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Search Results: 1 - 10 of 137627 matches for " Alexander T. Holmes "
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A Simple Bayesian Method for Improved Analysis of Quasi-2D Scattering Data
Alexander T. Holmes
Physics , 2014, DOI: 10.1103/PhysRevB.90.024514
Abstract: A new method is presented for the analysis of small angle neutron scattering data from quasi-2D systems such as flux lattices, Skyrmion lattices, and aligned liquid crystals. A significant increase in signal to noise ratio, and a natural application of the Lorentz factor can be achieved by taking advantage of the knowledge that all relevant scattering is centered on a plane in reciprocal space. The Bayesian form ensures that missing information is treated in a controlled way and can be subsequently included in the analysis. A simple algorithm based on Gaussian probability assumptions is provided which provides very satisfactory results. Finally, it is argued that a generalised model-independent Bayesian data analysis method would be highly advantageous for the processing of neutron and x-ray scattering data.
Spin and Valence-Fluctuation Mediated Superconductivity in Pressurized Fe and CeCu2(Si/Ge)2
D. Jaccard,Alexander T. Holmes
Physics , 2004,
Abstract: We review the evidence supporting valence-fluctuation mediated superconductivity in CeCu2Si2 and CeCu2Ge2, where Tc reaches 2.4 K at high pressure. In these systems the valence and magnetic critical points, at p_V and p_c respectively, are well separated. Characteristic signatures associated with both phenomena are distinct. In contrast, the valence and spin fluctuation regions appear much closer in most Ce based compounds. Concerning d-transition metals, superconductivity in pure iron emerges in the pressure window 15-30 GPa with the onset of Tc up to almost 3 K. All relevant observations point to unconventional superconductivity, likely mediated by ferromagnetic spin fluctuations.
Valence fluctuation mediated superconductivity in CeCu2Si2
Alexander T. Holmes,Didier Jaccard
Physics , 2005, DOI: 10.1016/j.physb.2006.01.123
Abstract: It has been proposed that there are two types of superconductivity in CeCu2Si2, mediated by spin fluctuations at ambient pressure, and by critical valence fluctuations around a charge instability at a pressure P_v \simeq 4.5 GPa. We present in detail some of the unusual features of this novel type of superconducting state, including the coexistence of superconductivity and huge residual resistivity of the order of the Ioffe-Regel limit, large and pressure dependent resistive transition widths in a single crystal measured under hydrostatic conditions, asymmetric pressure dependence of the specific heat jump shape, unrelated to the resistivity width, and negative temperature dependence of the normal state resistivity below 10 K at very high pressure.
Valence Instability and Superconductivity in Heavy Fermion Systems
Alexander T. Holmes,Didier Jaccard,Kazumasa Miyake
Physics , 2006, DOI: 10.1143/JPSJ.76.051002
Abstract: Many cerium-based heavy fermion (HF) compounds have pressure-temperature phase diagrams in which a superconducting region extends far from a magnetic quantum critical point. In at least two compounds, CeCu2Si2 and CeCu2Ge2, an enhancement of the superconducting transition temperature was found to coincide with an abrupt valence change, with strong circumstantial evidence for pairing mediated by critical valence, or charge transfer, fluctuations. This pairing mechanism, and the valence instability, is a consequence of a f-c Coulomb repulsion term U_fc in the hamiltonian. While some non-superconducting Ce compounds show a clear first order valence instability, analogous to the Ce alpha-gamma transition, we argue that a weakly first order valence transition may be a general feature of Ce-based HF systems, and both magnetic and critical valence fluctuations may be responsible for the superconductivity in these systems.
A 17 T horizontal field cryomagnet with rapid sample change designed for beamline use
Alexander T. Holmes,Gary Walsh,Elizabeth Blackburn,Edward M. Forgan,Marc Savey-Bennett
Physics , 2011, DOI: 10.1063/1.3688657
Abstract: We describe a new 17 T cryomagnet for neutron, x-ray or optical experiments with rapid in-situ sample change. Sample temperatures are controllable from < 2 K to 300 K in vacuum. Alternatively a room temperature bore insert can be used for experiments in the field centre under atmospheric conditions. Some advantages of this system include very low background scattering due to the small amount of material in the beam path, rapid cooldown, and fast field ramping. Access is available in a ~10-11 degree cone around the field direction on both sides.
Influence of the Fermi Surface Morphology on the Magnetic Field-Driven Vortex Lattice Structure Transitions in YBa$_{2}$Cu$_{3}$O$_{7-δ}:δ=$0, 0.15
Nikola Galvan Leos,Jonathan S. White,Joshua A. Lim,Jorge L. Gavilano,Bernard Delley,Louis Lemberger,Alexander T. Holmes,Marisa Medarde,Toshinao Loew,Vladimir Hinkov,Chengtian Lin,Mark Laver,Charles D. Dewhurst,Edward M. Forgan
Physics , 2015, DOI: 10.7566/JPSJ.84.044709
Abstract: We report small-angle neutron scattering measurements of the vortex lattice (VL) structure in single crystals of the lightly underdoped cuprate superconductor YBa2Cu3O6.85. At 2 K, and for fields of up to 16 T applied parallel to the crystal c-axis, we observe a sequence of field-driven and first-order transitions between different VL structures. By rotating the field away from the c-axis, we observe each structure transition to shift to either higher or lower field dependent on whether the field is rotated towards the [100] or [010] direction. We use this latter observation to argue that the Fermi surface morphology must play a key role in the mechanisms that drive the VL structure transitions. Furthermore, we show this interpretation is compatible with analogous results obtained previously on lightly overdoped YBa2Cu3O7. In that material, it has long-been suggested that the high field VL structure transition is driven by the nodal gap anisotropy. In contrast, the results and discussion presented here bring into question the role, if any, of a nodal gap anisotropy on the VL structure transitions in both YBa2Cu3O6.85 and YBa2Cu3O7.
Nonlinear Muscles, Passive Viscoelasticity and Body Taper Conspire To Create Neuromechanical Phase Lags in Anguilliform Swimmers
T. McMillen ,T. Williams,P. Holmes
PLOS Computational Biology , 2008, DOI: 10.1371/journal.pcbi.1000157
Abstract: Locomotion provides superb examples of cooperation among neuromuscular systems, environmental reaction forces, and sensory feedback. As part of a program to understand the neuromechanics of locomotion, here we construct a model of anguilliform (eel-like) swimming in slender fishes. Building on a continuum mechanical representation of the body as an viscoelastic rod, actuated by a traveling wave of preferred curvature and subject to hydrodynamic reaction forces, we incorporate a new version of a calcium release and muscle force model, fitted to data from the lamprey Ichthyomyzon unicuspis, that interactively generates the curvature wave. We use the model to investigate the source of the difference in speeds observed between electromyographic waves of muscle activation and mechanical waves of body curvature, concluding that it is due to a combination of passive viscoelastic and geometric properties of the body and active muscle properties. Moreover, we find that nonlinear force dependence on muscle length and shortening velocity may reduce the work done by the swimming muscles in steady swimming.
Perceived Difficulty of Friendship Maintenance Online: Geographic Factors  [PDF]
Kristie Holmes
Advances in Applied Sociology (AASoci) , 2012, DOI: 10.4236/aasoci.2012.24040
Abstract: Geographic location has an effect on the perceived ease of friendship maintenance online and may reflect physical space. Participants from the Northeastern United States rated maintaining friendships online as more difficult than those from other regions. Those with the highest anxiety level ratings were from the largest and most densely populated areas (metropolitan) and those who were the least anxious about their image (both online and offline) were from rural areas with the least population density. Those residing in metropolitan areas were the most trusting of online information posted by others and the town/small city group were the least trusting of others’ online posted information (similar to the urban group), making those from rural areas nearly as trusting of others’ information as the metropolitan group, though probably the result of entirely different influences.
A Comparison of Statistics for Assessing Model Invariance in Latent Class Analysis  [PDF]
Holmes Finch
Open Journal of Statistics (OJS) , 2015, DOI: 10.4236/ojs.2015.53022
Abstract: Latent class analysis (LCA) is a widely used statistical technique for identifying subgroups in the population based upon multiple indicator variables. It has a number of advantages over other unsupervised grouping procedures such as cluster analysis, including stronger theoretical underpinnings, more clearly defined measures of model fit, and the ability to conduct confirmatory analyses. In addition, it is possible to ascertain whether an LCA solution is equally applicable to multiple known groups, using invariance assessment techniques. This study compared the effectiveness of multiple statistics for detecting group LCA invariance, including a chi-square difference test, a bootstrap likelihood ratio test, and several information indices. Results of the simulation study found that the bootstrap likelihood ratio test was the optimal invariance assessment statistic. In addition to the simulation, LCA group invariance assessment was demonstrated in an application with the Youth Risk Behavior Survey (YRBS). Implications of the simulation results for practice are discussed.
Discovering the Phylodynamics of RNA Viruses
Edward C. Holmes ,Bryan T. Grenfell
PLOS Computational Biology , 2009, DOI: 10.1371/journal.pcbi.1000505
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