original
article (German) can be found under http://www.phy.duke.edu/~msperl/Janssen
The Jamming Transition in Granular Systems
Recent simulations have predicted that near jamming for collections of
spherical particles, there will be a discontinuous increase in the mean contact
number, Z, at a critical volume fraction, phi_c. Above phi_c, Z and the
pressure, P are predicted to increase as power laws in phi-phi_c. In
experiments using photoelastic disks we corroborate a rapid increase in Z at
phi_c and power-law behavior above phi_c for Z and P. Specifically we find
power-law increase as a function of phi-phi_c for Z-Z_c with an exponent beta
around 0.5, and for P with an exponent psi around 1.1. These exponents are in
good agreement with simulations. We also find reasonable agreement with a
recent mean-field theory for frictionless particles.Comment: 4 pages, 4 figures, 2 pages supplement; minor changes and
clarifications, 2 addtl. refs., accepted for publication in Phys. Rev. Let
Evolution of unoccupied resonance during the synthesis of a silver dimer on Ag(111)
Silver dimers were fabricated on Ag(111) by single-atom manipulation using
the tip of a cryogenic scanning tunnelling microscope. An unoccupied electronic
resonance was observed to shift toward the Fermi level with decreasing
atom-atom distance as monitored by spatially resolved scanning tunnelling
spectroscopy. Density functional calculations were used to analyse the
experimental observations and revealed that the coupling between the adsorbed
atoms is predominantly direct rather than indirect via the Ag(111) substrate.Comment: 9 pages, 3 figure
Glass glass transition and new dynamical singularity points in an analytically solvable p-spin glass like model
We introduce and analytically study a generalized p-spin glass like model
that captures some of the main features of attractive glasses, recently found
by Mode Coupling investigations, such as a glass/glass transition line and
dynamical singularity points characterized by a logarithmic time dependence of
the relaxation. The model also displays features not predicted by the Mode
Coupling scenario that could further describe the attractive glasses behavior,
such as aging effects with new dynamical singularity points ruled by
logarithmic laws or the presence of a glass spinodal line
Anomaly Detection by Recombining Gated Unsupervised Experts
Inspired by mixture-of-experts models and the analysis of the hidden
activations of neural networks, we introduce a novel unsupervised anomaly
detection method called ARGUE. Current anomaly detection methods struggle when
the training data does contain multiple notions of normal. We designed ARGUE as
a combination of multiple expert networks, which specialise on parts of the
input data. For its final decision, ARGUE fuses the distributed knowledge
across the expert systems using a gated mixture-of-experts architecture. ARGUE
achieves superior detection performance across several domains in a purely
data-driven way and is more robust to noisy data sets than other
state-of-the-art anomaly detection methods
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