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Search Results: 1 - 10 of 325610 matches for " S. Kirkpatrick "
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Blue Quantum Fog: Chiral Condensation in Quantum Helimagnets
S. Tewari,D. Belitz,T. R. Kirkpatrick
Physics , 2005, DOI: 10.1103/PhysRevLett.96.047207
Abstract: It is shown that a condensation transition involving a chiral order parameter can occur in itinerant helimagnets, in analogy to the transition between the isotropic phase and the phase known as blue fog or blue phase III in cholesteric liquid crystals. It is proposed that such a transition is the explanation for recent neutron scattering results in MnSi. Predictions are made that will allow to experimentally test this proposal.
Electronic relaxation rates in metallic ferromagnets
S. Bharadwaj,D. Belitz,T. R. Kirkpatrick
Physics , 2014, DOI: 10.1103/PhysRevB.89.134401
Abstract: We show that the single-particle and transport relaxation rates in ferromagnetic metals, which determine the thermal and electrical conductivity, respectively, at asymptotically low temperature do not obey a power law as previously thought, but rather show an exponential temperature dependence. The reason is the splitting of the conduction band that inevitably results from a nonzero magnetization. At higher temperatures there is a sizable temperature window where the transport rate shows a T^2 temperature dependence, in accord with prior results. This window is separated from the asymptotic regime by a temperature scale that is estimated to range from tens of mK to tens of K for typical ferromagnets. We motivate and derive a very general effective theory for metallic magnets that we then use to derive these results. Comparisons with existing experiments are discussed, and predictions for future experiments at low temperatures are made.
Optimal State-Space Reduction for Pedigree Hidden Markov Models
Bonnie Kirkpatrick,Kay Kirkpatrick
Computer Science , 2012,
Abstract: To analyze whole-genome genetic data inherited in families, the likelihood is typically obtained from a Hidden Markov Model (HMM) having a state space of 2^n hidden states where n is the number of meioses or edges in the pedigree. There have been several attempts to speed up this calculation by reducing the state-space of the HMM. One of these methods has been automated in a calculation that is more efficient than the naive HMM calculation; however, that method treats a special case and the efficiency gain is available for only those rare pedigrees containing long chains of single-child lineages. The other existing state-space reduction method treats the general case, but the existing algorithm has super-exponential running time. We present three formulations of the state-space reduction problem, two dealing with groups and one with partitions. One of these problems, the maximum isometry group problem was discussed in detail by Browning and Browning. We show that for pedigrees, all three of these problems have identical solutions. Furthermore, we are able to prove the uniqueness of the solution using the algorithm that we introduce. This algorithm leverages the insight provided by the equivalence between the partition and group formulations of the problem to quickly find the optimal state-space reduction for general pedigrees. We propose a new likelihood calculation which is a two-stage process: find the optimal state-space, then run the HMM forward-backward algorithm on the optimal state-space. In comparison with the one-stage HMM calculation, this new method more quickly calculates the exact pedigree likelihood.
Non-Focal Neurological Symptoms Associated with Classical Presentations of Transient Ischaemic Attack: Qualitative Analysis of Interviews with Patients
Susan Kirkpatrick, Louise Locock, Matthew F. Giles, Daniel S. Lasserson
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0066351
Abstract: Background Improving the recognition of transient ischaemic attack (TIA) at initial healthcare contact is essential as urgent specialist assessment and treatment reduces stroke risk. Accurate TIA detection could be achieved with clinical prediction rules but none have been validated in primary care. An alternative approach using qualitative analysis of patients' experiences of TIA may identify novel features of the TIA phenotype that are not detected routinely, as such techniques have revealed novel early features of other important conditions such as meningococcaemia. We sought to determine whether the patient's experience of TIA would reveal additional deficits that can be tested prospectively in cohort studies to determine their additional diagnostic and prognostic utility at the first healthcare contact. Methodology and Findings Qualitative semi-structured interviews with 25 patients who had experienced definite TIA as determined by a stroke specialist; framework analysis to map symptoms and key words or descriptive phrases used against each individual, with close attention to the detail of the language used. All interview transcripts were reviewed by a specialist clinician with experience in TIA/minor stroke. Patients described non-focal symptoms consistent with higher function deficits in spatial perception and awareness of deficit, as well as feelings of disconnection with their immediate surroundings. Of the classical features, weakness and speech disturbance were described in ways that did not meet the readily recognisable phenotype. Conclusion/Significance Analysis of patients' narrative accounts reveals a set of overlooked features of the experience of TIA which may provide additional diagnostic utility so that providers of first contact healthcare can recognise TIA more easily. Future research is required in a prospective cohort of patients presenting with transient neurological symptoms to determine how frequent these features are, what they add to diagnostic information and whether they can refine measures to predict stroke risk.
2+p-SAT: Relation of Typical-Case Complexity to the Nature of the Phase Transition
R. Monasson,R. Zecchina,S. Kirkpatrick,B. Selman,L. Troyansky
Physics , 1999,
Abstract: Heuristic methods for solution of problems in the NP-Complete class of decision problems often reach exact solutions, but fail badly at "phase boundaries", across which the decision to be reached changes from almost always having one value to almost having a different value. We report an analytic solution and experimental investigations of the phase transition that occurs in the limit of very large problems in K-SAT. The nature of its "random first-order" phase transition, seen at values of K large enough to make the computational cost of solving typical instances increase exponenitally with problem size, suggest a mechanism for the cost increase. There has been evidence for features like the "backbone" of frozen inputs which characterizes the UNSAT phase in K-SAT in the study of models of disordered materials, but this feature and this transition are uniquely accessible to analysis in K-SAT. The random first order transition combines properties of the 1st order (discontinuous onset of order) and 2nd order (with power law scaling, e.g. of the width of the the critical region in a finite system) transitions known in the physics of pure solids. Such transitions should occur in other combinatoric problems in the large N limit. Finally, improved search heuristics may be developed when a "backbone" is known to exist.
How and Why Chromosome Inversions Evolve
Mark Kirkpatrick
PLOS Biology , 2012, DOI: 10.1371/journal.pbio.1000501
Abstract:
Taking Stock of the New Managerialism in English Social Services
Kirkpatrick, Ian
Social Work and Society , 2006,
Abstract:
Between Markets and Networks: The Reform of Social Care Provision in the UK
Ian Kirkpatrick
Revista de Análisis Económico (RAE) , 2006,
Abstract: For over two decades there have been attempts across many countries to reform the management of public services and substitute market based provision for bureaucracy. But while these changes have been pursed vigorously, doubts about their appropriateness, feasibility and effectiveness remain. The aim of this paper is to contribute to this debate focusing on the specific case of social care markets in the UK. Drawing on ideas from institutional theory and a range of secondary sources it is argued that, in the UK, broad policy objectives of moving towards a mixed economy have been largely successful. However this review also points to costs associated with implementation and the reliance on low trust arms length contractual relations. Social care organisations are now seeking to manage these costs by attempting to move towards more collaborative networks, although the effectiveness of this change is open to question given prevailing institutional conditions in the UK.
How and Why Chromosome Inversions Evolve
Mark Kirkpatrick
PLOS Biology , 2010, DOI: 10.1371/journal.pbio.1000501
Abstract:
Hot Charge Transfer States and Charge Generation in Donor Acceptor Blends
James Kirkpatrick
Physics , 2010,
Abstract: In an organic blend the vibrational normal mode excited by exciton splitting is the same as the one coupled to charge hopping. Excess driving force for exciton splitting can therefore aid charge transfer, if vibrational relaxation is slow compared to charge transfer. A model is developed that takes this into account and hence explains the experimentally observed relation of driving force for exciton splitting and charge yield and that high charge yields can be achieved with the observed fast rates of recombination.
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