497 research outputs found
An experimental study of the effect of off-fault damage on the velocity of a slip pulse
The effect of off-fault damage on the speed of ruptures propagating on faults in photoelastic Homalite plates was measured using high-speed digital photography. The off-fault damage was composed of a network of fractures introduced by thermally shocking the Homalite in liquid nitrogen. The mode II rupture speed measured in damaged Homalite was significantly lower than the limiting Rayleigh speed of v_r = 0.92 v_s, even after the shear wave speed v_s was reduced to a value appropriate for the fracture-damaged Homalite. The additional slowing is most likely caused by frictional sliding on preexisting cracks, especially since we did not observe the generation of new fractures. The spatial extent of the interaction between the rupture and the off-fault damage was measured using samples in which the damage was limited to a band of width 2w centered on the fault and also using damaged samples containing a band of undamaged Homalite centered on the fault. By measuring the rupture velocity as a function of w, the interaction between the rupture and off-fault damage was observed to be limited to a distance of about 1 cm from the fault plane. This agrees with the spatial extent of Coulomb failure near the tip of a dynamic slip pulse predicted by the analytic model developed by Rice et al. (2005)
Spectrum and transition rates of the XX chain analyzed via Bethe ansatz
As part of a study that investigates the dynamics of the s=1/2 XXZ model in
the planar regime |Delta|<1, we discuss the singular nature of the Bethe ansatz
equations for the case Delta=0 (XX model). We identify the general structure of
the Bethe ansatz solutions for the entire XX spectrum, which include states
with real and complex magnon momenta. We discuss the relation between the
spinon or magnon quasiparticles (Bethe ansatz) and the lattice fermions
(Jordan-Wigner representation). We present determinantal expressions for
transition rates of spin fluctuation operators between Bethe wave functions and
reduce them to product expressions. We apply the new formulas to two-spinon
transition rates for chains with up to N=4096 sites.Comment: 11 pages, 4 figure
Linkage analysis in a large kindred with autosomal dominant transmission of polyglandular autoimmune disease type II (Schmidt syndrome)
Schmidt syndrome (PGA syndrome type II) is a rare condition characterized by polyglandular failure. It is an autosomal dominant trait with variable expressivity that was inherited over four generations in an Indiana kindred. Association of HLA-B8 has been reported with Schmidt syndrome. Our proband is a 12-year-old boy with Addison disease, insulin dependent diabetes mellitus (IDDM), and vitiligo. Two of his eight sibs had either IDDM (sister) or vitiligo and hyperthyroidism (brother). His mother had hypothyroidism. Seven members of earlier generations apparently were also affected. We obtained peripheral blood for HLA and genetic analysis from 21 relatives in a family with 8 Schmidt syndrome individuals in three generations. HLA studies on 15 affected and unaffected relatives showed only 2 of 7 persons with B8-containing haplotypes. Therefore, no association exists between the B8-containing haplotype and the syndrome.
We identified informative marker loci. No evidence for linkage of the Schmidt locus to any of the 14 markers was found and close linkage to esterase D and adenylate kinase and possibly properdin factor B was excluded
The two-spinon transverse structure factor of the gapped Heisenberg antiferromagnetic chain
We consider the transverse dynamical structure factor of the anisotropic
Heisenberg spin-1/2 chain (XXZ model) in the gapped antiferromagnetic regime
(). Specializing to the case of zero field, we use two independent
approaches based on integrability (one valid for finite size, the other for the
infinite lattice) to obtain the exact two-spinon part of this correlator. We
discuss in particular its asymmetry with respect to the momentum line,
its overall anisotropy dependence, and its contribution to sum rules.Comment: 19 pages, 6 figure
Quasiparticles governing the zero-temperature dynamics of the 1D spin-1/2 Heisenberg antiferromagnet in a magnetic field
The T=0 dynamical properties of the one-dimensional (1D)
Heisenberg antiferromagnet in a uniform magnetic field are studied via Bethe
ansatz for cyclic chains of sites. The ground state at magnetization
, which can be interpreted as a state with spinons or as a
state of magnons, is reconfigured here as the vacuum for a different
species of quasiparticles, the {\em psinons} and {\em antipsinons}. We
investigate three kinds of quantum fluctuations, namely the spin fluctuations
parallel and perpendicular to the direction of the applied magnetic field and
the dimer fluctuations. The dynamically dominant excitation spectra are found
to be sets of collective excitations composed of two quasiparticles excited
from the psinon vacuum in different configurations. The Bethe ansatz provides a
framework for (i) the characterization of the new quasiparticles in relation to
the more familiar spinons and magnons, (ii) the calculation of spectral
boundaries and densities of states for each continuum, (iii) the calculation of
transition rates between the ground state and the dynamically dominant
collective excitations, (iv) the prediction of lineshapes for dynamic structure
factors relevant for experiments performed on a variety of quasi-1D
antiferromagnetic compounds, including KCuF,
Cu(CHN, and CuGeO.Comment: 13 pages, 12 figure
Computation of dynamical correlation functions of Heisenberg chains: the gapless anisotropic regime
We compute all dynamical spin-spin correlation functions for the spin-1/2
anisotropic Heisenberg model in the gapless antiferromagnetic regime,
using numerical sums of exact determinant representations for form factors of
spin operators on the lattice. Contributions from intermediate states
containing many particles and string (bound) states are included. We present
modified determinant representations for the form factors valid in the general
case with string solutions to the Bethe equations. Our results are such that
the available sum rules are saturated to high precision. We Fourier transform
our results back to real space, allowing us in particular to make a comparison
with known exact formulas for equal-time correlation functions for small
separations in zero field, and with predictions for the zero-field asymptotics
from conformal field theory.Comment: 14 page
A deeply branching thermophilic bacterium with an ancient acetyl-CoA pathway dominates a subsurface ecosystem
<div><p>A nearly complete genome sequence of <em>Candidatus</em> ‘Acetothermum autotrophicum’, a presently uncultivated bacterium in candidate division OP1, was revealed by metagenomic analysis of a subsurface thermophilic microbial mat community. Phylogenetic analysis based on the concatenated sequences of proteins common among 367 prokaryotes suggests that <em>Ca.</em> ‘A. autotrophicum’ is one of the earliest diverging bacterial lineages. It possesses a folate-dependent Wood-Ljungdahl (acetyl-CoA) pathway of CO<sub>2</sub> fixation, is predicted to have an acetogenic lifestyle, and possesses the newly discovered archaeal-autotrophic type of bifunctional fructose 1,6-bisphosphate aldolase/phosphatase. A phylogenetic analysis of the core gene cluster of the acethyl-CoA pathway, shared by acetogens, methanogens, some sulfur- and iron-reducers and dechlorinators, supports the hypothesis that the core gene cluster of <em>Ca.</em> ‘A. autotrophicum’ is a particularly ancient bacterial pathway. The habitat, physiology and phylogenetic position of <em>Ca.</em> ‘A. autotrophicum’ support the view that the first bacterial and archaeal lineages were H<sub>2</sub>-dependent acetogens and methanogenes living in hydrothermal environments.</p> </div
Preliminary definition of CORTEX interaction model
As scheduled in the Technical Annex, WP2-D3 comprises work on the basic communication abstractions and the context and environmental awareness. It is structured in an introduction, providing a short survey of the content and four technical chapters. Chapter 2 describes the notion of event channels as a basic middleware abstraction of the interaction model. The concept of event channels accommodates an event-based, generative, many-to-many, anonymous communication model. It contributes to the resolution of the trade-off between autonomy and the need of coordination. Rather than explicitly coordinating actions by transferring control, an event channel allows interaction via a shared data space, thereby maintaining the autonomy of components. A comparison with alternative schemes is presented in chapter 3. Here, the impact of the interaction scheme on the modelling and implementation of a complex robotic application is analysed. It provides additional arguments in favour of a publisher/subscriber communication architecture. One of the challenges in CORTEX is to integrate the cooperation of components through the environment into the general interaction concept. The sensor capabilities of the sentient components and their ability to interact with the environment open new ways of cooperation. A mechanism called Stigmergy which is borrowed from biology and discussed in the CORTEX context is presented in chapter 4. Any activity which is carried out in the physical world needs to adapt to the pace and dependability requirements dictated by the environment. In technical terms this means that non-functional properties of the system, as timeliness and reliability of operation have to be included. These Quality of Service (QoS) attributes have to be guaranteed even in an environment where unanticipated dynamic change is one of the inherent properties. Chapter 5 introduces an adaptive QoS mechanism based on a reliable and timely system service. This service, called the Timely Computing Base (TCB) is able to monitor distributed system activities and to provide an "early warning system" for temporal and functional failures. The TCB thus provides part of the context and environmental awareness needed for adaptatio
Spinons and triplons in spatially anisotropic frustrated antiferromagnets
The search for elementary excitations with fractional quantum numbers is a
central challenge in modern condensed matter physics. We explore the
possibility in a realistic model for several materials, the spin-1/2 spatially
anisotropic frustrated Heisenberg antiferromagnet in two dimensions. By
restricting the Hilbert space to that expressed by exact eigenstates of the
Heisenberg chain, we derive an effective Schr\"odinger equation valid in the
weak interchain-coupling regime. The dynamical spin correlations from this
approach agree quantitatively with inelastic neutron measurements on the
triangular antiferromagnet Cs_2CuCl_4. The spectral features in such
antiferromagnets can be attributed to two types of excitations: descendents of
one-dimensional spinons of individual chains, and coherently propagating
"triplon" bound states of spinon pairs. We argue that triplons are generic
features of spatially anisotropic frustrated antiferromagnets, and arise
because the bound spinon pair lowers its kinetic energy by propagating between
chains.Comment: 16 pages, 6 figure
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