S P Cottrell

University of British Columbia - Vancouver, Vancouver, British Columbia, Canada

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Publications (146)244.49 Total impact

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    ABSTRACT: The study of kinetic isotope effects for H-atom abstraction rates by incident H-atoms from the homologous series of lower mass alkanes (CH4 , C2H6 and, here, C3H8 ) provide important tests of reaction rate theory on polyatomic systems. With a mass of only 0.114 amu, the most sensitive test is provided by the rates of the Mu atom. Abstraction of H by Mu can be highly endoergic, due to the large zero-point energy shift in the MuH bond formed, which also gives rise to high activation energies from similar zero-point energy corrections at the transition state. Rates are then far too slow near 300 K to be measured by conventional TF-µSR techniques that follow the disappearance of the spin-polarised Mu atom with time. Reported here is the first measurement of a slow Mu reaction rate in the gas phase by the technique of diamagnetic Radio Frequency (RF) resonance, where the amplitude of the MuH product formed in the Mu + C3H8 reaction is followed with time. The measured rate constant, kMu = (6.8 ± 0.5)× 10^−16 cm^3 s^−1 at 300 K, is surprisingly only about a factor of three slower than that for H + C3H8 , indicating a dominant contribution from quantum tunneling in the Mu reaction, consistent with elementary transition state theory calculations of the kMu /kH kinetic isotope effect.
    No preview · Article · Jun 2015 · Physical Chemistry Chemical Physics
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    ABSTRACT: For the last 27 years, muon experiments at ISIS have been making a significant contribution to a number of scientific fields. However, as a community of researchers, we are always aiming to improve and extend the instruments' capabilities. In this paper we will review the current significant developments at the ISIS muon facility, namely the primary beamline upgrade, proton pulse compression and the MANTID muon analysis package.
    No preview · Article · Dec 2014 · Journal of Physics Conference Series
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    ABSTRACT: We report muon spin rotation/relaxation measurements of muonium in mesoporous silica (SBA-15) with a high specific surface area of 600 m2/g. Up to 70 percent of the incoming muons form muonium and escape efficiently into the open pores at all temperatures between 3 and 300K. We present evidence that the interaction with the silica surfaces involves both spin exchange and a transition to a diamagnetic state, possibly due to dangling bonds on the surface. At very low temperatures, below 20K, the interaction between muonium and the silica surfaces is suppressed due to a He film coating the surfaces. These results indicate that it should be possible to use muonium to probe the surfaces of uncapped nanoparticles supported in silica.
    Full-text · Article · Dec 2014 · Journal of Physics Conference Series
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    ABSTRACT: μSR and ALCR techniques have been used to investigate the structure and dynamics of the Mu-cyclohexadienyl radical interacting with Au and Pt metal nanoparticles (MNPs) supported in mesoporous silica (SBA-15). Surprisingly, coherent precession signals are observed and the isotropic hyperfine coupling constants are almost the same in loaded and unloaded samples, implying that the electronic structure of MuC6H6 is only weakly perturbed by the presence of the MNPs. We propose the observed radicals are shielded from the metallic surfaces by a benzene coating on the MNPs. The Δ1 resonance is observable in MNP-loaded samples at higher temperatures than in the unloaded SBA-15. This is attributed to stronger binding of MuC6H6 to the benzene coated MNPs.
    No preview · Article · Dec 2014 · Journal of Physics Conference Series
  • I McKenzie · S P Cottrell
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    ABSTRACT: Longitudinal field muon spin relaxation (LF-μSR) has been used to measure the magnetic field dependence of the longitudinal field muon spin relaxation rates (λμ = 1/Tμ1) of two muoniated radicals, 2-muoxyprop-2-yl radical (formed by Mu addition to acetone) and the muoniated 1,2-dicarboxyvinyl radical dianion (formed by Mu addition to the 1,2- dicarboxyacetylene dianion), in aqueous solution. The rotational correlations time of the radicals in solution at 298 K were determined from the magnetic field dependence of λμ and were found to be 32±2 ps for the 2-muoxyprop-2-yl radical and 55±3 ps for the muoniated 1,2-dicarboxyvinyl radical dianion.
    No preview · Article · Dec 2014 · Journal of Physics Conference Series
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    ABSTRACT: Polyether ether ketone (PEEK) is a thermoplastic polymer with a wide range of applications due to its chemical inertness and thermal stability, and for these reasons sample cells for gas and liquid phase μSR have been constructed from PEEK. Muon levelcrossing resonance (μLCR) studies of PEEK revealed a broad, strong μLCR signal that, we hypothesize, is due to multiple overlapping resonances from the various muonium (Mu) adducts of PEEK. To investigate this, two monomer units from PEEK (4,4'-dihydroxybenzophenone and para-dimethoxybenzene) were studied in solution using transverse-field muon spin rotation (TF-μSR) and μLCR. Two different muoniated radicals were formed by Mu addition to 4,4/- dihydroxybenzophenone and one radical was formed in para-dimethoxybenzene. The μSR spectra were assigned by comparing the experimentally measured muon and proton hyperfine coupling constants with values calculated for the possible structures using Gaussian-09 software with the B3LYP functional and 6-31G basis set. Good agreement was found for cyclohexadienyl- type radicals formed by Mu addition to the benzene rings of the monomer units. We can also infer that these radicals are being formed in PEEK, and based on this we conclude that sample cells made of PEEK are unsuitable for many types of μSR experiment.
    No preview · Article · Dec 2014 · Journal of Physics Conference Series
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    ABSTRACT: This work describes the development of a state-of-the-art muon spectrometer for the ISIS pulsed muon source. Conceived as a major upgrade of the highly successful EMU instrument, emphasis has been placed on making effective use of the enhanced flux now available at the ISIS source. This has been achieved both through the development of a highly segmented detector array and enhanced data acquisition electronics. The pulsed nature of the ISIS beam is particularly suited to the development of novel experiments involving external stimuli, and therefore the ability to sequence external equipment has been added to the acquisition system. Finally, the opportunity has also been taken to improve both the magnetic field and temperature range provided by the spectrometer, to better equip the instrument for running the future ISIS user programme.
    Preview · Article · Apr 2014 · Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment
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    ABSTRACT: This paper marks the first 25 years of muon production at ISIS and the creation in that time of a facility dedicated to the use of these elementary particles as unique microscopic probes in condensed matter and molecular science. It introduces the basic techniques of muon spin rotation, relaxation and resonance, collectively known as μSR, that were already in use by specialist groups at other accelerator labs by the mid-1980s. It describes how these techniques have been implemented and made available at ISIS, beginning in 1987, and how they have evolved and improved since then. Ever widening applications embrace magnetism, superconductivity, interstitial diffusion and charge transport, semiconductors and dielectrics, chemical physics and radical chemistry. Over these first 25 years, a fully supported user facility has been established, open to all academic and industrial users. It presently comprises four scheduled instruments, optimized for different types of measurement, together with auxiliary equipment for radiofrequency or microwave spin manipulation and future plans for pump-probe laser excitation.
    No preview · Article · Dec 2013 · Physica Scripta
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    ABSTRACT: The muon hyperfine coupling constant (Aμ) of the muoniated cyclohexadienyl radical (C6H6Mu) has been directly measured in a 5 mM solution of benzene in water by the radio-frequency muon spin resonance (RF-μSR) technique. The relative shift of Aμ in aqueous solution compared with the value in neat benzene (ΔAμ/Aμ = +0.98(5)% at 293 K) can now be compared directly with theoretical predictions. Application of the RF-μSR method to other dilute systems will provide extremely important information on understanding solvent effects.
    No preview · Article · Oct 2013 · The Journal of Physical Chemistry B
  • Donald G Fleming · Stephen P Cottrell · Iain McKenzie · Roderick M Macrae
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    ABSTRACT: New evidence is presented for the observation of a muoniated radical in the Mu + Br(2) system, from μSR longitudinal field (LF) repolarisation studies in the gas phase, at Br(2) concentrations of 0.1 bar in a Br(2)/N(2) mixture at 300 K and at 10 bar total pressure. The LF repolarisation curve, up to a field of 4.5 kG, reveals two paramagnetic components, one for the Mu atom, formed promptly during the slowing-down process of the positive muon, with a known Mu hyperfine coupling constant (hfcc) of 4463 MHz, and one for a muoniated radical formed by fast Mu addition. From model fits to the Br(2)/N(2) data, the radical component is found to have an unusually high muon hfcc, assessed to be ∼3300 MHz with an overall error due to systematics expected to exceed 10%. This high muon hfcc is taken as evidence for the observation of either the Br-Mu-Br radical, and hence of vibrational bonding in this H[combining low line]-L[combining low line]-H[combining low line] system, or of a MuBr(2) van der Waals complex formed in the entrance channel. Preliminary ab initio electronic structure calculations suggest the latter is more likely but fully rigorous calculations of the effect of dynamics on the hfcc for either system have yet to be carried out.
    No preview · Article · Jul 2012 · Physical Chemistry Chemical Physics
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    ABSTRACT: Originally investigated as a candidate common data format, NeXus was an easy choice when ISIS developed the present PC-based acquisition system. After over ten years of use there was a clear need to revisit the original instrument definition to improve utility, and this work is discussed in the paper. Using NeXus has opened up the possibility of accessing analysis codes developed by the wider scientific community. The application of Mantid for analysingmuon data is particularly interesting, as this offers the muon community access to an analysis framework that is attracting broad international support. Recently, we have worked with the Mantid development team to program an interface for manipulatingmuon data that has confirmed the platform as an ideal tool for mu SR analysis.
    No preview · Article · Jan 2012 · Physics Procedia
  • Nigel J Clayden · Stephen P Cottrell · Iain McKenzie
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    ABSTRACT: The application of composite inversion pulses to a novel area of magnetic resonance, namely muon spin resonance, is demonstrated. Results confirm that efficient spin inversion can readily be achieved using this technique, despite the challenging experimental setup required for beamline measurements and the short lifetime (≈2.2μs) associated with the positive muon probe. Intriguingly, because the muon spin polarisation is detected by positron emission, the muon magnetisation can be monitored during the radio-frequency (RF) pulse to provide a unique insight into the effect of the RF field on the spin polarisation. This technique is used to explore the application of RF inversion sequences under the non-ideal conditions typically encountered when setting up pulsed muon resonance experiments.
    No preview · Article · Nov 2011 · Journal of Magnetic Resonance
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    ABSTRACT: The high magnetic field (HiFi) muon instrument at the ISIS pulsed neutron and muon source is a state-of-the-art spectrometer designed to provide applied magnetic fields up to 5 T for muon studies of condensed matter and molecular systems. The spectrometer is optimised for time-differential muon spin relaxation studies at a pulsed muon source. We describe the challenges involved in its design and construction, detailing, in particular, the magnet and detector performance. Commissioning experiments have been conducted and the results are presented to demonstrate the scientific capabilities of the new instrument.
    No preview · Article · Jul 2011 · Review of Scientific Instruments
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    ABSTRACT: Muonium (μ+e− or Mu) atoms have long been believed to exhibit band-like propagation at low temperatures in ionic insulators and semiconductors. However, new muon spin relaxation measurements in transverse magnetic field reveal strong localization of Mu atoms at low temperatures in the ionic insulator KCl, similar to that observed in van der Waals cryocrystals. This discrepancy with previous results challenges our understanding of muonium quantum dynamics in solids: can Mu atoms ever truly delocalize at low temperature, or is low-temperature Mu localization a universal phenomenon?
    Full-text · Article · Apr 2011 · EPL (Europhysics Letters)
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    ABSTRACT: Palladium bionanomaterial was manufactured using the sulfate-reducing bacterium, Desulfovibrio desulfuricansm, to reduce soluble Pd(II) ions to cell-bound Pd(0) in the presence of hydrogen. The biomaterial was examined using a Superconducting Quantum Interference Device (SQUID) to measure bulk magnetisation and by Muon Spin Rotation Spectroscopy (µSR) which is uniquely able to probe the local magnetic environment inside the sample. Results showed behaviour attributable to interaction of muons both with palladium electrons and the nuclei of hydrogen trapped in the particles during manufacture. Electronic magnetism, also suggested by SQUID, is not characteristic of bulk palladium and is consistent with the presence of nanoparticles previously seen in electron micrographs. We show the first use of μSR as a tool to probe the internal magnetic environment of a biologically-derived nanocatalyst material.
    No preview · Article · Feb 2011 · Biotechnology Letters
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    ABSTRACT: We have examined the magnetic properties of polycrystalline, superconducting YBa2(Cu0.96Ni0.04)3Oy (y∼7, Tsc∼75 K) using two local probe techniques: 170Yb Mössbauer down to 0.1 K and muon-spin relaxation (μSR) down to 1.5 K. At 0.1 K, the 170Yb measurements show the Cu(2) over essentially all the sample volume carry magnetically correlated moments which are static on the time scale 10−9 s. The moments show a distribution in size. The correlations are probably short range. As the temperature increases, the correlated moments are observed to fluctuate with measurable rates (in the GHz range) which increase as the temperature increases and which show a wide distribution. The μSR measurements also evidence that the fluctuation rates increase with increasing temperature and there is a distribution. The evidenced fluctuating, correlated Cu(2) moments coexist at an atomic level with superconductivity.
    Preview · Article · Dec 2009
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    ABSTRACT: We have examined the magnetic properties of superconducting YBa2(Cu0.96Co0.04)3Oy(y7,Tsc=65K) using elastic neutron scattering and muon spin relaxation (muSR) on single-crystal samples. The elastic-neutron-scattering measurements evidence magnetic reflections, which correspond to a commensurate antiferromagnetic Cu(2) magnetic structure with an associated Néel temperature TN˜400K . This magnetically correlated state is not evidenced by the muSR measurements. We suggest that this apparent anomaly arises because the magnetically correlated state is dynamic in nature. It fluctuates with rates that are low enough for it to appear static on the time scale of the elastic-neutron-scattering measurements, whereas on the time scale of the muSR measurements, at least down to ˜50K , it fluctuates too fast to be detected. The different results confirm the conclusions reached from work on equivalent polycrystalline compounds: the evidenced fluctuating, correlated Cu(2) moments coexist at an atomic level with superconductivity.
    Full-text · Article · Dec 2009 · Physical review. B, Condensed matter
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    D G Eshchenko · V G Storchak · S P Cottrell · E Morenzoni
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    ABSTRACT: The charge dynamics of hydrogenlike centers in semi-insulating GaAs have been studied by muon spin resonance in the presence of electric field and RF excitation. Electric-field-enhanced neutralization of deep electron and hole traps by track-induced hot carriers results in an increase of the excess electron's or hole's lifetimes. Similar processes may take place in semiconductor devices working at high voltages and/or under irradiation. As a consequence of the deep traps neutralization, the muonium (mu{+} + e{-}) center can capture a hole.
    Full-text · Article · Nov 2009 · Physical Review Letters
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    ABSTRACT: We have investigated a single crystal of the wide bandgap II–VI semiconductor ZnSe. The sample was highly resistive due to heavy compensation of this n-type semiconductor. In low transverse fields, clear signs of conversion from a paramagnetic to a diamagnetic fraction are observed, at about 60 K. The data are interpreted as delayed electron capture by paramagnetic muonium, forming the negatively charged state Mu-. The implications with respect to the electrical activity of muonium, and by analogy hydrogen, in this semiconductor are analyzed.
    Full-text · Article · Apr 2009 · Physica B Condensed Matter
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    D.G. Eshchenko · V.G. Storchak · S.P. Cottrell
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    ABSTRACT: We report on combined RF-μSR and electric fields (EF) μSR measurements in semi-insulating GaP. At low temperature the capture of excess carriers by a neutral muonium (Mu=μ++e-) center is suggested to be a dominant process in muonium charge dynamics. The charged muonium fraction (Mu- or Mu+) formed on the microsecond time-scale is increased with lowering the temperature. Contrary to the usually observed scenario, at T40K an EF prevents formation of the final charged state.
    Full-text · Article · Apr 2009 · Physica B Condensed Matter

Publication Stats

1k Citations
244.49 Total Impact Points

Institutions

  • 2014
    • University of British Columbia - Vancouver
      Vancouver, British Columbia, Canada
  • 2011
    • Science and Technology Facilities Council
      Swindon, England, United Kingdom
  • 2001-2009
    • University of Coimbra
      • Departamento de Física
      Coimbra, Distrito de Coimbra, Portugal
    • University College London
      • Department of Physics and Astronomy
      Londinium, England, United Kingdom
  • 2003-2006
    • Institute of Science In Society
      Abingdon-on-Thames, England, United Kingdom