S P Cottrell

University of Coimbra, Coimbra, Distrito de Coimbra, Portugal

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Publications (138)227.25 Total impact

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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.
    04/2014; 751.
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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.
    Physica Scripta 12/2013; 88(6):8502-. · 1.03 Impact Factor
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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.
    The Journal of Physical Chemistry B 10/2013; · 3.61 Impact Factor
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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.
    Physical Chemistry Chemical Physics 07/2012; 14(31):10953-66. · 4.20 Impact Factor
  • 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.
    Journal of Magnetic Resonance 11/2011; 214(1):144-50. · 2.30 Impact Factor
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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.
    Review of Scientific Instruments 07/2011; 82(7):073904-073904-10. · 1.60 Impact Factor
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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?
    EPL (Europhysics Letters) 04/2011; 94(2):26001. · 2.26 Impact Factor
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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.
    Phys. Rev. B. 12/2009; 80(21).
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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.
    Physical Review Letters 11/2009; 103(21):216601. · 7.73 Impact Factor
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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.
    Physica B Condensed Matter 04/2009; · 1.28 Impact Factor
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    Physica B Condensed Matter 01/2009; 404(5-7):827-830. · 1.28 Impact Factor
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    ABSTRACT: A paramagnetic muonium centre is detected in elemental boron at cryogenic temperatures and characterized by hyperfine decoupling in longitudinal magnetic fields. The contact term is only 0.52 GHz or 12% of the free muonium value. An effective ionization energy of 0.1 eV is estimated from polarization transfer to the muon Larmor precession signal in transverse field, beginning around 100 K and complete by room temperature. AIMPRO density functional calculations identify candidate sites for monatomic hydrogen defect centres in the rather complex boron lattice and predict the local distribution of unpaired electron spin density. One of these, bridging between two boron atoms in the inter-icosahedral space, provides an adequate match to the muonium hyperfine parameters, as well as its relatively weak binding energy. Negative-U behaviour is predicted, implying that H0 centres are not thermodynamically stable. Identification of the high diamagnetic muon fraction found at all temperatures is also discussed.
    Physica B Condensed Matter 01/2009; · 1.28 Impact Factor
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    ABSTRACT: Recently the spin of the electron has become the focus of a new direction in electronics - semiconductor spintronics - which utilizes mechanisms of strong mutual influence of magnetic and electrical properties in magnetic semiconductors. These mechanisms are still a matter of considerable debate; however, all of them involve a concept of magnetic polaron - a microscopic cloud of magnetization made of several neighbouring magnetic ions and a carrier(s) - which determines most of the electrical, magnetic and optical properties of the material. Although a great number of experiments indicate the existence of magnetic polaron in magnetic semiconductors and related materials it has eluded direct observation until now. Using the positive muon as both a donor centre and a local magnetic probe, we have been able to generate and detect the magnetic polaron and determine its size in the magnetic semiconductor EuS.
    Physica B Condensed Matter 01/2009; · 1.28 Impact Factor
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    ABSTRACT: We report on the stability and dynamics of muonium centers in heavily doped p-type and n-type Si crystals. The measurements were conducted in the absence of an applied magnetic field and were carried out at the ISIS facility in the United Kingdom. This allows us to detect weak dipolar interactions between the probe and its surrounding host nuclei. The Mu+ center was successfully observed in the p-type Si:B sample, where its dipolar interaction with the 29Si neighboring nuclei was detected. The stability, diffusivity and reactivity of this positive diamagnetic center were clearly identified. On the other hand, the neutral center appear to dominate the dynamics in the n-type Si:As sample due to its very rapid spin exchange rate , and thus it conceals the effect due to the Mu- center.
    Physica B Condensed Matter 01/2009; 404:831-833. · 1.28 Impact Factor
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    D. G. Eshchenko, V. G. Storchak, S. P. Cottrell
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    ABSTRACT: We report on preliminary muon-track-induced current measurements in semi-insulating (SI-) GaAs. At T=70K, after simultaneous treatment of the sample by muon irradiation and a strong electric field (a square wave with |E|>10kV/cm and the polarity changed every 150s) for approximately 2h, the sample is transferred to a metastable-like state which is characterized by increased life-times for non-equilibrium electrons and holes. The sample can be returned to the original state by heating up to 250K. Our results for SI-GaAs suggest a muon-track-induced electric-field-assisted neutralization process for the deep traps.
    Physica B Condensed Matter 01/2009; 404(5):880-883. · 1.28 Impact Factor
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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.
    Physica B Condensed Matter 01/2009; 404(5):876-879. · 1.28 Impact Factor
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    ABSTRACT: A new muSR spectrometer (HiFi) is being constructed at the ISIS pulsed muon source. The spectrometer is intended for measurements in longitudinal magnetic fields up to 5 T. Here we discuss various aspects of the design and future operation of the HiFi spectrometer. Detailed analysis of the detector design is presented, and results from tests on detector prototypes are compared to simulations. Potential applications of this spectrometer in the fields of condensed matter and chemistry are also discussed.
    Physica B Condensed Matter 01/2009; · 1.28 Impact Factor
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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.
    Physical review. B, Condensed matter 01/2009; 80. · 3.77 Impact Factor
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    ABSTRACT: The use of scintillating fibres as positron detectors in muSR experiments is discussed. Experimental tests and numerical simulations show that 1 mm thick fibres represent an optimum choice between a sufficiently high signal level and a good spatial resolution. A comparison of decay positron detection efficiency with that of electrons from common 90Sr sources shows that the existing results with beta emitters could be transferred almost unvaried to fibres for use in muSR spectrometers. Thanks to a quantum efficiency above 50% and to a continuously improving gain (currently at 1×10), the newly developed Geiger-mode avalanche photodiodes (G-APDs) represent the most promising choice as associated photon detectors.
    Physica B Condensed Matter 01/2009; 404:982-985. · 1.28 Impact Factor
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    ABSTRACT: The charge dynamics of hydrogen-like centers formed by the implantation of energetic (4 MeV) muons in semi-insulating GaAs have been studied by muon spin resonance in electric fields. The results point to the significant role of deep hole traps in the compensation mechanism of GaAs. Electric-field-enhanced neutralization of deep electron and hole traps by muon-track-induced hot carriers results to an increase of the non-equilibrium carrier life-times. As a consequence, the muonium ($\mu^+ + e^-$) center at the tetrahedral As site can capture the track's holes and therefore behaves like a donor.
    11/2008;

Publication Stats

554 Citations
227.25 Total Impact Points

Institutions

  • 2009
    • University of Coimbra
      • Departamento de Física
      Coimbra, Distrito de Coimbra, Portugal
    • Paul Scherrer Institut
      • Laboratory for Muon Spin Spectroscopy (LMU)
      Aargau, Switzerland
  • 2006–2009
    • University of Alberta
      • Department of Physics
      Edmonton, Alberta, Canada
  • 2007
    • TRIUMF
      Vancouver, British Columbia, Canada
  • 2003–2006
    • University of East Anglia
      • School of Chemistry
      Norwich, ENG, United Kingdom
    • University of British Columbia - Vancouver
      • Department of Physics and Astronomy
      Vancouver, British Columbia, Canada
    • Delft University Of Technology
      • Faculty of Applied Sciences (AS)
      Delft, South Holland, Netherlands
    • University of Southampton
      • Department of Physics and Astronomy
      Southampton, England, United Kingdom
  • 1998–2006
    • University College London
      • Department of Physics and Astronomy
      Londinium, England, United Kingdom