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Search Results: 1 - 10 of 433078 matches for " D. J. Hepburn "
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Light quark masses from UKQCD's dynamical simulations with O(a)-improved Wilson fermions
D. J. Hepburn,UKQCD Collaboration
Physics , 2002, DOI: 10.1016/S0920-5632(03)01546-9
Abstract: I present preliminary results on the light quark masses from a partially quenched analysis of UKQCD's dynamical datasets.
First principles study of helium, carbon and nitrogen in austenite, dilute austenitic iron alloys and nickel
D. J. Hepburn,D. Ferguson,S. Gardner,G. J. Ackland
Physics , 2013,
Abstract: An extensive set of first-principles density functional theory calculations have been performed to study the behaviour of He, C and N solutes in austenite, dilute Fe-Cr-Ni austenitic alloys and Ni in order to investigate their influence on the microstructural evolution of austenitic steel alloys under irradiation.
Interatomic forces for transition metals including magnetism
G. J. Ackland,D. J. Hepburn,J. Wallenius
Physics , 2009,
Abstract: }We present a formalism for extending the second moment tight-binding model, incorporating ferro- and anti-ferromagn etic interaction terms which are needed for the FeCr system. For antiferromagnetic and paramagnetic materials, an explicit additional variable representing the spin is required. In a mean-field approximation this spin can be eliminated, and the potential becomes explicitly temperature dependent. For ferromagnetic interactions, this degree of freedom can be eliminated, and the formalism reduces to the embedded atom method (EAM) and we show the equivale nce of existing EAM potentials to "magnetic" potentials.
Mechanism for Radiation Damage Resistance in Yttrium Oxide Dispersion Strengthened Steels
J. Brodrick,D. J. Hepburn,G. J. Ackland
Physics , 2013, DOI: 10.1016/j.jnucmat.2013.10.045
Abstract: ODS steels based on yttrium oxide have been suggested as potential fusion reactor wall materials due to their observed radiation resistance properties. Presumably this radiation resistance can be related to the interaction of the particle with vacancies,self-interstitial atoms (SIAs) and other radiation damage debris. Density functional theory has been used to investigate this at the atomic scale. Four distinct interfaces, some based on HRTEM observations, between iron and yttrium oxide were investigated. It is been shown that the Y$_2$O$_3$-Fe interface acts as a strong trap with long-range attraction for both interstitial and vacancy defects, allowing recombination without altering the interface structure. The catalytic elimination of defects without change to the microstructure explains the improved behaviour of ODS steels with respect to radiation creep and swelling.
Rescaled potential for transition metal solutes in a-iron
D. J. Hepburn,G. J. Ackland,P. Olsson
Physics , 2010,
Abstract: We present empirical potentials for dilute transition metal solutes in a-iron. It is in the Finnis-Sinclair form and is therefore suitable for billion atom molecular dynamics simulations. First principles calculation shows that there are clear trends across the transition metal series which enable us to relate the rescaling parameters to principal quantum number and number of d electrons. The potential has been developed using a rescaling technique to provide solute-iron and solute-solute interactions from an existing iron potential.
Defect and solute properties in dilute Fe-Cr-Ni austenitic alloys from first principles
T. P. C. Klaver,D. J. Hepburn,G. J. Ackland
Physics , 2011, DOI: 10.1103/PhysRevB.85.174111
Abstract: We present results of an extensive set of first-principles density functional theory calculations of point defect formation, binding and clustering energies in austenitic Fe with dilute concentrations of Cr and Ni solutes.
Transition metal solute interactions with point defects in austenitic iron from first principles
D. J. Hepburn,E. MacLeod,G. J. Ackland
Physics , 2014,
Abstract: We present a comprehensive set of first principles electronic structure calculations to study transition metal solutes and their interactions with point defects in austenite. Clear trends were observed across the series. Solute-defect interactions were strongly correlated to the solute size factors, consistent with local strain field effects. Strong correlations with results in ferrite show insensitivity to the underlying crystal structure in Fe. Oversized solutes act as strong traps for vacancy and self-interstitial defects and as nucleation sites for the development of proto-voids and small self-interstitial loops. The reduction in defect mobility and net defect concentrations explains the observed radiation-damage resistance in austenitic steels doped with oversized solutes. Oversized solutes remaining dissolved in oxide dispersion-strengthened (ODS) steels could contribute to their radiation-damage resistance. Ni and Co diffuse more slowly than Fe, along with any vacancy flux produced under irradiation below a critical temperature, which is 400 K for Co and their concentrations should be enhanced at defect sinks. Cr and Cu diffuse more quickly than Fe, against a vacancy flux and will be depleted at defect sinks. Oversized solutes early in the transition metal series form highly-stable solute-centred divacancy (SCD) defects with a nearest-neighbour vacancy. The vacancy-mediated diffusion of these solutes is dominated by the dissociation and reassociation of the SCDs, with a lower activation energy than for self-diffusion, which has important implications for the nucleation and growth of complex oxide nanoparticles containing these solutes in ODS steels. Interstitial-mediated solute diffusion is energetically disfavoured for all except Cr, Mn, Co and Ni. The central role that solute size plays in the results presented here means they should apply to other solvent metals and alloys.
First Principles Calculations of Defects in Unstable Crystals: Austenitic Iron
G. J. Ackland,T. P. C. Klaver,D. J. Hepburn
Physics , 2011,
Abstract: First principles calculations have given a new insight into the energies of point defects in many different materials, information which cannot be readily obtained from experiment. Most such calculation are done at zero Kelvin, with the assumption that finite temperature effects on defect energies and barriers are small. In some materials, however, the stable crystal structre of interest is mechanically unstable at 0K. In such cases, alternate approaches are needed. Here we present results of first principles calculations of austenitic iron using the VASP code. We determine an appropriate reference state for collinear magnetism to be the antiferromagnetic double-layer (AFM-d) which is both stable and lower in energy than other possible models for the low temperature limit of paramagnetic fcc iron. We then consider the energetics of dissolving typical alloying impurities (Ni, Cr) in the materials, and their interaction with point defects typical of the irradiated environment. We show that using standard methods there is a very strong dependence of calculated defect formation energies on the reference state chosen. Furthermore, there is a correlation between local free volume magnetism and energetics. The effect of substitutional Ni and Cr on defect properties is weak, rarely more than tenths of eV, so it is unlikely that small amounts of Ni and Cr will have a significant effect on the radiation damage in austenitic iron at high temperatures.
An Atraumatic Femoral Fracture in a Patient with Rheumatoid Arthritis and Osteoporosis Treated with Denosumab
J. Villiers,D. W. Clark,T. Jeswani,S. Webster,A. L. Hepburn
Case Reports in Rheumatology , 2013, DOI: 10.1155/2013/249872
Abstract: Osteoporosis is responsible for a significant burden both individually and socially, but is readily treated with antiresorptive agents and mineral supplementation. However, long-term usage of these agents, notably bisphosphonates, is rarely associated with atypical fractures. Denosumab is a monoclonal antibody that reduces osteoclast activity and thus increases bone mineral density. In this case report, we present a 78-year-old woman with a background of rheumatoid arthritis and osteoporosis who presented with an atypical diaphyseal femoral fracture. 1. Introduction Osteoporosis is a significant cause of morbidity and mortality. Following a neck of femur fracture, there is up to 50% loss of independence [1] and 20% mortality in the first 12 months [2]. Pharmacological therapy has been shown to significantly reduce fracture risk and therefore the subsequent morbidity and mortality [3]. Denosumab is a fully humanized monoclonal immunoglobulin that binds the receptor activator of nuclear factor-κB ligand (RANK-L), a member of the tumour necrosis factor cytokine family, essential for the formation, function, and survival of osteoclasts [4]. Denosumab is thought to sequester RANK-L, preventing it from activating NF-κB and subsequently reducing the resorption of bone. In the United Kingdom, Denosumab is recommended by the National Institute for Health and Care Excellence (NICE) for the treatment of postmenopausal women with osteoporosis who are unable to tolerate or comply with the administration of oral bisphosphonates, which remains the first line treatment option. However, since the introduction of antiresorptive therapy in the management of osteoporosis, concern has arisen that these medicines are associated with atypical fractures with long-term use [5]. The extension of the phase 3 FREEDOM trial, which had shown the benefits of Denosumab in the treatment of osteoporosis [6], has suggested that atypical femoral fractures do rarely occur after prolonged exposure. In this case report we present a 78-year-old female with a history of rheumatoid arthritis and osteoporosis who presented with an atraumatic diaphysial fracture of her right femur. 2. Case Report A 78-year-old female had been followed up in our Rheumatology outpatient clinic since being diagnosed with rheumatoid arthritis in 1987. Treatment has included occasional intraarticular and intramuscular corticosteroid injections, but never long-term oral corticosteroids. Current immunosuppressive therapy consists of methotrexate and hydroxychloroquine. She also has a background history of an idiopathic
Control of molecular rotation in the limit of extreme rotational excitation
V. Milner,J. W. Hepburn
Physics , 2015,
Abstract: Laser control of molecular rotation is an area of active research. A number of recent studies has aimed at expanding the reach of rotational control to extreme, previously inaccessible rotational states, as well as controlling the directionality of molecular rotation. Dense ensembles of molecules undergoing ultrafast uni-directional rotation, known as molecular superrotors, are anticipated to exhibit unique properties, from spatially anisotropic diffusion and vortex formation to the creation of powerful acoustic waves and tuneable THz radiation. Here we describe our recent progress in controlling molecular rotation in the regime of high rotational excitation. We review two experimental techniques of producing uni-directional rotational wave packets with a "chiral train" of femtosecond pulses and an "optical centrifuge". Three complementary detection methods, enabling the direct observation, characterization and control of the superrotor states, are outlined: the one based on coherent Raman scattering, and two other methods employing both resonant and non-resonant multi-photon ionization. The capabilities of the described excitation and detection techniques are demonstrated with a few examples. The paper is concluded with an outlook for future developments.
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