3,716 research outputs found
Dispersion interactions between semiconducting wires
The dispersion energy between extended molecular chains (or equivalently
infinite wires) with non-zero band gaps is generally assumed to be expressible
as a pair-wise sum of atom-atom terms which decay as . Using a model
system of two parallel wires with a variable band gap, we show that this is not
the case. The dispersion interaction scales as for large interwire
separations , as expected for an insulator, but as the band gap decreases
the interaction is greatly enhanced; while at shorter (but non-overlapping)
separations it approaches a power-law scaling given by , \emph{i.e.}
the dispersion interaction expected between \emph{metallic} wires. We
demonstrate that these effects can be understood from the increasing length
scale of the plasmon modes (charge fluctuations), and their increasing
contribution to the molecular dipole polarizability and the dispersion
interaction, as the band gaps are reduced. This result calls into question
methods which invoke locality assumptions in deriving dispersion interactions
between extended small-gap systems.Comment: 8 pages, 5 figure
ISA-Pol: distributed polarizabilities and dispersion models from a basis-space implementation of the iterated stockholder atoms procedure
Recently we have developed a robust, basis-space implementation of the
iterated stockholder atoms (BS-ISA) approach for defining atoms in a molecule.
This approach has been shown to yield rapidly convergent distributed multipole
expansions with a well-defined basis-set limit. Here we use this method as the
basis of a new approach, termed ISA-Pol, for obtaining non-local distributed
frequency-dependent polarizabilities. We demonstrate how ISA-Pol can be
combined with localization methods to obtain distributed dispersion models that
share the many unique properties of the ISA: These models have a well-defined
basis-set limit, lead to very accurate dispersion energies, and, remarkably,
satisfy commonly used combination rules to a good accuracy. As these models are
based on the ISA, they can be expected to respond to chemical and physical
changes naturally, and thus they may serve as the basis for the next generation
of polarization and dispersion models for ab initio force-field development.Comment: 18 pages, 9 figure
Ab Initio Atom-Atom Potentials Using CAMCASP: Theory and Application to Many-Body Models for the Pyridine Dimer
Creating accurate, analytic atom−atom potentials for small organic molecules from first principles can be a
time-consuming and computationally intensive task, particularly if we also require them to include explicit polarization
terms, which are essential in many systems. We describe how
the CamCASP suite of programs can be used to generate such
potentials using some of the most accurate electronic structure
methods currently applicable. We derive the long-range terms
from monomer properties and determine the short-range
anisotropy parameters by a novel and robust method based on
the iterated stockholder atom approach. Using these
techniques, we develop distributed multipole models for the
electrostatic, polarization, and dispersion interactions in the
pyridine dimer and develop a series of many-body potentials for the pyridine system. Even the simplest of these potentials
exhibits root mean square errors of only about 0.6 kJ mol −1 for the low-energy pyridine dimers, significantly surpassing the best
empirical potentials. Our best model is shown to support eight stable minima, four of which have not been reported before in the
literature. Further, the functional form can be made systematically more elaborate so as to improve the accuracy without a
significant increase in the human-time spent in their generation. We investigate the effects of anisotropy, rank of multipoles, and
choice of polarizability and dispersion models
X-ray diffraction from bone employing annular and semi-annular beams
This is the final version of the article. Available from the publisher via the DOI in this record.There is a compelling need for accurate, low cost diagnostics to identify osteo-tissues that are associated with a high risk of fracture within an individual. To satisfy this requirement the quantification of bone characteristics such as 'bone quality' need to exceed that provided currently by densitometry. Bone mineral chemistry and microstructure can be determined from coherent x-ray scatter signatures of bone specimens. Therefore, if these signatures can be measured, in vivo, to an appropriate accuracy it should be possible by extending terms within a fracture risk model to improve fracture risk prediction.In this preliminary study we present an examination of a new x-ray diffraction technique that employs hollow annular and semi-annular beams to measure aspects of 'bone quality'. We present diffractograms obtained with our approach from ex vivo bone specimens at Mo Kα and W Kα energies. Primary data is parameterized to provide estimates of bone characteristics and to indicate the precision with which these can be determined.We acknowledge gratefully the funding provided by the UK Engineering and Physical
Sciences Research Council (EPSRC) grant number EP/K020196/
Direct mapping of surface plasmon dispersion using imaging scatterometry
© 2013 American Institute of PhysicsThe iso-frequency contours of diffracted surface plasmons on metallic bigratings have been directly recorded using imaging scatterometry. Metallic rectangular bigratings, formed of two “crossed” surface relief gratings, are used to demonstrate this measurement technique. By deepening one of the constituent gratings, control of the surface plasmon dispersion anisotropy is shown in the recorded iso-frequency maps. Collating the iso-frequency contours over a range of wavelengths from 500 nm to 700 nm leads to a three-dimensional map of the surface plasmon dispersion
In search of multipolar order on the Penrose tiling
Based on Monte Carlo calculations, multipolar ordering on the Penrose tiling,
relevant for two-dimensional molecular adsorbates on quasicrystalline surfaces
and for nanomagnetic arrays, has been analyzed. These initial investigations
are restricted to multipolar rotors of rank one through four - described by
spherical harmonics Ylm with l=1...4 and restricted to m=0 - positioned on the
vertices of the rhombic Penrose tiling. At first sight, the ground states of
odd-parity multipoles seem to exhibit long-range multipolar order, indicated by
the appearance of a superstructure in the form of the decagonal
Hexagon-Boat-Star tiling, in agreement with previous investigations of dipolar
systems. Yet careful analysis establishes that long-range multipolar order is
absent in all cases investigated here, and only short-range order exists. This
result should be taken as a warning for any future analysis of order in either
real or simulated arrangements of multipoles on quasiperiodic templates
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Impact of meltwater on high-latitude early Last Interglacial climate
Recent data compilations of the early Last Interglacial period have indicated a bipolar temperature response at 130 ka, with colder-than-present temperatures in the North Atlantic and warmer-than-present temperatures in the Southern Ocean and over Antarctica. However, climate model simulations of this period have been unable to reproduce this response, when only orbital and greenhouse gas forcings are considered in a climate model framework. Using a full-complexity general circulation model we perform climate model simulations representative of 130 ka conditions which include a magnitude of freshwater forcing derived from data at this time. We show that this meltwater from the remnant Northern Hemisphere ice sheets during the glacial-interglacial transition produces a modelled climate response similar to the observed colder-than-present temperatures in the North Atlantic at 130 ka and also results in warmer-than-present temperatures in the Southern Ocean via the bipolar seesaw mechanism. Further simulations in which the West Antarctic Ice Sheet is also removed lead to warming in East Antarctica and the Southern Ocean but do not appreciably improve the model-data comparison. This integrated model-data approach provides evidence that Northern Hemisphere freshwater forcing is an important player in the evolution of early Last Interglacial climate.This work was carried out with funding from the UK-NERC consortium iGlass (NE/I009906/1) and is also a contribution to the European Union’s Seventh Framework programme (FP7/2007–2013) under grant agreement 243908, “Past4Future. Climate change – Learning from the past climate”. This is Past4Future contribution no. 85
Omacetaxine may have a role in chronic myeloid leukaemia eradication through downregulation of Mcl-1 and induction of apoptosis in stem/progenitor cells
Chronic myeloid leukaemia (CML) is maintained by a rare population of tyrosine kinase inhibitor (TKI)-insensitive malignant stem cells. Our long-term aim is to find a BcrAbl-independent drug that can be combined with a TKI to improve overall disease response in chronic-phase CML. Omacetaxine mepesuccinate, a first in class cetaxine, has been evaluated by clinical trials in TKI-insensitive/resistant CML. Omacetaxine inhibits synthesis of anti-apoptotic proteins of the Bcl-2 family, including (myeloid cell leukaemia) Mcl-1, leading to cell death. Omacetaxine effectively induced apoptosis in primary CML stem cells (CD34<sup>+</sup>38<sup>lo</sup>) by downregulation of Mcl-1 protein. In contrast to our previous findings with TKIs, omacetaxine did not accumulate undivided cells <i>in vitro</i>. Furthermore, the functionality of surviving stem cells following omacetaxine exposure was significantly reduced in a dose-dependant manner, as determined by colony forming cell and the more stringent long-term culture initiating cell colony assays. This stem cell-directed activity was not limited to CML stem cells as both normal and non-CML CD34<sup>+</sup> cells were sensitive to inhibition. Thus, although omacetaxine is not leukaemia stem cell specific, its ability to induce apoptosis of leukaemic stem cells distinguishes it from TKIs and creates the potential for a curative strategy for persistent disease
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