689 research outputs found
Ab-initio study of the anomalies in the He atom scattering spectra of H/Mo(110) and H/W(110)
Helium atom scattering (HAS) studies of the H-covered Mo(110) and W(110)
surfaces reveal a twofold anomaly in the respective dispersion curves. In order
to explain this unusual behavior we performed density functional theory
calculations of the atomic and electronic structure, the vibrational
properties, and the spectrum of electron-hole excitations of those surfaces.
Our work provides evidence for hydrogen adsorption induced Fermi surface
nesting. The respective nesting vectors are in excellent agreement with the HAS
data and recent angle resolved photoemission experiments of the H-covered alloy
system Mo_0.95Re_0.05(110). Also, we investigated the electron-phonon coupling
and discovered that the Rayleigh phonon frequency is lowered for those critical
wave vectors. Moreover, the smaller indentation in the HAS spectra can be
clearly identified as a Kohn anomaly. Based on our results for the
susceptibility and the recently improved understanding of the He scattering
mechanism we argue that the larger anomalous dip is due to a direct interaction
of the He atoms with electron-hole excitations at the Fermi level.Comment: RevTeX, 32 pages, 17 figures, submitted to Phys. Rev.
Anomalies in He atom scattering spectra of the H-covered Mo(110) and W(110) surfaces
Helium atom scattering (HAS) studies of the H-covered Mo(110) and W(110)
surfaces reveal a twofold anomaly in the respective dispersion curves. In order
to explain this unusual behavior we performed density-functional theory
calculations of the atomic and electronic structure, the vibrational
properties, and the electronic susceptibility of those surfaces. Our work
provides evidence for hydrogen adsorption induced Fermi-{}surface nesting. The
respective nesting vectors are in excellent agreement with the HAS data and
recent angle resolved photoemission experiments of the H-covered alloy system
Mo_0.95Re_0.05(110). Also, we investigated the electron-phonon coupling and
discovered that the Rayleigh phonon frequency is lowered for those critical
wave vectors compared to the clean surfaces. Moreover, the smaller indentation
in the HAS spectra can be clearly identified as a Kohn anomaly. Based on our
results for the susceptibility and the recently improved understanding of the
He scattering mechanism we argue that the larger anomalous dip is due to
electron-hole excitations by the He scattering.Comment: 7 pages, RevTeX, 7 figure
Reordering Buffer Management with a Logarithmic Guarantee in General Metric Spaces
In the reordering buffer management problem a sequence of requests arrive online in a finite metric space, and have to be processed by a single server. This server is equipped with a request buffer of size k and can decide at each point in time, which request from its buffer to serve next. Servicing of a request is simply done by moving the server to the location of the request. The goal is to process all requests while minimizing the total distance that the server is traveling inside the metric space.
In this paper we present a deterministic algorithm for the reordering buffer management problem that achieves a competitive ratio of O(log Delta + min {log n,log k}) in a finite metric space of n points and aspect ratio Delta. This is the first algorithm that works for general metric spaces and has just a logarithmic dependency on the relevant parameters. The guarantee is memory-robust, i.e., the competitive ratio decreases only slightly when the buffer-size of the optimum is increased to h=(1+epsilon)k. For memory robust guarantees our bounds are close to optimal
Enhanced electron-phonon coupling at the Mo and W(110) surfaces induced by adsorbed hydrogen
The possible occurrence of either a charge-density-wave or a Kohn anomaly is
governed by the presence of Fermi-surface nesting and the subtle interaction of
electrons and phonons. Recent experimental and theoretical investigations
suggest such an effect for the hydrogen covered Mo and W(110) surfaces. Using
density-functional theory we examine the electronic structure and the
electron-phonon coupling of these systems. Besides good agreement with the
experimental phonon frequencies our study provides a characterization and
quantitative analysis of an unusual scenario determining the electronic,
vibrational, and structural properties of these surfaces.Comment: 4 Pages, RevTe
Data-driven distributed MPC of dynamically coupled linear systems
In this paper, we present a data-driven distributed model predictive control (MPC) scheme to stabilise the origin of dynamically coupled discrete-time linear systems subject to decoupled input constraints. The local optimisation problems solved by the subsystems rely on a distributed adaptation of the Fundamental Lemma by Willems et al., allowing to parametrise system trajectories using only measured input-output data without explicit model knowledge. For the local predictions, the subsystems rely on communicated assumed trajectories of neighbours. Each subsystem guarantees a small deviation from these trajectories via a consistency constraint. We provide a theoretical analysis of the resulting non-iterative distributed MPC scheme, including proofs of recursive feasibility and (practical) stability. Finally, the approach is successfully applied to a numerical example
Material ecology
The world of design has been dominated since the Industrial Revolution by the rigors of manufacturing and mass production. Assembly lines have dictated a world made of standard parts framing the imagination of designers and builders who have been taught to think about their design objects and systems in terms of assemblies of parts with distinct functions. The assumption that parts are made of single material and fulfill predetermined specific functions is deeply rooted in design and usually goes unquestioned; it is also enforced by the way that industrial supply chains work. These age-old design paradigms have been reincarnated in Computer-aided Design (CAD) tools as well as Computer-aided Manufacturing (CAM) technologies where homogeneous materials are formed into pre-defined shapes at the service of pre-determined functions
Sleep-Dependent Memory Consolidation and Incremental Sentence Comprehension : Computational Dependencies during Language Learning as Revealed by Neuronal Oscillations
We hypothesize a beneficial influence of sleep on the consolidation of the combinatorial mechanisms underlying incremental sentence comprehension. These predictions are grounded in recent work examining the effect of sleep on the consolidation of linguistic information, which demonstrate that sleep-dependent neurophysiological activity consolidates the meaning of novel words and simple grammatical rules. However, the sleep-dependent consolidation of sentence-level combinatorics has not been studied to date. Here, we propose that dissociable aspects of sleep neurophysiology consolidate two different types of combinatory mechanisms in human language: sequence-based (order-sensitive) and dependency-based (order-insensitive) combinatorics. The distinction between the two types of combinatorics is motivated both by cross-linguistic considerations and the neurobiological underpinnings of human language. Unifying this perspective with principles of sleep-dependent memory consolidation, we posit that a function of sleep is to optimize the consolidation of sequence-based knowledge (thewhen) and the establishment of semantic schemas of unordered items (thewhat) that underpin cross-linguistic variations in sentence comprehension. This hypothesis builds on the proposal that sleep is involved in the construction of predictive codes, a unified principle of brain function that supports incremental sentence comprehension. Finally, we discuss neurophysiological measures (EEG/MEG) that could be used to test these claims, such as the quantification of neuronal oscillations, which reflect basic mechanisms of information processing in the brain
Tracking Brownian motion in three dimensions and characterization of individual nanoparticles using a fiber-based high-finesse microcavity
The dynamics of nanosystems in solution contain a wealth of information with
relevance for diverse fields ranging from materials science to biology and
biomedical applications. When nanosystems are marked with fluorophores or
strong scatterers, it is possible to track their position and reveal internal
motion with high spatial and temporal resolution. However, markers can be
toxic, expensive, or change the object's intrinsic properties. Here, we
simultaneously measure dispersive frequency shifts of three transverse modes of
a high-finesse microcavity to obtain the three-dimensional path of unlabeled
SiO nanospheres with s temporal and down to nm
spatial resolution. This allows us to quantitatively determine properties such
as the polarizability, hydrodynamic radius, and effective refractive index. The
fiber-based cavity is integrated in a direct-laser-written microfluidic device
that enables the precise control of the fluid with ultra-small sample volumes.
Our approach enables quantitative nanomaterial characterization and the
analysis of biomolecular motion at high bandwidth.Comment: 7 pages, 3 figure
Thermo-cyclically operated metal oxide gas sensor arrays for analysis of dissolved volatile organic compounds in fermentation processes: Part II – Quasi online monitoring in biogas fermentation
This study presents a quasi-online method for monitoring of dissolved volatile fatty acids (VFAs) in biogas fermentation processes with a carrier gas probe by use of thermo-cyclically operated metal oxide gas sensor arrays. Each of the two sensor arrays comprises a pure SnO and three different SnO/additive-composites (additives: alumina, YSZ, NASICON) but differ by SnO synthesis routes, namely Flame Spray Pyrolysis (FSP) and Sol-Gel (SG) technique, respectively. This allowed comparative studies of the influence of layer morphology on VFA sensing characteristics. For sensitive determination of the dissolved VFAs besides high concentrations of biogas components like CO or CH, first a pre-treatment routine of the fermentation sample was introduced to remove those physically dissolved gases without losing VFAs. The Conductance-over-Time-Profiles (CTPs) of eight different sensing layers were measured simultaneously at exposure to the gases extracted from the fermentation sample at different pH conditions. Almost all the investigated SnO/additive-composites show CTP-features clearly correlating with the undissociated VFA even at concentrations below 120 ppm as referenced by GC-analysis. The lower detection limit is well below inhibitory concentration for fermentation processes. As expected, most pronounced CTPs representing actual VFAs situation were measured at pH 3, well below the pKa of the VFAs. The FSP-layers highlighted clearly better sensitivity and CTP specificity of higher quality compared to SG-layers. Among the SnO/additives, the CTP-features of the SnO(FSP)/NASICON and SnO(SG)/NASICON layers showed the best specificity to acetic and propionic acid. For the first time, quasi-online analysis of VFAs using metal oxide gas sensors for early warning of VFA-development in biogas fermentation processes was demonstrated
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