C. Leighton

University of South Florida, Tampa, Florida, United States

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Publications (236)752.59 Total impact

  • 06/2015; 3(6):062509. DOI:10.1063/1.4919969
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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.
    Chemistry of Materials 03/2015; 27(7):150312115406007. DOI:10.1021/acs.chemmater.5b00108 · 8.54 Impact Factor
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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.
    New Journal of Physics 02/2015; 17(2). DOI:10.1088/1367-2630/17/2/023047 · 3.67 Impact Factor
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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.
    ACS Nano 01/2015; 9(2). DOI:10.1021/nn505731n · 12.03 Impact Factor
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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.
    Physical Review Letters 12/2014; 113(24):246602. DOI:10.1103/PhysRevLett.113.246602 · 7.73 Impact Factor
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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.
    Applied Physics Letters 09/2014; 105(11):112909-112909-4. DOI:10.1063/1.4896283 · 3.52 Impact Factor
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    Microscopy and Microanalysis 08/2014; 20(S3):556-557. DOI:10.1017/S1431927614004504 · 1.76 Impact Factor
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    ABSTRACT: Detailed experiments designed to optimize and understand the solvent vapor annealing of cylinder-forming poly(styrene)-block-poly(lactide) thin films for nanolithographic applications are reported. By combining climate-controlled solvent vapor annealing (including in situ probes of solvent concentration) with comparative small-angle X-ray scattering studies of solvent-swollen bulk polymers of identical composition, it is concluded that a narrow window of optimal solvent concentration occurs just on the ordered side of the order-disorder transition. In this window, the lateral correlation length of the hexagonally close-packed ordering, the defect density, and the cylinder orientation are simultaneously optimized, resulting in single-crystal-like ordering over 10 μm scales. The influences of polymer synthesis method, composition, molar mass, solvent vapor pressure, evaporation rate, and film thickness have all been assessed, confirming the generality of this behavior. Analogies to thermal annealing of elemental solids, in combination with an understanding of the effects of process parameters on annealing conditions, enable qualitative understanding of many of the key results and underscore the likely generality of the main conclusions. Pattern transfer via a Damascene-type approach verified the applicability for high-fidelity nanolithography, yielding large-area metal nanodot arrays with center-to-center spacing of 38 nm (diameter 19 nm). Finally, the predictive power of our findings was demonstrated by using small-angle X-ray scattering to predict optimal solvent annealing conditions for poly(styrene)-block-poly(lactide) films of low molar mass (18 kg mol(-1)). High-quality templates with cylinder center-to-center spacing of only 18 nm (diameter of 10 nm) were obtained. These comprehensive results have clear and important implications for optimization of pattern transfer templates and significantly advance the understanding of self-assembly in block copolymer thin films.
    ACS Applied Materials & Interfaces 07/2014; 6(16). DOI:10.1021/am503199d · 5.90 Impact Factor
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    ABSTRACT: The highest efficiency solar cells based on copper zinc tin sulfide (CZTS), a promising photovoltaic material comprised of earth abundant elements, are built on soda lime glass (SLG), a substrate which contains many impurities, including Na and K. These impurities may diffuse into CZTS films during processing and affect film structure and properties. We have investigated the effects of these impurities on the microstructure of CZTS films synthesized by ex situ sulfidation of Cu-Zn-Sn alloy films co-sputtered on SLG, Pyrex, and quartz. CZTS films synthesized on SLG were found to have significantly larger grains than films grown on the other substrates. Furthermore, we show that by including a bare additional piece of SLG in the sulfidation ampoule, the grain size of films grown on nominally impurity-free quartz increases from 100's of nm to greater than 1 μm. This demonstrates conclusively that impurities in SLG volatilize in S-containing atmospheres and incorporate into nearby CZTS films synthesized on other substrates. Impurity concentrations in these CZTS films were examined using depth profiling with time-of-flight secondary ion mass spectrometry (TOF-SIMS). Of all the impurities present in SLG, the TOF-SIMS experiments implicated Na, K, and Ca as possible elements responsible for the enhanced grain growth. To investigate the effects of these impurities individually, we introduced very small and controllable amounts of Na, K, or Ca into the sulfidation ampoule during CZTS synthesis. Impurity amounts as low as 10−6 moles of Na or 10−7 moles of K resulted in a dramatic increase in grain size, from 100's of nm to several microns, for films deposited on quartz, while Ca loading had no visible effect on the final microstructure. Based on this vapor transport mechanism, we thus demonstrate an approach for delivering precisely controlled amounts of specific impurities into CZTS films on arbitrary substrates to facilitate large-grain growth.
    Energy & Environmental Science 06/2014; 7(6):1931. DOI:10.1039/c3ee44130j · 15.49 Impact Factor
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    ABSTRACT: The non-local spin-valve is pivotal in spintronics, enabling separation of charge and spin currents, disruptive potential applications and the study of pressing problems in the physics of spin injection and relaxation. Primary among these problems is the perplexing non-monotonicity in the temperature-dependent spin accumulation in non-local ferromagnetic/non-magnetic metal structures, where the spin signal decreases at low temperatures. Here we show that this effect is strongly correlated with the ability of the ferromagnetic to form dilute local magnetic moments in the NM. This we achieve by studying a significantly expanded range of ferromagnetic/non-magnetic combinations. We argue that local moments, formed by ferromagnetic/non-magnetic interdiffusion, suppress the injected spin polarization and diffusion length via a manifestation of the Kondo effect, thus explaining all observations. We further show that this suppression can be completely quenched, even at interfaces that are highly susceptible to the effect, by insertion of a thin non-moment-supporting interlayer.
    Nature Communications 05/2014; 5:3927. DOI:10.1038/ncomms4927 · 10.74 Impact Factor
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    ABSTRACT: The Maxwell relation, the Clausius-Clapeyron equation, and a non-iterative method to obtain the critical exponents have been used to characterize the magnetocaloric effect (MCE) and the nature of the phase transitions in Pr0.5Sr0.5MnO3, which undergoes a second-order paramagnetic to ferromagnetic (PM-FM) transition at TC, and a first-order ferromagnetic to antiferromagnetic (FM-AFM) transition at TN. We find that around the second-order PM-FM transition, the MCE (as represented by the magnetic entropy change, deltaSM) can be precisely determined from magnetization measurements using the Maxwell relation. However, around the first-order FM-AFM transition, values of deltaSM calculated with the Maxwell relation deviate significantly from those calculated by the Clausius-Clapeyron equation at the magnetic field and temperature ranges where a conversion between the AFM and FM phases occurs. A detailed analysis of the critical exponents of the second-order PM-FM transition allows us to correlate the short-range type magnetic interactions with the MCE. Using the Arrott–Noakes equation of state with the appropriate values of the critical exponents, the field- and temperature-dependent magnetization curves, and hence the curves, have been simulated and compared with experimental data. A good agreement between the experimental and simulated data has been found in the vicinity of the Curie temperature TC, but a noticeable discrepancy is present for T << TC. This discrepancy arises mainly from the coexistence of AFM and FM phases and the presence of ferromagnetic clusters in the AFM matrix.
    Journal of Physics Condensed Matter 05/2014; 26(28). DOI:10.1088/0953-8984/26/28/286001 · 2.22 Impact Factor
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    ABSTRACT: Pr-based perovskite cobaltites, such as ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3$-${}$\delta${}}$ and (${\mathrm{Pr}}_{1$-${}y}{\mathrm{Y}}_{y}{)}_{1$-${}x}{\mathrm{Ca}}_{x}\mathrm{Co}{\mathrm{O}}_{3$-${}$\delta${}}$ have recently been discovered to undergo a first-order metal-insulator transition on cooling, thought to arise from an unusual shift in electron occupancy from Pr to hybridized Co-O orbitals. While this transition is known to generally suppress long-range ferromagnetic ordering, the true nature of the magnetic ground state remains unclear. In this work, we have performed structural, magnetic, transport, magnetotransport, and small-angle neutron scattering measurements on (${\mathrm{Pr}}_{1$-${}y}{\mathrm{Y}}_{y}{)}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Co}{\mathrm{O}}_{3$-${}$\delta${}}$ ($0.000$\le${}y$\le${}0.200$) in order to develop a complete microscopic picture of the ground state and a full appreciation of the evolution of the first-order transition with Y doping. Our magnetization and zero-field resistivity measurements confirm the presence of an abrupt temperature-dependent metal-insulator transition on cooling, accompanied by a sharp drop in magnetization, setting in between $y$ = 0.050 and 0.075. Small-angle neutron scattering measurements suggest the presence of critical ferromagnetic fluctuations above the metal-insulator transition for $y$ = 0.075, indicating that the system is poised to order ferromagnetically. This magnetic scattering is suppressed at the metal-insulator transition but reemerges at lower temperatures (below 40\char21{}50 K) due to the formation of a ferromagnetic cluster state. The clusters have a mean correlation length of $\sim${}50 \AA{} at 4 K, although magnetic inhomogeneity occurs across a broad spectrum of length scales, evidencing a highly inhomogeneous ground state, which we relate to the numerous sources of chemical and structural disorder. Interestingly, the magnetically inhomogeneous state manifests an intercluster magnetoresistance effect and a strong field-cooling effect on the low-temperature transport. We interpret these results, quite generally, in terms of the electronic shift from Pr to Co driving the system from the ferromagnetic side of the generic perovskite cobaltite magnetic phase diagram to the insulating side (where ferromagnetic clusters are well known to exist). These results thus shed significant light on the formation of the magnetically inhomogenous ground state of Pr-based cobaltites undergoing a first-order metal-insulator transition and indeed provide clear and direct evidence of such.
    Physical Review B 05/2014; 89(18). DOI:10.1103/PhysRevB.89.184427 · 3.66 Impact Factor
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    ABSTRACT: Strontium titanate (SrTiO3) is a foundational material in the emerging field of complex oxide electronics. Although its bulk electronic and optical properties are rich and have been studied for decades, SrTiO3 has recently become a renewed focus of materials research catalysed in part by the discovery of superconductivity and magnetism at interfaces between SrTiO3 and other non-magnetic oxides. Here we illustrate a new aspect to the phenomenology of magnetism in SrTiO3 by reporting the observation of an optically induced and persistent magnetization in slightly oxygen-deficient bulk SrTiO3-δ crystals using magnetic circular dichroism (MCD) spectroscopy and SQUID magnetometry. This zero-field magnetization appears below ~18 K, persists for hours below 10 K, and is tunable by means of the polarization and wavelength of sub-bandgap (400-500 nm) light. These effects occur only in crystals containing oxygen vacancies, revealing a detailed interplay between magnetism, lattice defects, and light in an archetypal complex oxide material.
    Nature Material 03/2014; DOI:10.1038/nmat3914 · 36.43 Impact Factor
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    ABSTRACT: Artificial spin ice is a class of lithographically created arrays of interacting ferromagnetic nanometre-scale islands. It was introduced to investigate many-body phenomena related to frustration and disorder in a material that could be tailored to precise specifications and imaged directly. Because of the large magnetic energy scales of these nanoscale islands, it has so far been impossible to thermally anneal artificial spin ice into desired thermodynamic ensembles; nearly all studies of artificial spin ice have either treated it as a granular material activated by alternating fields or focused on the as-grown state of the arrays. This limitation has prevented experimental investigation of novel phases that can emerge from the nominal ground states of frustrated lattices. For example, artificial kagome spin ice, in which the islands are arranged on the edges of a hexagonal net, is predicted to support states with monopolar charge order at entropies below that of the previously observed pseudo-ice manifold. Here we demonstrate a method for thermalizing artificial spin ices with square and kagome lattices by heating above the Curie temperature of the constituent material. In this manner, artificial square spin ice achieves unprecedented thermal ordering of the moments. In artificial kagome spin ice, we observe incipient crystallization of the magnetic charges embedded in pseudo-ice, with crystallites of magnetic charges whose size can be controlled by tuning the lattice constant. We find excellent agreement between experimental data and Monte Carlo simulations of emergent charge-charge interactions.
    Nature 08/2013; 500(7464):553-7. DOI:10.1038/nature12399 · 42.35 Impact Factor
  • D. Phelan, Y. Suzuki, S. Wang, A. Huq, C. Leighton
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    ABSTRACT: Low bandwidth Pr-based cobalt perovskites, such as Pr1-xCaxCoO3-δ, have received significant recent attention as they undergo first-order spin-state transitions with a strong influence on magnetic and transport properties. The unique nature of the Pr-O bond has been implicated as the impetus for these transitions, as it is thought that temperature-dependent charge transfer can occur between Pr and Co ions, i.e., a partial Pr3+→Pr4+ and Co4+→Co3+ valence shift. In the present work, we have studied the related compound Nd1-xCaxCoO3-δ. The Nd3+ ions have very similar ionic radius to Pr3+ but do not induce a temperature-dependent valence shift (at least in the composition range studied here), enabling deconvolution of the intrinsic low bandwidth physics from the unique effects of Pr-O bonding in Pr1-xCaxCoO3-δ. To this end, we have characterized the structural, magnetic, and electronic transport characteristics of Nd1-xCaxCoO3-δ bulk polycrystals, using neutron diffraction, small-angle neutron scattering, dc and ac magnetometry, and magnetotransport, and have established the Nd1-xCaxCoO3-δ magnetic phase diagram. This phase diagram contains regimes of short-range ferromagnetism and long-range ferromagnetism, in addition to ferrimagnetism. We argue that, with the exception of the valence transition that occurs at high x (e.g., x = 0.5) in Pr1-xCaxCoO3-δ and the low-temperature ordering of Nd3+moments that results in the ferrimagnetism in Nd1-xCaxCoO3-δ, the two systems are nearly isostructural and have similar magnetic and transport properties. The low bandwidth physics intrinsic to both systems is summarized as encompassing long-range ferromagnetism with a relatively low Curie temperature due to Co-O-Co bond buckling (<60 K for Nd1-xCaxCoO3-δ), short-range ferromagnetism that emerges at much higher temperatures (∼270 K for Nd1-xCaxCoO3-δ), and likely stems from oxygen deficiency, exchange-spring behavior related to magnetoelectronic phase separation, and a doping-driven insulator-metal transition. In addition to elucidating the essential physics of narrow bandwidth perovskite cobaltites, the results thus further confirm the importance of the unique features of the Pr-O bond in driving the abrupt spin-state transition in Pr1-xCaxCoO3-δ.
    Physical Review B 08/2013; 88(7). DOI:10.1103/PhysRevB.88.075119 · 3.66 Impact Factor
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    ABSTRACT: The properties of complex oxide films depend sensitively on epitaxial strain. This strain affects bond lengths and angles, and defect types and densities, thus impacting physical properties. In this work we perform detailed characterization of depth-dependent strain in epitaxial La0.5Sr0.5CoO3-δ (LSCO) films on SrTiO3(001), SrTiO3(110), and LaAlO3(001) substrates, combining high resolution x-ray diffraction and scanning transmission electron microscopy, in addition to geometric phase analysis. We elucidate a fundamental link between strain state and O vacancy ordering in LSCO films, where lattice mismatch and crystallographic orientation can be used to manipulate the modulation vector of the long-range vacancy order, thus providing a new approach to tailor the properties of such films.
    06/2013; 1(1). DOI:10.1063/1.4809547
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    ABSTRACT: Despite rapid recent progress, controlled dopant incorporation and attainment of high mobility in thin films of the prototypical complex oxide semiconductor SrTiO3 remain problematic. Here, analytical scanning transmission electron microscopy is used to study the local atomic and electronic structure of Nb-doped SrTiO3 both in ideally substitutionally-doped bulk single crystals, and epitaxial thin films. The films are deposited under conditions that would yield highly stoichiometric undoped SrTiO3, but are nevertheless insulating. The Nb incorporation in such films was found to be highly inhomogeneous on nanoscopic length-scales, with large quantities of what we deduce to be interstitial Nb. Electron energy loss spectroscopy reveals changes in the electronic density of states in Nb-doped SrTiO3 films compared to undoped SrTiO3, but without the clear shift in the Fermi edge seen in bulk single crystal Nb-doped SrTiO3. Analysis of atomic-resolution annular dark-field images allows us to conclude that the interstitial Nb is in the Nb(0) state, confirming that it is electrically inactive. We argue that this approach should enable future work establishing the vitally needed relationships between synthesis/processing conditions and electronic properties of Nb-doped SrTiO3 thin films.
    ACS Nano 04/2013; 7(5). DOI:10.1021/nn401101y · 12.03 Impact Factor
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    ABSTRACT: We investigate the thermodynamics of first order non-ferromagnetic to ferromagnetic phase transformation in low thermal hysteresis alloys with compositions near Ni44Co6Mn40Sn10 as a basis for the study of multiferroic energy conversion. We develop a Gibbs free energy function based on magnetic and calorimetric measurements that accounts for the magnetic behavior and martensitic phase transformation. The model predicts temperature and field induced phase transformations in agreement with experiments. The model is used to analyze a newly discovered method for the direct conversion of heat to electricity [Srivastava et al., Adv. Energy Mater., 2011, 1, 97], which is suited for the small temperature difference regime, about 10-100 K. Using the model, we explore the efficiency of energy conversion thermodynamic cycles based on this method. We also explore the implications of these predictions for future alloy development. Finally, we relate our simple free energy to more sophisticated theories that account
    Energy & Environmental Science 04/2013; DOI:10.1039/C3EE24021E · 15.49 Impact Factor
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    ABSTRACT: Pyrite FeS2 is receiving a resurgence of interest as a uniquely attractive thin film solar absorber based on abundant, low-cost, nontoxic elements. Here we address, via ex situ sulfidation synthesis, the long-standing problem of understanding conduction and doping in FeS2 films, an elusive prerequisite to successful solar cells. We find that an abrupt improvement in crystallinity at intermediate sulfidation temperatures is accompanied by unanticipated crossovers from intergranular hopping to conventional transport, and, remarkably, from hole-like to electron-like Hall coefficients. The hopping is found to occur between a small volume fraction of conductive nanoscopic sulfur-deficient grain cores (beneath our X-ray diffraction detection limits), embedded in nominally stoichiometric FeS2. In addition to placing constraints on the conditions under which useful properties can be obtained from FeS2 synthesized in diffusion-limited situations, these results also emphasize that FeS2 films are not universally p-type. Indeed, with no knowledge of the active transport mechanism we demonstrate that the Hall coefficient alone is insufficient to determine the sign of the carriers. These results elucidate the possible transport mechanisms in thin film FeS2 in addition to their influence on the deduced carrier type, an enabling advancement with respect to understanding and controlling doping in pyrite films.
    ACS Nano 03/2013; 7(3). DOI:10.1021/nn4003264 · 12.03 Impact Factor

Publication Stats

4k Citations
752.59 Total Impact Points


  • 2014
    • University of South Florida
      • Department of Physics
      Tampa, Florida, United States
  • 2002–2014
    • University of Minnesota Duluth
      • • Department of Electrical Engineering
      • • Department of Chemistry and Biochemistry
      Duluth, Minnesota, 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