P J C King

University of the Witwatersrand, Johannesburg, Gauteng, South Africa

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Publications (83)140.01 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: The current paper discusses possible designs for a stand alone muon target for MuSR studies of condensed matter science. Considering the ISIS 7 mm graphite target as a reference, Geant4 simulations have been performed in order to optimize the target parameters with respect to muon and pion yield. Previous studies suggested that the muon production can be optimized by using a thin graphite slab target with an incident proton energy significantly lower than initially considered. Surface muon production obtained by firing an 800 MeV proton beam energy onto the target is simulated and potential improvements to the target material, geometry and angle orientation with respect to the incoming proton beam as well as an estimated performance of the muon target are presented in this paper. Implications for the ISIS muon facility are also discussed. A comparison of the pion production cross section between experimental data and three theoretical models for the latest four Geant4 versions is also included in this paper.
    Physical Review Special Topics - Accelerators and Beams 02/2014; 17(3). · 1.57 Impact Factor
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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: From magnetic, specific heat, ^{170}Yb Mössbauer effect, neutron diffraction, and muon spin relaxation measurements on polycrystalline Yb_{2}Sn_{2}O_{7}, we show that below the first order transition at 0.15 K all of the Yb^{3+} ions are long-range magnetically ordered and each has a moment of 1.1μ_{B} which lies at ≃10° to a common fourfold cubic axis. The four sublattice moments have four different directions away from this axis and are therefore noncoplanar. We term this arrangement splayed ferromagnetism. This ground state has a dynamical component with a fluctuation rate in the megahertz range. The net ferromagnetic exchange interaction has an anisotropy that favors the local threefold axis. We discuss our results in terms of the phase diagram proposed by Savary and Balents [Phys. Rev. Lett. 108, 037202 (2012)] for a pyrochlore lattice of Kramers 1/2 effective spins.
    Physical Review Letters 03/2013; 110(12):127207. · 7.73 Impact Factor
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    Adriana Bungau, Philip King, James Lord
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    ABSTRACT: In this paper we present the results of GEANT4 simulations of the production of surface muons as a function of energy of the incident protons on a graphite target. A validation of the GEANT4 hadronic physics models has been performed by comparing the results with experimental data from the Lawrence Radiation Laboratory, United States. Considering the ISIS muon target as a reference, simulations have been performed to optimize the pion and muon production. Of particular significance, we predict that optimal surface muon production occurs at a relatively modest proton energy of 500 MeV. This will be of importance for the development of future μSR facilities.
    Physical Review Special Topics - Accelerators and Beams 01/2013; 16(1). · 1.57 Impact Factor
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    ABSTRACT: From magnetic, specific heat, 170Yb Mössbauer effect, neutron diffraction, and muon spin relaxation measurements on polycrystalline Yb2Sn2O7, we show that below the first order transition at 0.15 K all of the Yb3+ ions are long-range magnetically ordered and each has a moment of 1.1μB which lies at ≃10° to a common fourfold cubic axis. The four sublattice moments have four different directions away from this axis and are therefore noncoplanar. We term this arrangement splayed ferromagnetism. This ground state has a dynamical component with a fluctuation rate in the megahertz range. The net ferromagnetic exchange interaction has an anisotropy that favors the local threefold axis. We discuss our results in terms of the phase diagram proposed by Savary and Balents [ Phys. Rev. Lett. 108 037202 (2012)] for a pyrochlore lattice of Kramers 1/2 effective spins.
    Physical Review Letters 01/2013; 110:127207. · 7.73 Impact Factor
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    ABSTRACT: Standard muon spin rotation (μSR) spectroscopy implants 4 MeV spin-polarized positive muons to investigate the bulk properties of matter. Success in producing epithermal muons opens interesting possibilities for studying ultrathin films, interfaces, and even surfaces. At the ISIS Facility, Rutherford Appleton Laboratory (Chilton, UK), we have produced a pulsed ultraslow muon beam (E< 20 eV) and have performed the first μSR experiments. Due to the pulsed feature, the implantation time is automatically determined and, by adjusting the final muon energy between about 8 and 20 eV, depth slicing experiments are possible down to monolayers distances. We describe slicing experiments across a 20 nm copper film on quartz substrate with evidence for a 2 nm copper oxide surface layer. A preliminary experiment on a hexagonal cobalt film suggests the existence of muon precession in the local magnetic field. The results are discussed in relation to the morphological features of the film.
    Applied Magnetic Resonance 04/2012; 19(3):471-477. · 0.83 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: In the pyrochlore Yb(2)GaSbO(7), the Yb(3+) sublattice forms a network of corner sharing tetrahedra and the second sublattice is made up of disordered, non-magnetic Ga(3+) and Sb(5+) ions. We have examined this compound using magnetic susceptibility, (170)Yb Mössbauer spectroscopy (down to 0.03 K) and muon spin relaxation (μSR) (down to 0.02 K) measurements. We establish the size of the Yb(3+) magnetic moments and that of the Yb(3+)-Yb(3+) coupling. At low temperatures, the correlated moments fluctuate between directions that are well tilted relative to the local [111] axis. The lattice disorder does not quench the frustration induced low temperature spin fluctuations but it does remove the first order dynamic transition that is present in the crystallographically ordered counterpart Yb(2)Ti(2)O(7). Below 1.0 K, the fluctuation rate of the correlated moments decreases progressively as the temperature is reduced and the moments remain dynamic down to 0.02 K where the rate is 7 × 10(7) s(-1). Magnetic frustration is operative in Yb(2)GaSbO(7) where the Yb(3+)-Yb(3+) interaction is antiferromagnetic as it is in Yb(2)Ti(2)O(7) where the interaction is ferromagnetic.
    Journal of Physics Condensed Matter 04/2011; 23(16):164217. · 2.22 Impact Factor
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    ABSTRACT: A limiting factor in the ability to interpret isotope effect measurements in cuprates is the absence of sufficiently accurate data for the whole phase diagram; there is precise data for Tc, but not for the antiferromagnetic transition temperature T_N. Extreme sensitivity of T_N to small changes in the amount of oxygen in the sample is the major problem. This problem is solved here by using the novel compound (Ca 0.1 La 0.9)(Ba 1.65 La 0.35)Cu 3 O y for which there is a region where T_N is independent of oxygen doping. Meticulous measurements of T_N for samples with oxygen 16 and oxygen 18 find the absence of an oxygen isotope effect on T_N with unprecedented accuracy. A possible interpretation of our finding and existing data is that isotope substitution affects the normal state charge carrier density.
    Advances in Condensed Matter Physics 02/2011; · 1.18 Impact Factor
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    01/2010;
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    01/2010;
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    ABSTRACT: We report Muonium (Mu) donor and acceptor levels in Czochralski-grown Silicon Germanium alloys (Cz-Si1−xGex). Measurement of these defect energies provides an analogous examination of Hydrogen defects that are otherwise inaccessible. Temperature-dependent Muonium fractions in several alloy samples (x=0.20,0.45,0.77,0.81,0.84,0.90,0.91,0.94,0.98) show charge-state transitions assigned to Mu donor and acceptor ionizations. Our results indicate a deep Mu donor level across the alloy system. The Mu acceptor level is deep in pure Si and valence-band resonant in pure Ge; we specifically examine the compositional dependence of the MuT0 acceptor ionization energy in Ge-rich alloys, where this level crosses into the valence band.
    Physical review. B, Condensed matter 01/2010; 82(20). · 3.77 Impact Factor
  • Neutron News 01/2010; 21(2):30-32.
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    ABSTRACT: Preliminary low-field data on the diamagnetic muon spin rotation signal in 4H-SiC is presented. The initial results on all three electrical types of 4H-SiC are compared with those for the 6H polytype. Analysis of amplitude transitions at low temperatures in n-and p-type samples indicate carrier capture. At temperatures above 600 K, an increase in diamagnetic amplitude and an associated dip in the phase indicate a slowly formed state in the p-type material. We discuss possible identification of transitions evident from temperature dependent diamagnetic amplitudes and correlations with neutral muonium centers.
    Physica B Condensed Matter 04/2009; · 1.28 Impact Factor
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    ABSTRACT: The behaviour of monatomic hydrogen defect centres is modelled using implanted positive muons in both allotropes of elemental tin. A new search for paramagnetic muonium is made in αα-Sn, the semiconducting allotrope with the diamond structure. No deep state is found, nor any enhanced Korringa relaxation that would suggest localized moment formation on interstitial protons in this intriguing material. In relation to muonium in the other Group-IV semiconductors, tantalizing indications of shallow state formation below 30 K suggest that a deep-to-shallow transition may be responsible. The diamagnetic state remains static to 200 K and may be tentatively identified as the bond-centred positive ion, by comparison with recent data for silicon. This behaviour contrasts with the abrupt onset of mobility at 50 K for muons in β-Snβ-Sn, the normal metallic allotrope with tetragonal structure, for which a more conventional interstitial site has lately been determined. Mobility studies are admittedly difficult in the presence of such weak nuclear magnetism but a discrepancy arises between reported migration barriers in white tin.
    Physica B Condensed Matter 04/2009; 404(s 5–7):884–887. · 1.28 Impact Factor
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    ABSTRACT: We discuss possible designs for a high-intensity, stand-alone muon source dedicated to and fully optimised for μSR studies. In particular, we focus upon the potential implementation of a new generation of high-power, but relatively compact and cost effective, proton drivers based on non-scaling fixed-field alternating gradient (ns-FFAG) accelerator technology. Initial considerations suggest that a facility with multiple optimised pion targets, each affording positron count rates approximately two orders of magnitude higher than existing pulsed muon sources, together with the potential of steady state operation at count rates comparable to the best existing sources, should be achievable at reasonable cost. The relative merits of a stand-alone muon facility with respect to those of current facilities which operate in symbiotic mode with other users of the proton driver are highlighted. The outstanding technical issues which must be addressed by both muon scientists and accelerator technologists are also considered.
    Physica B Condensed Matter 04/2009; · 1.28 Impact Factor
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    ABSTRACT: The Muonium (Mu) [0/-] transition energy level is predicted to become valance band resonant in the Silicon Germanium alloy system for a Ge content of roughly 92%. The comparison of observations from various Muon Spin Research (MuSR) techniques indicate a Mu signal consistent with a shallow acceptor state. We report our MuSR measurements of diamagnetic and paramagnetic Muonium states in Czochralski grown Si0.09Ge0.91 as part of an on-going effort to characterize the Mu shallow acceptor as well as testing the prediction of a universal Hydrogen (Muonium) defect level. Muonium states have been examined by means of Transverse Field (TF), Longitudinal Field (LF), and Radio Frequency (RF) MuSR techniques that provide information on Mu charge states, hyperfine distributions, and motional properties. Characterization of the ubiquitous H defect is important in the development of high mobility components of novel transistors and optoelectronic devices based on SiGe alloy heterojunctions.
    04/2009;
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    ABSTRACT: The charge state of muonium has been investigated in p-type doped, nominally undoped (low n-type) and heavily n-type doped InAs. The donor Mu(+) state is shown to be the dominant defect in all cases. Consequently, muonium does not simply counteract the prevailing conductivity in this material. This is consistent with the charge neutrality level lying above the conduction band minimum in InAs.
    Journal of Physics Condensed Matter 02/2009; 21(7):075803. · 2.22 Impact Factor
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    01/2009;

Publication Stats

458 Citations
140.01 Total Impact Points

Institutions

  • 2009
    • University of the Witwatersrand
      • School of Physics
      Johannesburg, Gauteng, South Africa
  • 2003–2009
    • Texas Tech University
      • Department of Physics
      Lubbock, TX, United States
    • University of Oxford
      • Department of Physics
      Oxford, ENG, United Kingdom
  • 2007
    • University of Alberta
      • Department of Physics
      Edmonton, Alberta, Canada
  • 2002
    • Argonne National Laboratory
      Lemont, Illinois, United States
  • 2001
    • Tohoku University
      • Institute for Materials Research
      Sendai, Kagoshima-ken, Japan
  • 1999–2000
    • Università degli studi di Parma
      • Department of Chemistry
      Parma, Emilia-Romagna, Italy