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Search Results: 1 - 10 of 582834 matches for " D. A. Ritchie "
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Multifactor dimensionality reduction: An analysis strategy for modelling and detecting gene - gene interactions in human genetics and pharmacogenomics studies
Alison A Motsinger, Marylyn D Ritchie
Human Genomics , 2006, DOI: 10.1186/1479-7364-2-5-318
Abstract: As this method is more frequently applied, and was gained acceptance in the study of human disease and pharmacogenomics, it is becoming increasingly important that the implementation of the MDR approach is properly understood. As with all statistical methods, MDR is only powerful and useful when implemented correctly. Concerns regarding dataset structure, configuration parameters and the proper execution of permutation testing in reference to a particular dataset and configuration are essential to the method's effectiveness.The detection, characterisation and interpretation of gene - gene and gene - environment interactions are expected to improve the diagnosis, prevention and treatment of common human diseases. MDR can be a powerful tool in reaching these goals when used appropriately.
Mesoscopic Fluctuations of Coulomb Drag of Composite Fermions
A. S. Price,A. K. Savchenko,D. A. Ritchie
Physics , 2009, DOI: 10.1103/PhysRevB.81.193303
Abstract: We present the first experimental study of mesoscopic fluctuations of Coulomb drag in a system with two layers of composite fermions, which are seen when either the magnetic field or carrier concentration are varied. These fluctuations cause an alternating sign of the average drag. We study these fluctuations at different temperatures to establish the dominant dephasing mechanism of composite fermions.
A dynamic localization of 2D electrons at mesoscopic length scales
A. Ghosh,M. Pepper,H. E. Beere,D. A. Ritchie
Physics , 2004, DOI: 10.1103/PhysRevB.70.233309
Abstract: We have investigated the local magneto-transport in high-quality 2D electron systems at low carrier densities. The positive magneto-resistance in perpendicular magnetic field in the strongly insulating regime has been measured to evaluate the spatial concentration of localized states within a mesoscopic region of the samples. An independent measurement of the electron density within the same region shows an unexpected correspondence between the density of electrons in the metallic regime and that of the localized states in the insulating phase. We have argued that this correspondence manifests a rigid distribution of electrons at low densities.
Neural networks for genetic epidemiology: past, present, and future
Alison A Motsinger-Reif, Marylyn D Ritchie
BioData Mining , 2008, DOI: 10.1186/1756-0381-1-3
Abstract: In the current review, we consider how NN have been used for both linkage and association analyses in genetic epidemiology. We discuss both the successes of these initial NN applications, and the questions that arose during the previous studies. Finally, we introduce evolutionary computing strategies, Genetic Programming Neural Networks (GPNN) and Grammatical Evolution Neural Networks (GENN), for using NN in association studies of complex human diseases that address some of the caveats illuminated by previous work.The identification of disease susceptibility genes for complex, multifactorial disease is arguably the most difficult challenge facing human geneticists today [1]. Most common diseases are the result of complex interactions among multiple genetic factors in addition to a collection of environmental exposures [2]. This has been documented by Ming and Muenke who compiled a list of diseases with known epistatic interactions [3]. Traditional gene mapping studies utilize one of two possible research strategies: linkage or association. Linkage analysis determines whether a chromosomal region is preferentially inherited by offspring with the trait of interest by using genotype and phenotype data from multiple biologically-related family members. Linkage analysis capitalizes on the fact that, as a causative gene(s) segregates through a family kindred, other markers nearby on the same chromosome tend to segregate together (are in linkage) with the causative gene due to the lack of recombination in that region. Association analysis, on the other hand, describes the use of case-control, cohort, or even family data to statistically relate genetic variations to a disease/phenotype. While each of these approaches has been very effective in identifying disease genes in rare, Mendelian disorders, there are additional challenges when studying common, complex diseases. To aid readers less familiar with the terminology used in genetic epidemiology, Table 1 provides a glossar
Oxidation State of a Polyurethane Membrane after Plasma Etching
Matthew D. Moles,Colin A. Scotchford,Alastair Campbell Ritchie
Conference Papers in Science , 2014, DOI: 10.1155/2014/347979
Abstract: Low moduli cell culture substrates can be used to apply dynamic mechanical strain to cells, by surface deformation. Understanding the surface interaction with cells is critical to improving cell adhesion and normal growth. A medical grade polyurethane (PU), Chronoflex AL 80A, was modified by oxygen plasma etching and characterised by X-ray photoelectron spectroscopy. Etching resulted in increased cross-linking at the isocyanate bond and formation of new oxygen moieties. The model, derived from patent data and XPS data of the unetched PU, indicated that the additional oxygen was likely to be hydroxyl and carbonyl groups. Etched membranes enhanced protein adhesion, resulting in full surface coverage compared to unetched PU. The etched PU supported cell adhesion and spreading, while the unetched PU was not conducive to monolayer formation. 1. Introduction The human body comprises tissues with a range of elastic moduli. Improving the response of in vitro cell- and tissue-based investigations can be achieved by culturing cells upon a substrate with a modulus closer to that of living tissue, as opposed to tissue culture polystyrene that has a modulus an order of magnitude greater than smooth muscle, for example, [1]. In addition, dynamic modulation of the substrate can provide mechanical signals that drive differentiation or proliferation. Previously, a stable biocompatible polyurethane (PU), Chronoflex AL 80A (AdvanSource Biomaterials, Wilmington, MA), was tested as a low modulus candidate substrate for a bioreactor capable of subjecting cells to a dynamic mechanical environment [2]. PU was selected as it is more resilient [3] and has a better cell response than similar polymers [4]. It was shown that plasma etching is a key factor to the success of cell adhesion and normal cell growth: the wettability was found to be dependent on etching power and duration, while roughness was more affected by the duration [2]. Therefore the state of oxidation of the PU membrane has been examined, in order to understand the effects of plasma etching on protein and cell adhesion. 2. Method 2.1. Membrane Manufacture and Modification PU membranes were produced as described previously [2]. In brief, polyurethane membranes of thickness ?μm (mean ± standard deviation (SD)) were formed by solvent casting with tetrahydrofuran (10% w/w) and drying in a vacuum oven at 50°C. Surface modification was carried out by etching using an inductively coupled RF-source (13.56?MHz) plasma barrel etcher (Biorad PT7100). The sample chamber was evacuated to 8?Pa, purged with oxygen (grade 2
Influence of parallel magnetic fields on a single-layer two-dimensional electron system with a hopping mechanism of conductivity
I. Shlimak,S. I. Khondaker,M. Pepper,D. A. Ritchie
Physics , 1999, DOI: 10.1103/PhysRevB.61.7253
Abstract: Large positive (P) magnetoresistance (MR) has been observed in parallel magnetic fields in a single 2D layer in a delta-doped GaAs/AlGaAs heterostructure with a variable-range-hopping (VRH) mechanism of conductivity. Effect of large PMR is accompanied in strong magnetic fields by a substantial change in the character of the temperature dependence of the conductivity. This implies that spins play an important role in 2D VRH conductivity because the processes of orbital origin are not relevant to the observed effect. A possible explanation involves hopping via double occupied states in the upper Hubbard band, where the intra-state correlation of spins is important.
Electrically addressing a single self-assembled quantum dot
D. J. P. Ellis,A. J. Bennett,P. Atkinson,D. A. Ritchie,A. J. Shields
Physics , 2006, DOI: 10.1063/1.2190451
Abstract: We report on the use of an aperture in an aluminum oxide layer to restrict current injection into a single self-assembled InAs quantum dot, from an ensemble of such dots within a large mesa. The insulating aperture is formed through the wet-oxidation of a layer of AlAs. Under photoluminescence we observe that only one quantum dot in the ensemble exhibits a Stark shift, and that the same single dot is visible under electroluminescence. Autocorrelation measurements performed on the electroluminescence confirm that we are observing emission from a single quantum dot.
Oxide-apertured microcavity single-photon emitting diode
D. J. P. Ellis,A. J. Bennett,A. J. Shields,P. Atkinson,D. A. Ritchie
Physics , 2007, DOI: 10.1063/1.2747200
Abstract: We have developed a microcavity single-photon source based on a single quantum dot within a planar cavity in which wet-oxidation of a high-aluminium content layer provides lateral confinement of both the photonic mode and the injection current. Lateral confinement of the optical mode in optically pumped structures produces a strong enhancement of the radiative decay rate. Using microcavity structures with doped contact layers, we demonstrate a single-photon emitting diode where current may be injected into a single dot.
Electron Correlations in an Electron Bilayer at Finite Temperature: Landau Damping of the Acoustic Plasmon
D. S. Kainth,D. Richards,H. P. Hughes,M. Y. Simmons,D. A. Ritchie
Physics , 1999, DOI: 10.1088/0953-8984/12/4/306
Abstract: We report angle-resolved Raman scattering observations of the temperature dependent Landau damping of the acoustic plasmon in an electron bilayer system realised in a GaAs double quantum well structure. Corresponding calculations of the charge-density excitation spectrum of the electron bilayer using forms of the random phase approximation (RPA), and the static local field formalism of Singwi, Tosi, Land and Sj\"{o}lander (STLS) extended to incorporate non-zero electron temperature $T_{\rm e}$ and phenomenological damping, are also presented. The STLS calculations include details of the temperature dependence of the intra- and inter-layer local field factors and pair-correlation functions. Good agreement between experiment and the various theories is obtained for the acoustic plasmon energy and damping for $T_{\rm e} \lesssim T_{\rm F}/2$, the Fermi temperature. However, contrary to current expectations, all of the calculations show significant departures from our experimental data for $T_{\rm e} \gtrsim T_{\rm F}/2$. From this, we go on to demonstrate unambiguously that real local field factors fail to provide a physically accurate description of exchange correlation behaviour in low dimensional electron gases. Our results suggest instead that one must resort to a {\em{dynamical}} local field theory, characterised by a {\em{complex}} field factor to provide a more accurate description.
Energy-level pinning and the 0.7 spin state in one dimension: GaAs quantum wires studied using finite-bias spectroscopy
A. C. Graham,D. L. Sawkey,M. Pepper,M. Y. Simmons,D. A. Ritchie
Physics , 2007, DOI: 10.1103/PhysRevB.75.035331
Abstract: We study the effects of electron-electron interactions on the energy levels of GaAs quantum wires (QWs) using finite-bias spectroscopy. We probe the energy spectrum at zero magnetic field, and at crossings of opposite-spin-levels in high in-plane magnetic field B. Our results constitute direct evidence that spin-up (higher energy) levels pin to the chemical potential as they populate. We also show that spin-up and spin-down levels abruptly rearrange at the crossing in a manner resembling the magnetic phase transitions predicted to occur at crossings of Landau levels. This rearranging and pinning of subbands provides a phenomenological explanation for the 0.7 structure, a one-dimensional (1D) nanomagnetic state, and its high-B variants.
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