332 research outputs found
Optimal Parallel Construction of Hamiltonian Cycles and Spanning Trees in Random Graphs
We give tight bounds on the parallel complexity of some problems involving random graphs. Specifically, we show that a Hamiltonian cycle, a breadth first spanning tree, and a maximal matching can all be constructed in \Theta(log n) expected time using n= log n processors on the CRCW PRAM. This is a substantial improvement over the best previous algorithms, which required \Theta((log log n) 2 ) time and n log 2 n processors. We then introduce a technique which allows us to prove that constructing an edge cover of a random graph from its adjacency matrix requires \Omega\Gammaequ n) expected time on a CRCW PRAM with O(n) processors. Constructing an edge cover is implicit in constructing a spanning tree, a Hamiltonian cycle, and a maximal matching, so this lower bound holds for all these problems, showing that our algorithms are optimal. This new lower bound technique is one of the very few lower bound techniques known which apply to randomized CRCW PRAM algorithms, and it pro..
Monitoring the introduction of pneumococcal conjugate vaccines into West Africa: design and implementation of a population-based surveillance system.
Routine use of pneumococcal conjugate vaccines (PCVs) in developing countries is expected to lead to a significant reduction in childhood deaths. However, PCVs have been associated with replacement disease with non-vaccine serotypes. We established a population-based surveillance system to document the direct and indirect impact of PCVs on the incidence of invasive pneumococcal disease (IPD) and radiological pneumonia in those aged 2 months and older in The Gambia, and to monitor changes in serotype-specific IPD. Here we describe how this surveillance system was set up and is being operated as a partnership between the Medical Research Council Unit and the Gambian Government. This surveillance system is expected to provide crucial information for immunisation policy and serves as a potential model for those introducing routine PCV vaccination in diverse settings
Infrared spectra and fragmentation dynamics of isotopologue-selective mixed-ligand complexes â
Isolated mixed-ligand complexes provide tractable model systems in which to study competitive and cooperative binding effects as well as controlled energy flow. Here, we report spectroscopic and isotopologue-selective infrared photofragmentation dynamics of mixed gas-phase Au(12/13CO)n(N2O)m+ complexes. The rich infrared action spectra, which are reproduced well using simulations of calculated lowest energy structures, clarify previous ambiguities in the assignment of vibrational bands, especially accidental coincidence of CO and N2O bands. The fragmentation dynamics exhibit the same unexpected behaviour as reported previously in which, once CO loss channels are energetically accessible, these dominate the fragmentation branching ratios, despite the much lower binding energy of N2O. We have investigated the dynamics computationally by considering anharmonic couplings between a relevant subset of normal modes involving both ligand stretch and intermolecular modes. Discrepancies between correlated and uncorrelated model fit to the ab initio potential energy curves are quantified using a Boltzmann sampled root mean squared deviation providing insight into efficiency of vibrational energy transfer between high frequency ligand stretches and the softer intermolecular modes which break during fragmentation
Avoided metallicity in a hole-doped Mott insulator on a triangular lattice
Charge carrier doping of a Mott insulator is known to give rise to a wide
variety of exotic emergent states, from high-temperature superconductivity to
various charge, spin, and orbital orders. The physics underpinning their
evolution is, however, poorly understood. A major challenge is the chemical
complexity associated with traditional routes to the addition or removal of
carriers. Here, we study the Mott insulating CrO layer of the delafossite
oxide PdCrO, where an intrinsic polar catastrophe provides a clean route to
induce substantial doping of the surface layer. Despite this, from scanning
tunneling microscopy and angle-resolved photoemission, we find that the surface
retains an insulating character, but with a modified electronic structure and
the development of a short-range ordered state with a distinct
periodicity. From density
functional theory, we demonstrate how this reflects the formation of an
intricate charge disproportionation that results in an insulating ground state
of the surface layer that is disparate from the hidden Mott insulator found in
the bulk. By applying voltage pulses to the surface layer, we induce
substantial local modifications to this state, which we find relax on a time
scale of tens of minutes, pointing to a glassy nature of the
charge-disproportionated insulator realised here.Comment: manuscript and supplementary, 37 pages in total, 4 figures in the
main text and 9 in the supplementar
Directional ballistic transport in the two-dimensional metal PdCoO2
In an idealized infinite crystal, the material properties are constrained by
the symmetries of its unit cell. Naturally, the point-group symmetry is broken
by the sample shape of any finite crystal, yet this is commonly unobservable in
macroscopic metals. To sense the shape-induced symmetry lowering in such
metals, long-lived bulk states originating from anisotropic Fermi surfaces are
needed. Here we show how strongly facetted Fermi surfaces and long
quasiparticle mean free paths present in microstructures of PdCoO2 yield an
in-plane resistivity anisotropy that is forbidden by symmetry on an infinite
hexagonal lattice. Bar shaped transport devices narrower than the mean free
path are carved from single crystals using focused ion beam (FIB) milling, such
that the ballistic charge carriers at low temperatures frequently collide with
both sidewalls defining a channel. Two symmetry-forbidden transport signatures
appear: the in-plane resistivity anisotropy exceeds a factor of 2, and
transverse voltages appear in zero magnetic field. We robustly identify the
channel direction as the source of symmetry breaking via ballistic Monte- Carlo
simulations and numerical solution of the Boltzmann equation
h/e oscillations in interlayer transport of delafossites
Funding: This project was funded by the European Research Council (ERC) under the European Unionâs Horizon 2020 research and innovation program (grant no. 715730, MiTopMat) and also was supported by the Max Planck Society. A.P.M. and R.M. acknowledge support from the WĂŒrzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter (EXC 2147). M.D.B., P.M., and V.S. acknowledge studentship funding from the EPSRC under grant no. EP/L015110/1. A.S. was supported by the Israel Science Foundation, the European Research Council (Project LEGOTOP), and the DFG through projectno.CRC-183. M.K. acknowledges support from the SIRIUS irradiation facility through project no. EMIR 2019 18-7099.Microstructures can be carefully designed to reveal the quantum phase of the wave-like nature of electrons in a metal. Here, we report phase-coherent oscillations of out-of-plane magnetoresistance in the layered delafossites PdCoO2 and PtCoO2 The oscillation period is equivalent to that determined by the magnetic flux quantum, h/e, threading an area defined by the atomic interlayer separation and the sample width, where h is Planck's constant and e is the charge of an electron. The phase of the electron wave function appears robust over length scales exceeding 10 micrometers and persisting up to temperatures of T > 50 kelvin. We show that the experimental signal stems from a periodic field modulation of the out-of-plane hopping. These results demonstrate extraordinary single-particle quantum coherence lengths in delafossites.PostprintPeer reviewe
Hierarchy of Lifshitz transitions in the surface electronic structure of Sr2RuO4 under uniaxial compression
Funding: We gratefully acknowledge support from the Engineering and Physical Sciences Research Council (Grant Nos. EP/T02108X/1 and EP/R031924/1), the European Research Council (through the QUESTDO project, 714193), and the Leverhulme Trust (Grant No. RL-2016-006). E.A.M., A.Z., and I.M. gratefully acknowledge studentship support from the International Max-Planck Research School for Chemistry and Physics of Quantum Materials. N.K. is supported by a KAKENHI Grants-in-Aids for Scientific Research (Grant Nos.18K04715, and 21H01033), and Core-to-Core Program (No. JPJSCCA20170002) from the Japan Society for the Promotion of Science (JSPS) and by a JST-Mirai Program (Grant No. JPMJMI18A3). APM and CWH acknowledge support from the Deutsche Forschungsgemeinschaft - TRR 435 288 - 422213477 (project A10). We thank Diamond Light Source for access to Beamline I05 (Proposals SI27471 and SI28412), which contributed to the results presented here.We report the evolution of the electronic structure at the surface of the layered perovskiteSr2RuO4 under large in-plane uniaxial compression, leading to anisotropic B1g strains of Δxx â Δyy = â0.9 ± 0.1%. From angle-resolved photoemission, we show how this drives a sequence of Lifshitz transitions, reshaping the low-energy electronic structure and the rich spectrum of van Hove singularities that the surface layer of Sr2RuO4 hosts. From comparison to tight-binding modelling, we find that the strain is accommodated predominantly by bond-length changes rather than modifications of octahedral tilt and rotation angles. Our study sheds new light on the nature of structural distortions at oxide surfaces, and how targeted control of these can be used to tune density of states singularities to the Fermi level, in turn paving the way to the possible realisation of rich collective states at the Sr2RuO4 surface.PostprintPeer reviewe
Tuneable electron-magnon coupling of ferromagnetic surface states in PdCoO2
Funding: We gratefully acknowledge support from the European Research Council (through the QUESTDO project, 714193), the Royal Society, the Max Planck Society, and the UKRI Engineering and Physical Sciences Research Council (Grant No. EP/S005005/1). V.S., O.J.C., and L.B. acknowledge the EPSRC for PhD studentship support through Grants EP/L015110/1, EP/K503162/1, and EP/G03673X/1, respectively. I.M. and D.C. acknowledge studentship support from the International Max-Planck Research School for Chemistry and Physics of Quantum Materials.Controlling spin wave excitations in magnetic materials underpins the burgeoning field of magnonics. Yet, little is known about how magnons interact with the conduction electrons of itinerant magnets, or how this interplay can be controlled. Via a surface-sensitive spectroscopic approach, we demonstrate a strong and highly-tuneable electron-magnon coupling at the Pd-terminated surface of the delafossite oxide PdCoO2, where a polar surface charge mediates a Stoner transition to itinerant surface ferromagnetism. We show how the coupling can be enhanced 7-fold with increasing surface disorder, and concomitant charge carrier doping, becoming sufficiently strong to drive the system into a polaronic regime, accompanied by a significant quasiparticle mass enhancement. Our study thus sheds new light on electron-magnon interactions in solid-state materials, and the ways in which these can be controlled.Publisher PDFPeer reviewe
Hierarchical Hough all-sky search for periodic gravitational waves in LIGO S5 data
We describe a new pipeline used to analyze the data from the fifth science
run (S5) of the LIGO detectors to search for continuous gravitational waves
from isolated spinning neutron stars. The method employed is based on the Hough
transform, which is a semi-coherent, computationally efficient, and robust
pattern recognition technique. The Hough transform is used to find signals in
the time-frequency plane of the data whose frequency evolution fits the pattern
produced by the Doppler shift imposed on the signal by the Earth's motion and
the pulsar's spin-down during the observation period. The main differences with
respect to previous Hough all-sky searches are described. These differences
include the use of a two-step hierarchical Hough search, analysis of
coincidences among the candidates produced in the first and second year of S5,
and veto strategies based on a test.Comment: 7 pages, 2 figures, Amaldi08 proceedings, submitted to JPC
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