396 research outputs found
Performance of Several Low‐Cost Accelerometers
Several groups are implementing low-cost host-operated systems of strong-motion accelerographs to support the somewhat divergent needs of seismologists and earthquake engineers. The Advanced National Seismic System Technical Implementation Committee (ANSS TIC, 2002), managed by the U.S. Geological Survey (USGS) in cooperation with other network operators, is exploring the efficacy of such systems if used in ANSS networks. To this end, ANSS convened a working group to explore available Class C strong-motion accelerometers (defined later), and to consider operational and quality control issues, and the means of annotating, storing, and using such data in ANSS networks. The working group members are largely coincident with our author list, and this report informs instrument-performance matters in the working group’s report to ANSS. Present examples of operational networks of such devices are the Community Seismic Network (CSN; csn.caltech.edu), operated by the California Institute of Technology, and Quake-Catcher Network (QCN; Cochran et al., 2009; qcn.stanford.edu; November 2013), jointly operated by Stanford University and the USGS. Several similar efforts are in development at other institutions. The overarching goals of such efforts are to add spatial density to existing Class-A and Class-B (see next paragraph) networks at low cost, and to include many additional people so they become invested in the issues of earthquakes, their measurement, and the damage they cause
Probing structural relaxation in complex fluids by critical fluctuations
Complex fluids, such as polymer solutions and blends, colloids and gels, are
of growing interest in fundamental and applied soft-condensed-matter science. A
common feature of all such systems is the presence of a mesoscopic structural
length scale intermediate between atomic and macroscopic scales. This
mesoscopic structure of complex fluids is often fragile and sensitive to
external perturbations. Complex fluids are frequently viscoelastic (showing a
combination of viscous and elastic behaviour) with their dynamic response
depending on the time and length scales. Recently, non-invasive methods to
infer the rheological response of complex fluids have gained popularity through
the technique of microrheology, where the diffusion of probe spheres in a
viscoelastic fluid is monitored with the aid of light scattering or microscopy.
Here we propose an alternative to traditional microrheology that does not
require doping of probe particles in the fluid (which can sometimes drastically
alter the molecular environment). Instead, our proposed method makes use of the
phenomenon of "avoided crossing" between modes associated with the structural
relaxation and critical fluctuations that are spontaneously generated in the
system.Comment: 4 pages, 4 figure
Coherence length of an elongated condensate: a study by matter-wave interferometry
We measure the spatial correlation function of Bose-Einstein condensates in
the cross-over region between phase-coherent and strongly phase-fluctuating
condensates. We observe the continuous path from a gaussian-like shape to an
exponential-like shape characteristic of one-dimensional phase-fluctuations.
The width of the spatial correlation function as a function of the temperature
shows that the condensate coherence length undergoes no sharp transition
between these two regimes.Comment: 8 pages, 6 figure, submitted to EPJ
Phase transitions in the boson-fermion resonance model in one dimension
We study 1D fermions with photoassociation or with a narrow Fano-Feshbach
resonance described by the Boson-Fermion resonance model. Using thebosonization
technique, we derive a low-energy Hamiltonian of the system. We show that at
low energy, the order parameters for the Bose Condensation and fermion
superfluidity become identical, while a spin gap and a gap against the
formation of phase slips are formed. As a result of these gaps, charge density
wave correlations decay exponentially in contrast with the phases where only
bosons or only fermions are present. We find a Luther-Emery point where the
phase slips and the spin excitations can be described in terms of
pseudofermions. This allows us to provide closed form expressions of the
density-density correlations and the spectral functions. The spectral functions
of the fermions are gapped, whereas the spectral functions of the bosons remain
gapless. The application of a magnetic field results in a loss of coherence
between the bosons and the fermion and the disappearance of the gap. Changing
the detuning has no effect on the gap until either the fermion or the boson
density is reduced to zero. Finally, we discuss the formation of a Mott
insulating state in a periodic potential. The relevance of our results for
experiments with ultracold atomic gases subject to one-dimensional confinement
is also discussed.Comment: 31 pages, 8 EPS figures, RevTeX 4, long version of cond-mat/050570
Finite-temperature correlations in the one-dimensional trapped and untrapped Bose gases
We calculate the dynamic single-particle and many-particle correlation
functions at non-zero temperature in one-dimensional trapped repulsive Bose
gases. The decay for increasing distance between the points of these
correlation functions is governed by a scaling exponent that has a universal
expression in terms of observed quantities. This expression is valid in the
weak-interaction Gross-Pitaevskii as well as in the strong-interaction
Girardeau-Tonks limit, but the observed quantities involved depend on the
interaction strength. The confining trap introduces a weak center-of-mass
dependence in the scaling exponent. We also conjecture results for the
density-density correlation function.Comment: 18 pages, Latex, Revtex
Momentum distribution and correlation function of quasicondensates in elongated traps
We calculate the spatial correlation function and momentum distribution of a
phase-fluctuating, elongated three-dimensional condensate, in a trap and in
free expansion. We take the inhomogeneous density profile into account
{\it{via}} a local density approximation. We find an almost Lorentzian momentum
distribution, in stark contrast with a Heisenberg-limited Thomas-Fermi
condensate.Comment: 5 pages, 2 figures; final version, references update
Impact of actin filament stabilization on adult hippocampal and olfactory bulb neurogenesis
Rearrangement of the actin cytoskeleton is essential for dynamic cellular processes. Decreased actin turnover and rigidity of cytoskeletal structures have been associated with aging and cell death. Gelsolin is a Ca(2+)-activated actin-severing protein that is widely expressed throughout the adult mammalian brain. Here, we used gelsolin-deficient (Gsn(-/-)) mice as a model system for actin filament stabilization. In Gsn(-/-) mice, emigration of newly generated cells from the subventricular zone into the olfactory bulb was slowed. In vitro, gelsolin deficiency did not affect proliferation or neuronal differentiation of adult neural progenitors cells (NPCs) but resulted in retarded migration. Surprisingly, hippocampal neurogenesis was robustly induced by gelsolin deficiency. The ability of NPCs to intrinsically sense excitatory activity and thereby implement coupling between network activity and neurogenesis has recently been established. Depolarization-induced [Ca(2+)](i) increases and exocytotic neurotransmitter release were enhanced in Gsn(-/-) synaptosomes. Importantly, treatment of Gsn(-/-) synaptosomes with mycotoxin cytochalasin D, which, like gelsolin, produces actin disassembly, decreased enhanced Ca(2+) influx and subsequent exocytotic norepinephrine release to wild-type levels. Similarly, depolarization-induced glutamate release from Gsn(-/-) brain slices was increased. Furthermore, increased hippocampal neurogenesis in Gsn(-/-) mice was associated with a special microenvironment characterized by enhanced density of perfused vessels, increased regional cerebral blood flow, and increased endothelial nitric oxide synthase (NOS-III) expression in hippocampus. Together, reduced filamentous actin turnover in presynaptic terminals causes increased Ca(2+) influx and, subsequently, elevated exocytotic neurotransmitter release acting on neural progenitors. Increased neurogenesis in Gsn(-/-) hippocampus is associated with a special vascular niche for neurogenesis
Coherent matter wave inertial sensors for precision measurements in space
We analyze the advantages of using ultra-cold coherent sources of atoms for
matter-wave interferometry in space. We present a proof-of-principle experiment
that is based on an analysis of the results previously published in [Richard et
al., Phys. Rev. Lett., 91, 010405 (2003)] from which we extract the ratio h/m
for 87Rb. This measurement shows that a limitation in accuracy arises due to
atomic interactions within the Bose-Einstein condensate
TURBOMOLE: Modular program suite for ab initio quantum-chemical and condensed-matter simulations
TURBOMOLE is a collaborative, multi-national software development project aiming to provide highly efficient and stable computational tools for quantum chemical simulations of molecules, clusters, periodic systems, and solutions. The TURBOMOLE software suite is optimized for widely available, inexpensive, and resource-efficient hardware such as multi-core workstations and small computer clusters. TURBOMOLE specializes in electronic structure methods with outstanding accuracy–cost ratio, such as density functional theory including local hybrids and the random phase approximation (RPA), GW-Bethe–Salpeter methods, second-order Møller–Plesset theory, and explicitly correlated coupled-cluster methods. TURBOMOLE is based on Gaussian basis sets and has been pivotal for the development of many fast and low-scaling algorithms in the past three decades, such as integral-direct methods, fast multipole methods, the resolution-of-the-identity approximation, imaginary frequency integration, Laplace transform, and pair natural orbital methods. This review focuses on recent additions to TURBOMOLE’s functionality, including excited-state methods, RPA and Green’s function methods, relativistic approaches, high-order molecular properties, solvation effects, and periodic systems. A variety of illustrative applications along with accuracy and timing data are discussed. Moreover, available interfaces to users as well as other software are summarized. TURBOMOLE’s current licensing, distribution, and support model are discussed, and an overview of TURBOMOLE’s development workflow is provided. Challenges such as communication and outreach, software infrastructure, and funding are highlighted
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