1,817 research outputs found
Doping dependence of charge-transfer excitations in La_{2-x}Sr_xCuO_4
We report a resonant inelastic x-ray scattering (RIXS) study of the doping
dependence of charge-transfer excitations in . The mome
ntum dependence of these charge excitations are studied over the whole
Brillouin zone in underdoped (x=0.05) and optimally doped (x=0.17) samples, and
compared with that of the undoped (x=0) sample. We observe a large change in
the RIXS spectra between the x=0 and x=0.17 sample, while the RIXS spectra of
the x=0.05 sample are similar to that of the x=0 sample. The most prominent
effect of doped-holes on the charge excitation spectra is the appearance of a
continuum of intensity, which exhibits a strong momentum-dependence below 2 eV.
For the x=0.17 sample, some of the spectral weight from the lowest-lying
charge-transfer excitation of the undoped compound is transferred to the
continuum intensity below the gap, in agreement with earlier optical studies.
However, the higher energy charge-transfer excitation carries significant
spectral weight even for the x=0.17 sample. The doping dependence of the
dispersion of this charge-transfer excitation is also discussed and compared
with recent theoretical calculations.Comment: 7 pages, 6 figures, to appear in Phys. Rev.
Polydisperse suspensions : erosion, deposition, and flow capacity
Deposition from particle-laden flows is often described in terms of the capacity and competence of the flow, but robust definitions of these terms have proved elusive. In this paper we provide a mathematical modelling framework within which erosion and deposition of polydisperse sediment, and thus flow capacity and competence, can be rigorously defined. This framework explicitly captures the coupling between the suspension and an active layer of sediment at the top of the bed, and is capable of describing both depositional and erosional flows over both erodible and non-erodible beds. Crucially, the capacity of a flow is shown to depend on the erosional and depositional history because these processes determine the composition of the active layer. This dependence is explored within models of bidisperse and polydisperse suspensions. It is further demonstrated that monodisperse representations of suspended sediment transport may severely underpredict actual flow capacity. The polydisperse model is validated against recent experimental studies of the evolution of suspended material in waning turbulent flows, and is used to demonstrate that loss of capacity is the principal driver of sediment deposition
Resonant inelastic x-ray scattering study of charge excitations in La2CuO4
We report a resonant inelastic x-ray scattering study of the dispersion
relations of charge transfer excitations in insulating LaCuO. These
data reveal two peaks, both of which show two-dimensional characteristics. The
lowest energy excitation has a gap energy of eV at the zone center,
and a dispersion of eV. The spectral weight of this mode becomes
dramatically smaller around (, ). The second peak shows a smaller
dispersion ( eV) with a zone-center energy of eV. We argue
that these are both highly dispersive exciton modes damped by the presence of
the electron-hole continuum.Comment: 5 pages, 3 figure
The role of spatial variations of abiotic factors in mediating intratumour phenotypic heterogeneity
We present here a space- and phenotype-structured model of selection dynamics between cancer cells within a solid tumour. In the framework of this model, we combine formal analyses with numerical simulations to investigate in silico the role played by the spatial distribution of abiotic components of the tumour microenvironment in mediating phenotypic selection of cancer cells. Numerical simulations are performed both on the 3D geometry of an in silico multicellular tumour spheroid and on the 3D geometry of an in vivo human hepatic tumour, which was imaged using computerised tomography. The results obtained show that inhomogeneities in the spatial distribution of oxygen, currently observed in solid tumours, can promote the creation of distinct local niches and lead to the selection of different phenotypic variants within the same tumour. This process fosters the emergence of stable phenotypic heterogeneity and supports the presence of hypoxic cells resistant to cytotoxic therapy prior to treatment. Our theoretical results demonstrate the importance of integrating spatial data with ecological principles when evaluating the therapeutic response of solid tumours to cytotoxic therapy
Correlated Polarons in Dissimilar Perovskite Manganites
We report x-ray scattering studies of broad peaks located at a (0.5 0 0)/(0
0.5 0)-type wavevector in the paramagnetic insulating phases of
La_{0.7}Ca_{0.3}MnO_{3} and Pr_{0.7}Ca_{0.3}MnO_{3}. We interpret the
scattering in terms of correlated polarons and measure isotropic correlation
lengths of 1-2 lattice constants in both samples. Based on the wavevector and
correlation lengths, the correlated polarons are found to be consistent with
CE-type bipolarons. Differences in behavior between the samples arise as they
are cooled through their respective transition temperatures and become
ferromagnetic metallic (La_{0.7}Ca_{0.3}MnO_{3}) or charge and orbitally
ordered insulating (Pr_{0.7}Ca_{0.3}MnO_{3}). Since the primary difference
between the two samples is the trivalent cation size, these results illustrate
the robust nature of the correlated polarons to variations in the relative
strength of the electron-phonon coupling, and the sensitivity of the
low-temperature ground state to such variations.Comment: 13 pages, 6 figure
Modelling cell motility and chemotaxis with evolving surface finite elements
We present a mathematical and a computational framework for the modelling of cell motility. The cell membrane is represented by an evolving surface, with the movement of the cell determined by the interaction of various forces that act normal to the surface. We consider external forces such as those that may arise owing to inhomogeneities in the medium and a pressure that constrains the enclosed volume, as well as internal forces that arise from the reaction of the cells' surface to stretching and bending. We also consider a protrusive force associated with a reaction-diffusion system (RDS) posed on the cell membrane, with cell polarization modelled by this surface RDS. The computational method is based on an evolving surface finite-element method. The general method can account for the large deformations that arise in cell motility and allows the simulation of cell migration in three dimensions. We illustrate applications of the proposed modelling framework and numerical method by reporting on numerical simulations of a model for eukaryotic chemotaxis and a model for the persistent movement of keratocytes in two and three space dimensions. Movies of the simulated cells can be obtained from http://homepages.warwick.ac.uk/maskae/CV_Warwick/Chemotaxis.html
Orbital Correlations in Doped Manganites
We review our recent x-ray scattering studies of charge and orbital order in
doped manganites, with specific emphasis on the role of orbital correlations in
Pr_1-xCa_xMnO_3. For x=0.25, we find an orbital structure indistinguishable
from the undoped structure with long range orbital order at low temperatures.
For dopings 0.3<x<0.5, we find scattering consistent with a charge and
orbitally ordered CE-type structure. While in each case the charge order peaks
are resolution limited, the orbital order exhibits only short range
correlations. We report the doping dependence of the correlation length and
discuss the connection between the orbital correlations and the finite magnetic
correlation length observed on the Mn^3+ sublattice with neutron scattering
techniques. The physical origin of these domains, which appear to be isotropic,
remains unclear. We find that weak orbital correlations persist well above the
phase transitions, with a correlation length of 1-2 lattice constants at high
temperatures. Significantly, we observe similar correlations at high
temperatures in La_0.7Ca_0.3MnO_3, which does not have an orbitally ordered
ground state, and we conclude that such correlations are robust to variations
in the relative strength of the electron-phonon coupling.Comment: 22 pagegs, 7 figure
Antiapoptotic Actions of Methyl Gallate on Neonatal Rat Cardiac Myocytes Exposed to H 2
Reactive oxygen species trigger cardiomyocyte cell death via increased oxidative stress and have been implicated in the pathogenesis of cardiovascular diseases. The prevention of cardiomyocyte apoptosis is a putative therapeutic target in cardioprotection. Polyphenol intake has been associated with reduced incidences of cardiovascular disease and better overall health. Polyphenols like epigallocatechin gallate (EGCG) can reduce apoptosis of cardiomyocytes, resulting in better health outcomes in animal models of cardiac disorders. Here, we analyzed whether the antioxidant N-acetyl cysteine (NAC) or polyphenols EGCG, gallic acid (GA) or methyl gallate (MG) can protect cardiomyocytes from cobalt or H2O2-induced stress. We demonstrate that MG can uphold viability of neonatal rat cardiomyocytes exposed to H2O2 by diminishing intracellular ROS, maintaining mitochondrial membrane potential, augmenting endogenous glutathione, and reducing apoptosis as evidenced by impaired Annexin V/PI staining, prevention of DNA fragmentation, and cleaved caspase-9 accumulation. These findings suggest a therapeutic value for MG in cardioprotection
LINVIEW: Incremental View Maintenance for Complex Analytical Queries
Many analytics tasks and machine learning problems can be naturally expressed
by iterative linear algebra programs. In this paper, we study the incremental
view maintenance problem for such complex analytical queries. We develop a
framework, called LINVIEW, for capturing deltas of linear algebra programs and
understanding their computational cost. Linear algebra operations tend to cause
an avalanche effect where even very local changes to the input matrices spread
out and infect all of the intermediate results and the final view, causing
incremental view maintenance to lose its performance benefit over
re-evaluation. We develop techniques based on matrix factorizations to contain
such epidemics of change. As a consequence, our techniques make incremental
view maintenance of linear algebra practical and usually substantially cheaper
than re-evaluation. We show, both analytically and experimentally, the
usefulness of these techniques when applied to standard analytics tasks. Our
evaluation demonstrates the efficiency of LINVIEW in generating parallel
incremental programs that outperform re-evaluation techniques by more than an
order of magnitude.Comment: 14 pages, SIGMO
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