C. Leighton

University of Minnesota Duluth, Duluth, Minnesota, United States

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Publications (255)848.35 Total impact

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    ABSTRACT: Nonlocal spin valves (NLSVs) generate pure spin currents, providing unique insight into spin injection and relaxation at the nanoscale. Recently it was shown that the puzzling low temperature nonmonotonicity of the spin accumulation in all-metal NLSVs occurs due to a manifestation of the Kondo effect arising from dilute local-moment-forming impurities in the nonmagnetic material. Here it is demonstrated that precise control over interdiffusion in Fe/Cu NLSVs via thermal annealing can induce dramatic increases in this Kondo suppression of injection efficiency, observation of injector/detector separation-dependent Kondo effects in both charge and spin channels simultaneously, and, in the limit of large interdiffusion, complete breakdown of standard Valet-Fert-based models. The Kondo effect in the charge channel enables extraction of the exact interdiffusion profile, quantifying the influence of local moment density on the injection efficiency and presenting a well-posed challenge to theory.
    No preview · Article · Jan 2016
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    ABSTRACT: Recently, motivated both by basic scientific interest and technological applications, and utilizing both experiment and theory, a number of investigators have independently identified a small group of off-stoichiometric Heusler alloys as having unusually interesting magnetic properties. These alloys take the form Ni50−xCoxMn25+yZ25−y (Z = Sn, In, Ga, etc.), being Co-doped off-stoichiometric versions of the better-known Ni2MnZ full Heusler compounds. In certain critical composition ranges these alloys are found to display unusually reversible martensitic phase transformations, multiferroicity (due to coexisting ferroelasticity and magnetic order), heightened sensitivity to compositional changes, and acute magnetic phase competition, leading to such exotic phenomena as spontaneous nanoscale magnetic inhomogeneity, collective cluster freezing, and intrinsic exchange bias. In terms of applications they can exhibitmagnetic-field-induced phase transformations,magnetic shape memory behavior, magnetocaloric effects, and remarkably low thermal hysteresis, making them attractive for sensors and actuators, magnetic refrigeration, and energy conversion devices. In this chapter we briefly review the current state of knowledge on the magnetic properties of these alloys, before presenting new results on the prototypical Ni50−xCoxMn40Sn10 system in the critical composition range0 ≤ x ≤ 14. Combining comprehensive magnetometry, exchange bias studies, and both new and previously published neutron scattering data, we present a detailed picture of the magnetic phenomenology in this alloy system and construct a magnetic phase diagram. Most importantly, based on these results and the work of others, we discuss in detail potential origins of the unusual magnetic properties of these materials, most notably the magnetic phase competition and nanoscale inhomogeneity that dominate their low temperature magnetism.
    No preview · Article · Jan 2016
  • Yintao Song · Chris Leighton · Richard D. James
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    ABSTRACT: Heusler alloys, particularly in the family Ni\(_2\)MnZ (Z \(=\) Ga, In, Sn and Sb) and nearby compositions, often exhibit martensitic phase transformation from a high-temperature cubic (L2\(_1\)) structure to a low-temperature, low-symmetry martensitic phase. These transformations are commonly accompanied by a change in magnetic ordering, due to the sensitivity of spin interactions to the change in interatomic distances and local symmetry. Various forms of energy can be made to interconvert during this multiferroic phase transformation. A particularly interesting family of alloys for this purpose is Ni\(_{50 - x}\)Co\(_x\)Mn\(_{25-y}\)Z\(_{y}\) (Z \(=\) Ga, In, Sn and Sb). Over a small compositional range, the phase transformation in these alloys is accompanied by a large change in magnetization—up to 1.2 MA/m (1200 emu/cm\(^3\)) in Ni\(_{45}\)Co\(_5\)Mn\(_{40}\)Sn\(_{10}\). In such materials it is possible to design an energy conversion device that directly converts heat to electricity using Faraday’s law of induction and cyclic phase transformation. Both the efficiency and work output per volume of such an energy conversion device are significantly affected by the size of the hysteresis , which is however tunable in these systems. A thermodynamic theory for such a energy conversion method is presented based on a free energy function that includes contributions from phase transformation and magnetism. Material constants in the free energy functions are determined by magnetic and calorimetric measurements. We give the estimates of efficiency and power output in terms of material constants, design parameters and working conditions for this energy conversion method. The first-order nature of the phase transformation, leading to an effect of magnetic field on transformation temperature and a mixed-phase region, play a critical role for the effectiveness of these methods.
    No preview · Article · Jan 2016
  • Ahmet Gulec · Daniel P. Phelan · Chris Leighton · Robert F. Klie
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    ABSTRACT: Perovskite cobaltites have been studied for years as some of the few solids to exhibit thermally-driven spin-state crossovers. The unanticipated first-order spin and electronic transitions recently discovered in Pr-based cobaltites are notably different from these conventional crossovers, and are understood in terms of a unique valence transition. In essence, the Pr valence is thought to spontaneously shift from 3+ towards 4+ on cooling, driving subsequent transitions in Co valence and electronic/magnetic properties. Here, we apply temperature-dependent transmission electron microscopy and spectroscopy to study this phenomenon, for the first time with atomic spatial resolution, in the prototypical (Pr0.85Y0.15)0.7Ca0.3CoO3-δ . In addition to the direct spectroscopic observation of charge transfer between Pr and Co at the 165 K transition (on both the Pr and O edges), we also find a simultaneous order/disorder transition associated with O vacancies. Remarkably, the first-order valence change drives a transition between ordered and random O vacancies, at constant O vacancy density, demonstrating reversible crystallization of such vacancies even at cryogenic temperatures.
    No preview · Article · Nov 2015 · ACS Nano
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    ABSTRACT: Reducing the dimensionality of a physical system can have a profound effect on its properties, as in the ordering of low-dimensional magnetic materials, phonon dispersion in mercury chain salts, sliding phases, and the electronic states of graphene. Here we explore the emergence of quasi-one-dimensional behaviour in two-dimensional artificial spin ice, a class of lithographically fabricated nanomagnet arrays used to study geometrical frustration. We extend the implementation of artificial spin ice by fabricating a new array geometry, the so-called tetris lattice. We demonstrate that the ground state of the tetris lattice consists of alternating ordered and disordered bands of nanomagnetic moments. The disordered bands can be mapped onto an emergent thermal one-dimensional Ising model. Furthermore, we show that the level of degeneracy associated with these bands dictates the susceptibility of island moments to thermally induced reversals, thus establishing that vertex frustration can reduce the relevant dimensionality of physical behaviour in a magnetic system.
    No preview · Article · Oct 2015 · Nature Physics
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    ABSTRACT: Recent progress with the thermally driven spin-state crossover in LaCoO3 has made it increasingly apparent that the nominally nonmagnetic low-spin ground state of this material actually hosts defect-based magnetism. This is investigated here via a small-angle neutron scattering (SANS) study of LaCoO3-δ crystals. The results provide (i) the surprising finding that the spin-state crossover is clearly reflected in SANS via quasieleastic and inelastic scattering from paramagnetic spin fluctuations and excitations, and (ii) evidence for the formation, likely around oxygen defects, of local entities known as magnetic excitons. The latter generate distinct magnetic scattering below 60 K, providing valuable quantitative information on exciton densities and interactions. Potential relevance to the unexpected ferromagnetism recently discovered in epitaxial LaCoO3 films is discussed. © 2015 American Physical Society.
    No preview · Article · Aug 2015 · Physical Review B
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    ABSTRACT: Relaxation calorimetry experiments were used to measure the temperature dependence of the heat capacity near the order-disorder phase transition (ODT) in a low molecular weight volumetrically symmetric, lamellae-forming, poly(1,4-isoprene-b-dl-lactide) diblock copolymer with Mn = 2750 g/mol (dispersity cross sign D = 1.10) and composition fPLA = 0.51. This enabled accurate determination of the latent heat of the ODT, yielding a value ΔHODT = 0.26 ± 0.02 J/g, consistent with previous measurements using differential scanning calorimetry. The relatively small magnitude of the latent heat supports the results of recent molecular simulations and reinforces the importance of composition fluctuations on the thermodynamics of block polymers near the ODT. These thermal measurements reveal no signature of the fluctuation effects in the disordered state as T → TODT, consistent with previous experimental and theoretical work, which supports the notion that composition fluctuations develop over a relatively wide range of temperatures for T > TODT. The finite width of the heat capacity peak, ΔTODT ≈ 1 °C, is shown to be consistent with molecular dispersity. These measurements demonstrate the utility of relaxation calorimetry as a quantitative semistatic thermal characterization tool for block polymers in the vicinity of phase transitions. (Graph Presented).
    No preview · Article · Jul 2015 · Macromolecules
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    ABSTRACT: The use of pyrite FeS2 as an earth-abundant, low-cost, nontoxic thin film photovoltaic hinges on improved understanding and control of certain physical and chemical properties. Phase stability, phase purity, stoichiometry, and defects, are central in this respect, as they are frequently implicated in poor solar cell performance. Here, phase-pure polycrystalline pyrite FeS2 films, synthesized by ex situ sulfidation, are subject to systematic reduction by vacuum annealing (to 550 °C) to assess phase stability, stoichiometry evolution, and their impact on transport. Bulk probes reveal the onset of pyrrhotite (Fe1-δS) around 400 °C, rapidly evolving into the majority phase by 425 °C. This is supported by X-ray photoelectron spectroscopy on (001) crystals, revealing surface Fe1-δS formation as low as 160 °C, with rapid growth near 400 °C. The impact on transport is dramatic, with Fe1-δS minority phases leading to a crossover from diffusive transport to hopping (due to conductive Fe1-δS nanoregions in an FeS2 matrix), followed by metallicity when Fe1-δS dominates. Notably, the crossover to hopping leads to an inversion of the sign, and a large decrease in magnitude of the Hall coefficient. By tracking resistivity, magnetotransport, magnetization, and structural/chemical parameters vs annealing, we provide a detailed picture of the evolution in properties with stoichiometry. A strong propensity for S-deficient minority phase formation is found, with no wide window where S vacancies control the FeS2 carrier density. These findings have important implications for FeS2 solar cell development, emphasizing the need for (a) nanoscale chemical homogeneity, and (b) caution in interpreting carrier types and densities.
    No preview · Article · Jun 2015 · ACS Applied Materials & Interfaces
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    ABSTRACT: For certain compositions Ni-Mn-Sn and related magnetic shape-memory alloys undergo a martensitic transition at temperatures in the range 300-400 K, with the emergence of novel magnetic properties below the transition. While Ni50Mn50 is an antiferromagnet, substitution of Sn on some fraction of the Mn sites in Ni50Mn50-xSnx leads to competing ferromagnetic (F) and antiferromagnetic (AF) phases at low temperatures. Details of this magnetic phase coexistence are, however, significantly lacking, particularly with respect to the AF phase. The present investigations use zero applied magnetic field Mn55 NMR as a local probe of the magnetic properties of the alloy Ni50Mn50-xSnx with x=10. Rich multipeak spectra are observed, and the various components are definitively assigned to nanoscale F or AF regions. Measurements of the static nuclear hyperfine field distributions as a function of temperature, and in small applied fields, together with nuclear relaxation rates provide detailed information on the size distributions, relative concentrations, and physical natures of these F and AF regions. The results show that the nanoscale magnetic features of the x=10 system are substantially more complex than previous studies have suggested. We argue that the general approach used in these experiments is applicable to other such complex metal alloys, and could yield many additional insights.
    No preview · Article · Jun 2015 · Physical Review B
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    ABSTRACT: BaSnO3 has recently been identified as a high mobility wide gap semiconductor with significant potential for room temperature oxide electronics. Here, a detailed study of the high pressure oxygen sputter-deposition, microstructure, morphology, and stoichiometry of epitaxial BaSnO3 on SrTiO3(001) and MgO(001) is reported, optimized conditions resulting in single-phase, relaxed, close to stoichiometric films. Most significantly, vacuum annealing is established as a facile route to n-doped BaSnO3−δ, leading to electron densities above 1019 cm−3, 5 mΩ cm resistivities, and room temperature mobility of 20 cm2 V−1 s−1 in 300-Å-thick films on MgO(001). Mobility limiting factors, and the substantial scope for their improvement, are discussed.
    Full-text · Article · Jun 2015 · APL Materials
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    Chris Leighton
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    ABSTRACT: There is no abstract available for this article.
    Preview · Article · May 2015 · Journal of Applied Physics
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    Chris Leighton
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    ABSTRACT: There is no abstract available for this article.
    Preview · Article · May 2015 · Journal of Applied Physics
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    ABSTRACT: There has been a tremendous recent surge of interest in copper zinc tin sulfide (Cu2ZnSnS4, CZTS) as a photovoltaic material, because its optical and electronic properties are well-suited for solar cells, and its elemental constituents are abundant in the earth’s crust. Here we have studied the formation mechanisms of CZTS films, and the factors that control the cation stoichiometry during ex situ sulfidation of precursor Cu-Zn-Sn alloy films in a closed isothermal system. We find that the Cu/Sn ratio in CZTS is self-regulating and approaches 2, regardless of the initial composition of the precursor films, provided that adequate Sn is available in the sulfidation system. If precursor films are initially Sn-rich excess Sn evaporates in the form of SnS. If precursor films are initially Sn-deficient, the inclusion of solid Sn in the sulfida-tion ampoule readily generates SnS vapor, which mitigates the films' Sn deficiency to return the Cu/Sn ratio to 2. When sulfidized for sufficiently long times at sufficiently high temperatures (e.g., 600 oC, 8 hours), films with similar Cu/Zn ra-tios exhibit similar phase compositions, such that if Cu/Zn >2, a Cu2SnS3 impurity phase is present in addition to CZTS, and if Cu/Zn < 2, a ZnS impurity phase occurs. To achieve phase-pure, void-free films, Sn-deficient precursor films with Cu/Zn in the desired range (typically close to, but slightly less than 2) can be sulfidized with excess Sn in a closed system, or a system that maintains a SnS vapor pressure over the film. Time-dependent sulfidation experiments were performed to elucidate the mechanism of this Sn self-regulation. During the formation of CZTS, almost all of the Sn is found to leave the film as SnS, later reincorporation of the Sn occurring through reactions between SnS vapor and CuS to form Cu2SnS3. The ZnS and Cu2SnS3 phases within the films then interdiffuse to form CZTS. Because Cu/Sn is 2 in both Cu2SnS3 and CZTS, the Cu/Sn ratio tends to 2 when sufficient Sn is included in the system to consume all Cu. This strategy is useful for avoiding Cu-S minority phases, provided the films are sulfidized to the point of equilibrium phase composition.
    No preview · Article · Mar 2015 · Chemistry of Materials
  • B L Le · D W Rench · R Misra · L O’Brien · C Leighton · N Samarth · P Schiffer
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    ABSTRACT: We investigate the magnetotransport properties of connected kagome artificial spin ice networks composed of permalloy nanowires. Our data show clear evidence of magnetic switching among the wires, both in the longitudinal and transverse magnetoresistance. An unusual asymmetry with field sweep direction appears at temperatures below about 20 K that appears to be associated with exchange bias resulting from surface oxidation of permalloy, and which disappears in alumina-capped samples. These results demonstrate that exchange bias is a phenomenon that must be considered in understanding the physics of such artificial spin ice systems, and that opens up new possibilities for their control.
    No preview · Article · Feb 2015 · New Journal of Physics
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    ABSTRACT: Low temperature atomic layer deposition of conformal ZnO on a self-assembled block polymer lithographic template comprising well-ordered, vertically-aligned cylindrical pores within a poly(styrene) (PS) matrix was used to produce nanocrucible templates with pore diameters tunable via ZnO thickness. Starting from a PS template with a hexagonal array of 30 nm diameter pores on a 45 nm pitch, the ZnO thickness was progressively increased to narrow the pore diameter to as low as 14 nm. Upon removal of the PS by heat treatment in air at 500 °C to form an array of size-tunable ZnO nanocrucibles, permalloy (Ni80Fe20) was evaporated at normal incidence, filling the pores and creating an overlayer. Argon ion beam milling was then used to etch back the overlayer (a Damascene-type process), leaving a well-ordered array of isolated ZnO nanocrucibles filled with permalloy nanodots. Microscopy and temperature-dependent magnetometry verified the diameter reduction with increasing ZnO thickness. The largest diameter (30 nm) dots exhibit a ferromagnetic multidomain/vortex state at 300 K, with relatively weakly temperature-dependent coercivity. Reducing the diameter leads to a crossover to a single domain state, and eventually superparamagnetism at sufficiently high temperature, in quantitative agreement with expectations. We argue that this approach could render this form of block polymer lithography compatible with high temperature processing (as required for technologically important high perpendicular anisotropy ordered alloys, for instance), in addition to enabling separation-dependent studies to probe inter-dot magnetostatic interactions.
    Full-text · Article · Jan 2015 · ACS Nano
  • Wei Xie · Shun Wang · Xin Zhang · C Leighton · C Daniel Frisbie
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    ABSTRACT: We report the observation of the Hall effect at hole densities up to 6×10^{13} cm^{-2} (0.3 holes/molecule) on the surface of electrolyte-gated rubrene crystals. The perplexing peak in the conductance as a function of gate voltage is confirmed to result from a maximum in mobility, which reaches 4 cm^{2} V^{-1} s^{-1} at 2.5×10^{13} cm^{-2}. Measurements to liquid helium temperatures reveal that this peak is markedly asymmetric, with bandlike and hopping-type transport occurring on the low density side, while unconventional, likely electrostatic-disorder-affected transport dominates the high density side. Most significantly, near the mobility peak the temperature coefficient of the resistance remains positive to as low as 120 K, the low temperature resistance becomes weakly temperature dependent, and the conductance reaches within a factor of 2 of e^{2}/h, revealing conduction unprecedentedly close to a two-dimensional metallic state.
    No preview · Article · Dec 2014 · Physical Review Letters
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    Sangwoo Lee · Chris Leighton · Frank S Bates
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    ABSTRACT: Frank-Kasper phases are tetrahedrally packed structures occurring in numerous materials, from elements to intermetallics to self-assembled soft materials. They exhibit complex manifolds of Wigner-Seitz cells with many-faceted polyhedra, forming an important bridge between the simple close-packed periodic and quasiperiodic crystals. The recent discovery of the Frank-Kasper σ-phase in diblock and tetrablock polymers stimulated the experiments reported here on a poly(isoprene-b-lactide) diblock copolymer melt. Analysis of small-angle X-ray scattering and mechanical spectroscopy exposes an undiscovered competition between the tendency to form self-assembled particles with spherical symmetry, and the necessity to fill space at uniform density within the framework imposed by the lattice. We thus deduce surprising analogies between the symmetry breaking at the body-centered cubic phase to σ-phase transition in diblock copolymers, mediated by exchange of mass, and the symmetry breaking in certain metals and alloys (such as the elements Mn and U), mediated by exchange of charge. Similar connections are made between the role of sphericity in real space for polymer systems, and the role of sphericity in reciprocal space for metallic systems such as intermetallic compounds and alloys. These findings establish new links between disparate materials classes, provide opportunities to improve the understanding of complex crystallization by building on synergies between hard and soft matter, and, perhaps most significantly, challenge the view that the symmetry breaking required to form reduced symmetry structures (possibly even quasiperiodic crystals) requires particles with multiple predetermined shapes and/or sizes.
    Preview · Article · Nov 2014 · Proceedings of the National Academy of Sciences
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    ABSTRACT: Copper zinc tin sulfide (CZTS) thin films were synthesized by ex situ sulfidation of Cu-Zn-Sn metal alloy precursor films cosputtered from Cu, Cu-Zn, and Cu-Sn targets onto five different substrate materials: single crystal quartz, fused quartz, sapphire, Pyrex, and soda lime glass (SLG). Cosputtered precursor films, which were found to consist of Cu, Zn, and Sn metals and Cu 6.26Sn5 ordered alloys, were sulfidized between 100 and 600 °C, corresponding to an S pressure range of 0.051–36 Torr. While CZTS forms at temperatures as low as 300 °C on all substrates, the film&apos;s phase composition is dominated by binary metal sulfides between 300 and 400 °C. Significant phase composition variations among films synthesized on different substrates begin to emerge at 400 °C. Films grown on SLG are nearly phase pure CZTS by 500 °C, with small amounts of ZnS. In contrast, films deposited on all other substrates persistently contain significant amounts of impurity phases such as SnS 2 and Cu 4Sn7S16 until the sulfidation temperature is increased to 600 °C. Significant grain growth also begins between 500 and 600 °C. At 600 °C, CZTS films synthesized on SLG were found to have significantly larger grains than films grown on any of the other substrates. These results demonstrate that CZTS phase purity and grain size, properties that may affect solar cell performance, are affected by impurity diffusion from the SLG substrate, further emphasizing the importance of selecting appropriate substrates.
    No preview · Article · Nov 2014 · Journal of Vacuum Science & Technology A Vacuum Surfaces and Films
  • S. Kelly · F. Galli · J. Aarts · Shameek Bose · M. Sharma · C. Leighton
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    ABSTRACT: Recent magnetotransport and neutron scattering measurements implicate interfacial magneto-electronic phase separation as the origin of the degradation in transport and magnetism in ultra-thin film La1−xSrxCoO3 on SrTiO3(001). Here, using low temperature scanning tunneling microscopy and spectroscopy the first direct, real space observation of this nanoscopic electronic inhomogeneity is provided. Films of thickness 12.4 nm (32 unit cells) are found to exhibit spatially uniform conductance, in stark contrast to 4.7 nm (12 unit cell) films that display rich variations in conductance, and thus local density of states. The electronic heterogeneity occurs across a hierarchy of length scales (5–50 nm), with complex correlations with both topography and applied magnetic fields. These results thus provide a direct observation of magneto-electronic inhomogeneity in SrTiO3(001)/La0.5Sr0.5CoO3 at thicknesses below 6–7 nm, in good agreement with less direct techniques.
    No preview · Article · Sep 2014 · Applied Physics Letters
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    Full-text · Article · Aug 2014 · Microscopy and Microanalysis

Publication Stats

5k Citations
848.35 Total Impact Points

Institutions

  • 2002-2016
    • University of Minnesota Duluth
      • • Department of Electrical Engineering
      • • Department of Chemistry and Biochemistry
      Duluth, Minnesota, United States
  • 2015
    • University of Illinois, Urbana-Champaign
      Urbana, Illinois, United States
    • University of Minnesota Twin Cities
      • Department of Chemical Engineering and Materials Science
      Minneapolis, Minnesota, United States
  • 2014
    • University of Oviedo
      • Department of Physics
      Oviedo, Asturias, Spain
  • 2012
    • University of Nebraska at Lincoln
      • Department of Physics and Astronomy
      Lincoln, Nebraska, United States
  • 2009
    • University of Bristol
      • School of Chemistry
      Bristol, England, United Kingdom
  • 1997-2007
    • Durham University
      • Department of Physics
      Durham, England, United Kingdom
  • 1999-2005
    • University of California, San Diego
      • Department of Physics
      San Diego, CA, United States