1,598 research outputs found
Coherent control of a surface structural phase transition
Active optical control over matter is desirable in many scientific disciplines, with prominent examples in all-optical magnetic switching1,2, light-induced metastable or exotic phases of solids3,4,5,6,7,8 and the coherent control of chemical reactions9,10. Typically, these approaches dynamically steer a system towards states or reaction products far from equilibrium. In solids, metal-to-insulator transitions are an important target for optical manipulation, offering ultrafast changes of the electronic4 and lattice11,12,13,14,15,16 properties. The impact of coherences on the efficiencies and thresholds of such transitions, however, remains a largely open subject. Here, we demonstrate coherent control over a metalâinsulator structural phase transition in a quasi-one-dimensional solid-state surface system. A femtosecond double-pulse excitation scheme17,18,19,20 is used to switch the system from the insulating to a metastable metallic state, and the corresponding structural changes are monitored by ultrafast low-energy electron diffraction21,22. To govern the transition, we harness vibrational coherence in key structural modes connecting both phases, and observe delay-dependent oscillations in the double-pulse switching efficiency. Mode-selective coherent control of solids and surfaces could open new routes to switching chemical and physical functionalities, enabled by metastable and non-equilibrium states
Deliverable D5.7 Validation of the LinkedTV Architecture
The LinkedTV architecture lays the foundation for the LinkedTV system. It consists of the integrating platform for the end-to-end functionality, the backend components and the supporting client components. Since the architecture of a software system has a fundamental impact on quality attributes, it is important to evaluate its design. The document at hand reports on the validation of the LinkedTV architecture
Lateral prefrontal model-based signatures are reduced in healthy individuals with high trait impulsivity
High impulsivity is an important risk factor for addiction with evidence from
endophenotype studies. In addiction, behavioral control is shifted toward the
habitual end. Habitual control can be described by retrospective updating of
reward expectations in âmodel-freeâ temporal-difference algorithms. Goal-
directed control relies on the prospective consideration of actions and their
outcomes, which can be captured by forward-planning âmodel-basedâ algorithms.
So far, no studies have examined behavioral and neural signatures of model-
free and model-based control in healthy high-impulsive individuals. Fifty
healthy participants were drawn from the upper and lower ends of 452
individuals, completing the Barratt Impulsiveness Scale. All participants
performed a sequential decision-making task during functional magnetic
resonance imaging (fMRI) and underwent structural MRI. Behavioral and fMRI
data were analyzed by means of computational algorithms reflecting model-free
and model-based control. Both groups did not differ regarding the balance of
model-free and model-based control, but high-impulsive individuals showed a
subtle but significant accentuation of model-free control alone. Right lateral
prefrontal model-based signatures were reduced in high-impulsive individuals.
Effects of smoking, drinking, general cognition or gray matter density did not
account for the findings. Irrespectively of impulsivity, gray matter density
in the left dorsolateral prefrontal cortex was positively associated with
model-based control. The present study supports the idea that high levels of
impulsivity are accompanied by behavioral and neural signatures in favor of
model-free behavioral control. Behavioral results in healthy high-impulsive
individuals were qualitatively different to findings in patients with the same
task. The predictive relevance of these results remains an important target
for future longitudinal studies
Finite mass self-similar blowing-up solutions of a chemotaxis system with non-linear diffusion
For a specific choice of the diffusion, the parabolic-elliptic
Patlak-Keller-Segel system with non-linear diffusion (also referred to as the
quasi-linear Smoluchowski-Poisson equation) exhibits an interesting threshold
phenomenon: there is a critical mass such that all the solutions with
initial data of mass smaller or equal to exist globally while the
solution blows up in finite time for a large class of initial data with mass
greater than . Unlike in space dimension 2, finite mass self-similar
blowing-up solutions are shown to exist in space dimension
The Presampler for the Forward and Rear Calorimeter in the ZEUS Detector
The ZEUS detector at HERA has been supplemented with a presampler detector in
front of the forward and rear calorimeters. It consists of a segmented
scintillator array read out with wavelength-shifting fibers. We discuss its
desi gn, construction and performance. Test beam data obtained with a prototype
presampler and the ZEUS prototype calorimeter demonstrate the main function of
this detector, i.e. the correction for the energy lost by an electron
interacting in inactive material in front of the calorimeter.Comment: 20 pages including 16 figure
Critical dynamics of self-gravitating Langevin particles and bacterial populations
We study the critical dynamics of the generalized Smoluchowski-Poisson system
(for self-gravitating Langevin particles) or generalized Keller-Segel model
(for the chemotaxis of bacterial populations). These models [Chavanis & Sire,
PRE, 69, 016116 (2004)] are based on generalized stochastic processes leading
to the Tsallis statistics. The equilibrium states correspond to polytropic
configurations with index similar to polytropic stars in astrophysics. At
the critical index (where is the dimension of space),
there exists a critical temperature (for a given mass) or a
critical mass (for a given temperature). For or
the system tends to an incomplete polytrope confined by the box (in a
bounded domain) or evaporates (in an unbounded domain). For
or the system collapses and forms, in a finite time, a Dirac peak
containing a finite fraction of the total mass surrounded by a halo. This
study extends the critical dynamics of the ordinary Smoluchowski-Poisson system
and Keller-Segel model in corresponding to isothermal configurations with
. We also stress the analogy between the limiting mass of
white dwarf stars (Chandrasekhar's limit) and the critical mass of bacterial
populations in the generalized Keller-Segel model of chemotaxis
Mathematical description of bacterial traveling pulses
The Keller-Segel system has been widely proposed as a model for bacterial
waves driven by chemotactic processes. Current experiments on {\em E. coli}
have shown precise structure of traveling pulses. We present here an
alternative mathematical description of traveling pulses at a macroscopic
scale. This modeling task is complemented with numerical simulations in
accordance with the experimental observations. Our model is derived from an
accurate kinetic description of the mesoscopic run-and-tumble process performed
by bacteria. This model can account for recent experimental observations with
{\em E. coli}. Qualitative agreements include the asymmetry of the pulse and
transition in the collective behaviour (clustered motion versus dispersion). In
addition we can capture quantitatively the main characteristics of the pulse
such as the speed and the relative size of tails. This work opens several
experimental and theoretical perspectives. Coefficients at the macroscopic
level are derived from considerations at the cellular scale. For instance the
stiffness of the signal integration process turns out to have a strong effect
on collective motion. Furthermore the bottom-up scaling allows to perform
preliminary mathematical analysis and write efficient numerical schemes. This
model is intended as a predictive tool for the investigation of bacterial
collective motion
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