6,483 research outputs found
CO adsorption on (111) and (100) surfaces of the Pt sub 3 Ti alloy. Evidence for parallel binding and strong activation of CO
The CO adsorption on a 40 atom cluster model of the (111) surface and a 36 atom cluster model of the (100) surface of the Pt3Ti alloy was studied. Parallel binding to high coordinate sites associated with Ti and low CO bond scission barriers are predicted for both surfaces. The binding of CO to Pt sites occurs in an upright orientation. These orientations are a consequence of the nature of the CO pi donation interactions with the surface. On the Ti sites the orbitals donate to the nearly empty Ti 3d band and the antibonding counterpart orbitals are empty. On the Pt sites, however, they are in the filled Pt 5d region of the alloy band, which causes CO to bond in a vertical orientation by 5 delta donation from the carbon end
Anomalous Excitation Spectra of Frustrated Quantum Antiferromagnets
We use series expansions to study the excitation spectra of spin-1/2
antiferromagnets on anisotropic triangular lattices. For the isotropic
triangular lattice model (TLM) the high-energy spectra show several anomalous
features that differ strongly from linear spin-wave theory (LSWT). Even in the
Neel phase, the deviations from LSWT increase sharply with frustration, leading
to roton-like minima at special wavevectors. We argue that these results can be
interpreted naturally in a spinon language, and provide an explanation for the
previously observed anomalous finite-temperature properties of the TLM. In the
coupled-chains limit, quantum renormalizations strongly enhance the
one-dimensionality of the spectra, in agreement with experiments on Cs_2CuCl_4.Comment: 4 pages, 5 figures. New Fig. 3 with higher-order series data, paper
shortened, references updated, one added (Ref. 28), minor changes otherwise.
Published versio
Comparison of the phase diagram of the half-filled layered organic superconductors with the phase diagram of the RVB theory of the Hubbard-Heisenberg model
We present an resonating valence bond (RVB) theory of superconductivity for
the Hubbard--Heisenberg model on an anisotropic triangular lattice. We show
that these calculations are consistent with the observed phase diagram of the
half-filled layered organic superconductors, such as the beta, beta', kappa and
lambda phases of (BEDT-TTF)_2X [bis(ethylenedithio)tetrathiafulvalene] and
(BETS)_2X [bis(ethylenedithio)tetraselenafulvalene]. We find a first order
transition from a Mott insulator to a d_{x^2-y^2} superconductor with a small
superfluid stiffness and a pseudogap with d_{x^2-y^2} symmetry. The
Mott--Hubbard transition can be driven either by increasing the on-site Coulomb
repulsion, U, or by changing the anisotropy of the two hopping integrals, t'/t.
Our results suggest that the ratio t'/t plays an important role in determining
the phase diagram of the organic superconductors.Comment: 4 pages, 3 figur
Quasiharmonic elastic constants corrected for deviatoric thermal stresses
The quasiharmonic approximation (QHA), in its simplest form also called the
statically constrained (SC) QHA, has been shown to be a straightforward method
to compute thermoelastic properties of crystals. Recently we showed that for
non-cubic solids SC-QHA calculations develop deviatoric thermal stresses at
high temperatures. Relaxation of these stresses leads to a series of
corrections to the free energy that may be taken to any desired order, up to
self-consistency. Here we show how to correct the elastic constants obtained
using the SC-QHA. We exemplify the procedure by correcting to first order the
elastic constants of MgSiO-perovskite and MgSiO-post-perovskite, the
major phases of the Earth's lower mantle. We show that this first order
correction is quite satisfactory for obtaining the aggregated elastic averages
of these minerals and their velocities in the lower mantle. This type of
correction is also shown to be applicable to experimental measurements of
elastic constants in situations where deviatoric stresses can develop, such as
in diamond anvil cells.Comment: 4 figures, 1 table, submitted to Phys. Rev. B, July 200
Truthmakers and modality
This paper attempts to locate, within an actualist ontology, truthmakers for modal truths: truths of the form or . In section 1 I motivate the demand for substantial truthmakers for modal truths. In section 2 I criticise Armstrong’s account of truthmakers for modal truths. In section 3 I examine essentialism and defend an account of what makes essentialist attributions true, but I argue that this does not solve the problem of modal truth in general. In section 4 I discuss, and dismiss, a theistic account of the source of modal truth proposed by Alexander Pruss. In section 5 I offer a means of (dis)solving the problem
Revised Distribution Records of Some Minnesota Fishes
Recent collections of fishes from Minnesota have resulted in the addition of one species, Fundulus sciadicus (Cyprinodontidae), to the state\u27s inland faunal list. Additional information on the distribution of 12 other species (Notropis topeka, Notropis spilooterus, Ictiobus byprinellus, Carpiodes byprinus, Moxostoma valenciennesi, Notorus flavus, Morone chrysops, Lepomis megalotis, Stizostedion canadense, Percina shumardi, Etheostoma caeruleum, and Cottus bairdi) is presented. These new records are the result of field work in the Missouri River, Red River, and Lake Superior drainage systems of Minnesota
Akt in the pathogenesis of COPD
In this review we consider the therapeutic potential of targeting Akt for the treatment of COPD. Akt is a serine/threonine protein kinase that functions as a signaling intermediate linked to multiple signaling programs involved in survival, inflammation, and growth. Akt is closely associated with key membrane-bound receptors and represents a convergent integration point for multiple stimuli implicated in COPD pathogenesis. Persistent activation of Akt secondary to somatic mutations in regulatory oncogenes, such as PTEN, may explain why inflammation in COPD does not resolve when smoking is ceased. Akt is also implicated in the systemic manifestations of COPD such as skeletal muscle wasting and metabolic disturbances. Furthermore, targeting Akt may provide a useful means of limiting the severity and duration of disease exacerbations in COPD. As such, Akt represents a particularly attractive therapeutic target for the treatment of COPD. Interestingly, current knowledge suggests that both inhibitors and activators of Akt may be useful for treating different clinical subpopulations of COPD patients
Resilience engineering as a quality improvement method in healthcare
Current approaches to quality improvement rely on the identification of past problems through incident reporting and audits or the use of Lean principles to eliminate waste, to identify how to improve quality. In contrast, Resilience Engineering (RE) is based on insights from complexity science, and quality results from clinicians’ ability to adapt safely to difficult situations, such as a surge in patient numbers, missing equipment or difficult unforeseen physiological problems. Progress in applying these insights to improve quality has been slow, despite the theoretical developments. In this chapter we describe a study in the Emergency Department of a large hospital in which we used RE principles to identify opportunities for quality improvement interventions. In depth observational fieldwork and interviews with clinicians were used to gather data about the key challenges faced, the misalignments between demand and capacity, adaptations that were required, and the four resilience abilities: responding, monitoring, anticipating and learning. Data were transcribed and used to write extended resilience narratives describing the work system. The narratives were analysed thematically using a combined deductive/inductive approach. A structured process was then used to identify potential interventions to improve quality. We describe one intervention to improve monitoring of patient flow and organisational learning about patient flow interventions. The approach we describe is challenging and requires close collaboration with clinicians to ensure accurate results. We found that using RE principles to improve quality is feasible and results in a focus on strengthening processes and supporting the challenges that clinicians face in their daily work
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Microstructures in Metasedimentary Rocks from the Neoproterozoic Bonahaven Formation, Scotland: Microconcretions, Impact Spherules, or Microfossils?
Microscopic spherules in relatively undeformed mudstones of the Neoproterozoic Bonahaven Formation, Islay, Scotland, are differentiated from their matrix by a sharp micron-scale, smoothly rounded boundary. These elongate spherules were earlier interpreted as hollow bodies filled penecontemporaneously by glauconite and subsequently metamorphosed to phengite, but their origin remains a matter of debate. Spherules observed in thin section are predominantly rounded (∼74%) but can exhibit a flat edge or protrusion at one end. In 11% of a sample population, two or more spherules are conjoined. X-ray diffraction indicates that spherule-bearing mudstones consist mainly of muscovite, with variable amounts of kaolin-group minerals and minor iron-chlorites. A range of physical origins for the spherules – including microconcretions or metamorphic microstructures; deposition from the sky as micrometeorites, microtektites/microkrystites, or accretionary volcanic ash particles; and detrital grains – is considered but rejected on distributional, morphological, and mineralogical evidence. Biological origins are considered most likely, especially protistan tests similar to the vase-shaped microfossils found in somewhat older Neoproterozoic rocks. If correct, this provides the first report of eukaryotic life in the Dalradian succession that passes critical tests for biogenicity and new evidence for testate microfossils in post-Sturtian but pre-Marinoan aged rocks.Earth and Planetary SciencesOrganismic and Evolutionary Biolog
Quantum phase space picture of Bose-Einstein Condensates in a double well: Proposals for creating macroscopic quantum superposition states and a study of quantum chaos
We present a quantum phase space model of Bose-Einstein condensate (BEC) in a
double well potential. In a two-mode Fock-state analysis we examine the
eigenvectors and eigenvalues and find that the energy correlation diagram
indicates a transition from a delocalized to a fragmented regime. Phase space
information is extracted from the stationary quantum states using the Husimi
distribution function. It is shown that the quantum states are localized on the
known classical phase space orbits of a nonrigid physical pendulum, and thus
the novel phase space characteristics of a nonrigid physical pendulum such as
the motions are seen to be a property of the exact quantum states. Low
lying states are harmonic oscillator like libration states while the higher
lying states are Schr\"odinger cat-like superpositions of two pendulum rotor
states. To study the dynamics in phase space, a comparison is made between a
displaced quantum wavepacket and the trajectories of a swarm of points in
classical phase space. For a driven double well, it is shown that the classical
chaotic dynamics is manifest in the dynamics of the quantum states pictured
using the Husimi distribution. Phase space analogy also suggests that a
phase displaced wavepacket put on the unstable fixed point on a separatrix will
bifurcate to create a superposition of two pendulum rotor states - a
Schr\"odinger cat state (number entangled state) for BEC. It is shown that the
choice of initial barrier height and ramping, following a phase
imprinting on the condensate, can be used to generate controlled entangled
number states with tunable extremity and sharpness.Comment: revised version, 13 pages, 13 figure
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