C. Leighton

University of South Florida, Tampa, Florida, United States

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Publications (230)674.76 Total impact

  • [Show abstract] [Hide abstract]
    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; · 5.90 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; · 2.22 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; · 35.75 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 01/2014; 5:3927. · 10.74 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 01/2014; 105(11):112909-112909-4. · 3.52 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. · 38.60 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). · 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.
    APL MATERIALS. 06/2013; 1(1).
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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; · 12.03 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; · 12.03 Impact Factor
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    ABSTRACT: Dilute hole-doping in La1-xSrxCoO3 leads to the formation of ``spin-state polarons'' where a non-zero spin-state is stabilized on the nearest Co3+ ions surrounding a hole [1]. Here, we discuss the development of electronic/magnetic properties of this system from non-magnetic x=0, through the regime of spin-state polarons, and into the region where longer-range spin correlations and phase separation develop. We present magnetometry, transport, heat capacity, and small-angle neutron scattering (SANS) on single crystals. Magnetometry indicates a crossover with x from Langevin-like behavior (polaronic) to a state with a freezing temperature and finite coercivity. Fascinating correlations with this behavior are seen in transport measurements, the evolution from polaronic to clustered states being accompanied by a crossover from Mott variable range hopping to intercluster hopping. SANS data shows Lorentzian scattering from short-range ferromagnetic clusters first emerging around x = 0.03 with correlation lengths of order two unit cells. We argue that this system provides a unique opportunity to understand in detail the crossover from polaronic to truly phase-separated states.[4pt] [1] A. Podlesnyak et al., Phys. Rev. Lett. 101, 247603.
    03/2013;
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    ABSTRACT: We recently showed that the degraded magnetic and electronic properties in very thin STO(001)/La1-xSrxCoO3 films is due to a form of magnetic phase separation. This is primarily due to the strain driven accumulation of O vacancies near the interface. In this work we demonstrate how this understanding allows us to engineer these interfacial properties via crystallographic orientation and strain control. Using PNR, magnetometry and transport, we show how this degradation can be significantly mitigated by using LAO(001) and STO(110) substrates cf. STO(001). PNR on 400å x=0.28 films reveals an interfacial layer with suppressed magnetism on all three substrates. However, while this layer is 150å on STO(001), it extends at most to 30å on LAO(001) and STO(110). Transport measurements on x=0.5 films show that at a thickness of ˜ 55å, films on STO(110) and LAO(001) exhibit AMR whereas films on STO(001) are dominated by inter-cluster GMR. Finally, thickness dependent magnetometry shows that the magnetic order deteriorates more quickly on STO(001) than on LAO(001) and STO(110). Our work thus opens up a possible new route to tailor interfacial magneto-electronic properties in oxide heterostructures.
    03/2013;
  • Palak Ambwani, Bharat Jalan, Chris Leighton
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    ABSTRACT: Defect management in epilayers of semiconducting complex oxides such as SrTiO3 is a topic of considerable contemporary interest. Recent work has shown that sufficiently precise control over stoichiometry and defects in SrTiO3 enables facile n-type doping, record high mobilities, and even simultaneous observation of quantum oscillations and superconductivity. Such progress has typically been made using techniques such as oxygen/LASER MBE or high-temperature PLD. In this work we demonstrate, via homoepitaxy on SrTiO3(001), that RF high pressure oxygen sputtering from a ceramic target is similarly capable of growth of high-quality, stoichiometric SrTiO3 films. We show that optimization of the deposition temperature (above 750 ^oC) and oxygen pressure (above 2.5 mBar) leads to the deposition of films indistinguishable from the substrate via grazing incidence and wide-angle x-ray scattering. The importance of a pre-treatment of the substrates in oxygen above 900 ^oC is emphasized. The defect density/stoichiometry was further probed via the transport properties of vacuum annealed samples with controlled O vacancy density. Finally, we also demonstrate that the stoichiometry and defect density of films deposited under non-optimal conditions can be remarkably improved via post-deposition heat treatment.
    03/2013;
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    ABSTRACT: We demonstrate the stabilization of a ferrimagnetic ground state in epitaxial films of PrCoO3 grown on SrTiO3 (001) substrates, in stark contrast to paramagnetic behavior observed in bulk. Heteroepitaxial strain is found to induce long-range ordering of the Co ions, which we deduce to be in a high-spin state. The ferromagnetic ordering of the CoO6 array is accompanied by ordering of the Pr sublattice in an antiparallel orientation to the Co. The ordering of the Pr sublattice provides evidence for significant Co-Pr exchange likely facilitated by the presence of high-spin Co.
    Physical review. B, Condensed matter 01/2013; 87(2). · 3.77 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 01/2013; · 11.65 Impact Factor
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    ABSTRACT: Crystals and polycryst. powders of two new oxide materials, Tl4CuTeO6 and Tl6CuTe2O10, have been synthesized by hydrothermal and solid-state methods. The materials were structurally characterized by single-crystal X-ray diffraction. Tl4CuTeO6 and Tl6CuTe2O10 exhibit one dimensional anionic slabs of [CuTeO6]4- and [CuTe2O10]6-, resp. Common to both slabs is the occurrence of Cu2+O4 distorted squares and Te6+O6 octahedra. The slabs are sepd. by Tl+ cations. For Tl4CuTeO6, magnetic measurements indicate a max. at ∼8 K in the temp. dependence of the susceptibility. Low temp. neutron diffraction data confirm no long-range magnetic ordering occurs and the susceptibility was adequately accounted for by fits to a Heisenberg alternating chain model. For Tl6CuTe2O10 on the other hand, magnetic measurements revealed paramagnetism with no evidence of long-range magnetic ordering. IR, UV-vis spectra, thermogravimetric, and differential thermal analyses are also reported. Crystal data: Tl4CuTeO6, Triclinic, space group P-1 (No. 2), a=5.8629(8) Å, b=8.7848(11) Å, c=9.2572(12) Å, α=66.0460(10), β=74.2010(10), γ=79.254(2), V=417.70(9) Å3, and Z=2; Tl6CuTe2O10, orthorhombic, space group Pnma (No. 62), a=10.8628(6) Å, b=11.4962(7) Å, c=10.7238(6) Å, V=1339.20(13) Å3, and Z=4. [on SciFinder(R)]
    Journal of Solid State Chemistry 12/2012; 196:607-613. · 2.04 Impact Factor
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    ABSTRACT: Despite 35 years of investigation, much remains to be understood regarding charge transport in semiconducting polymers, including the ultimate limits on their conductivity. Recently developed ion gel gating techniques provide a unique opportunity to study such issues at very high charge carrier density. Here we have probed the benchmark polymer semiconductor poly(3-hexylthiophene) at electrochemically induced three-dimensional hole densities approaching 10(21) cm(-3) (up to 0.2 holes per monomer). Analysis of the hopping conduction reveals a remarkable phenomenon where wavefunction delocalization and Coulomb gap collapse are disrupted by doping-induced disorder, suppressing the insulator-metal transition, even at these extreme charge densities. Nevertheless, at the highest dopings, we observe, for the first time in a polymer transistor, a clear Hall effect with the expected field, temperature and gate voltage dependencies. The data indicate that at such mobilities (~0.8 cm(2)V(-1) s(-1)), despite the extensive disorder, these polymers lie close to a regime of truly diffusive band-like transport.
    Nature Communications 11/2012; 3:1210. · 10.74 Impact Factor
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    ABSTRACT: The valence band occupied state electronic structure of Co(1-x)Fe(x)S(2) in the region of the Fe/Co 3d bands has been investigated using photoemission and spin-polarized photoemission. As measured by using spin-polarized ultraviolet photoemission, the surface Fermi level spin polarization of Co(1-x)Fe(x)S(2) thin films at 50 K, specifically at x = 0, 0.05, 0.10 and 0.15, was found to be much reduced compared to that of the bulk. The spin polarization nonetheless increases with Fe concentration. The resonant photoemission spectroscopy provides evidence that S bands have a strong resonance at the photon energy corresponding to the Co 2p core level, indicating strong hybridization between Co and S bands in Co(1-x)Fe(x)S(2) (at small x). Similar evidence exists for Fe hybridization with the S bands.
    Journal of Physics Condensed Matter 11/2012; 25(1):012001. · 2.22 Impact Factor
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    ABSTRACT: The valence band occupied state electronic structure of Co1−xFexS2 in the region of the Fe/Co 3d bands has been investigated using photoemission and spin-polarized photoemission. As measured by using spin-polarized ultraviolet photoemission, the surface Fermi level spin polarization of Co1−xFexS2 thin films at 50 K, specifically at x = 0, 0.05, 0.10 and 0.15, was found to be much reduced compared to that of the bulk. The spin polarization nonetheless increases with Fe concentration. The resonant photoemission spectroscopy provides evidence that S bands have a strong resonance at the photon energy corresponding to the Co 2p core level, indicating strong hybridization between Co and S bands in Co1−xFexS2 (at small x). Similar evidence exists for Fe hybridization with the S bands.
    Journal of Physics Condensed Matter 11/2012; 25(1). · 2.22 Impact Factor

Publication Stats

3k Citations
674.76 Total Impact Points

Institutions

  • 2014
    • University of South Florida
      • Department of Physics
      Tampa, Florida, United States
  • 2002–2014
    • University of Minnesota Duluth
      • Department of Chemistry and Biochemistry
      Duluth, Minnesota, United States
  • 2009–2012
    • University of Nebraska at Lincoln
      • Department of Physics and Astronomy
      Lincoln, NE, United States
    • University of Bristol
      Bristol, England, United Kingdom
  • 2002–2012
    • University of Minnesota Twin Cities
      • • Department of Chemistry
      • • Department of Chemical Engineering and Materials Science
      Minneapolis, MN, United States
  • 2008
    • Complutense University of Madrid
      • Departamento de Física Aplicada III (Electricidad y Electrónica)
      Madrid, Madrid, Spain
  • 2007
    • University of Illinois at Chicago
      • Department of Physics
      Chicago, IL, United States
  • 1997–2007
    • Durham University
      • Department of Physics
      Durham, ENG, United Kingdom
  • 2006
    • Florida State University
      • Department of Physics
      Tallahassee, FL, United States
  • 1999–2005
    • University of California, San Diego
      • Department of Physics
      San Diego, CA, United States
  • 2003
    • National High Magnetic Field Laboratory
      Tallahassee, Florida, United States