J. R. Thompson

Oak Ridge National Laboratory, Oak Ridge, Florida, United States

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Publications (307)569.02 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The free-flux-flow (FFF) phase has been attained successfully in a number of clean, weak-pinning, low-anisotropy, low-Tc, single-crystal samples as a unique probe into type II superconductivity that is independent of composition. The ``clean'' quality of the samples have been confirmed by reversible magnetization, high residual resistivity ratio, and low critical current densities Jc with a re-entrant ``peak'' effect in Jc(H) just below the critical field Hc2. The necessity of high current densities presented technical challenges that had been successfully addressed, and FFF is confirmed by a field-dependent ohmic state that is also well below the normal state. In these studies, the FFF resistivity ρf(H) has been measured in order to observe the field-dependent core size of the quantized magnetic flux vortices as modeled recently by Kogan and Zelezhina (KZ) who predicted a specific deviation from Bardeen-Stephen flux flow, dependent on normalized temperature and scattering parameter λ. The compounds studied are: V3Si, LuNi2B2C, and NbSe2, and results have shown consistency with the KZ model. Other applications of this method could also be used to probe normal-state properties, especially for the new iron arsenides, as will be discussed.
    02/2012;
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    ABSTRACT: The critical current density Jc and the magnetic relaxation (‘creep’) properties have been studied for a set of NdBa2Cu3O7−δ (NdBCO) films doped with BaZrO3 (BZO) nanoparticles to form columnar defects. The dependence of Jc on the magnitude and orientation of the applied magnetic field Happ (0–6.5 T) and temperature T (5 K–Tc) was investigated. The normalized flux-creep rate S =− dln(J)/dln(t) was determined as a function of T. The current dependence of the effective activation energy Ueff(J) was derived using the formalism developed by Maley. The results are well described by an inverse power law type barrier of the form Ueff(J) ∼ U0(J0/J)μ with fitted values for the pinning energy scale U0 and the glassy exponent μ. When comparing values for these parameters in the BZO-doped samples with those for their undoped control counterparts, the most striking difference is the larger scale of current density J0 in the doped samples (a factor of 2.4 higher), while the other pinning parameters do not differ strongly. In the BZO-doped materials, the pinning energy scale U0 increases with vortex density and J0 decreases, with both following simple power law dependences on the field.
    Superconductor Science and Technology 01/2012; 25(4). · 2.76 Impact Factor
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    ABSTRACT: In the present work, composite LaMnO<sub>3</sub>:MgO (LMO:MgO) cap buffer layers with varying MgO contents 5 vol% up to 75 vol% have been grown on homo-epi MgO/IBAD(MgO) substrates to enhance the performance of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-x</sub> (YBCO) films. Results showed formation of phase separated MgO nanocolumns within the LMO matrix. The impact of these nanocolumns on the superconducting properties of YBCO films deposited using pulsed laser deposition (PLD) on the nanostructured layer was investigated by electrical transport measurements. Such YBCO films showed better in-field performance compared to that of YBCO films on standard LMO cap films. In particular, measurements of the field-angle dependence revealed c -axis correlated pinning for YBCO films on these composite cap layers. The present results demonstrate a practical approach to obtain high performance superconducting wires.
    IEEE Transactions on Applied Superconductivity 07/2011; · 1.20 Impact Factor
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    ABSTRACT: A generalized methodology is described for determining both the intragranular and through-grain-boundary critical current densities in practical coated conductors from contact-free magnetic hysteresis measurements. The model incorporates the vector nature of current density J within the superconducting grains and current conservation with respect to the grain boundaries. Using physically observed values for the grain aspect ratio as input, the analysis yields the low-field intragranular critical current density JcG from a single field-dependent measurement of the global Jc and provides a consistent description that spans the entire range from the weak-link () to the single-grain () limit. Results are given for ex situ processed RBCO coatings on RABiTS.
    Superconductor Science and Technology 03/2011; 24(6):062001. · 2.76 Impact Factor
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    ABSTRACT: Columnar defects (CDs) in a superconductor provide angularly selective vortex pinning, where the density of CDs provides a natural scale for the magnetic field, the "matching field." While dramatic changes in the critical current density Jc might be expected when the vortex density exceeds the CD density, observations of this phenomena has been elusive in systems with chemically produced, self-assembled CDs of BaSnO3, BaZrO3, SrZrO3, etc. Here we describe studies of two RBa2Cu3O˜7 systems containing these self-avoiding CDs, using either contact free magnetic or transport measurements. In magnetic measurements on a material with measured areal CD of ˜2.5 T, the Jc decreased abruptly when the applied field H exceeded this level; this feature was observed over a wide temperature range, from 77 to ˜ 40 K. All these features disappeared when the field was tilted away from the CD orientation. Research at ORNL sponsored by US DOE.
    03/2011;
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    ABSTRACT: The dependence of the critical current density Jc on temperature, magnetic field, and film thickness has been investigated in (Gd-Y)-Ba-Cu-O materials of 0.7, 1.4, and 2.8 μm thickness. Generally the Jc decreases with film thickness at investigated temperatures and magnetic fields. The nature and strength of the pinning centers for vortices have been identified through angular and temperature measurements, respectively. These films do not exhibit c-axis correlated vortex pinning, but do have correlated defects oriented near the ab planes. For all film thicknesses studied, strong pinning dominates at most temperatures. The vortex dynamics were investigated through magnetic relaxation studies in the temperature range of 5–77 K in 1 and 3 T applied magnetic fields, H ∥ surface normal. The creep rate S is thickness dependent at high temperatures, implying that the pinning energy is also thickness dependent. Maley analyses of the relaxation data show an inverse power law variation for the effective pinning energy Ueff ∼ (J0/J)μ. Finally, the electric field-current density (E-J) characteristics were determined over a wide range of dissipation by combining experimental results from transport, swept field magnetometry (VSM), and superconducting quantum interference device (SQUID) magnetometry. We develop a self-consistent model of the combined experimental results, leading to an estimation of the critical current density Jc0(T) in the absence of flux creep.
    Physical review. B, Condensed matter 01/2011; 84(2). · 3.77 Impact Factor
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    ABSTRACT: The possibility of a new magnetic component to the superconductivity in the recently discovered iron-containing superconductors -- something previously deemed impossible -- has attracted a wide breadth of studies. One area of interest is in magnetic phase transitions in the mixed-state ``flux medium'' comprised of interacting magnetic flux quanta (or vortices) which are found in Type II superconductors. Not surprisingly, the flux dynamics in these materials already show novelties not yet completely understood. Recent work from various groups do agree on a vortex ``liquid'' phase at highest fields and temperatures, with a ``melting'' transition line from a phase in which interactions between vortices become significant -- all while still affected by pinning mechanisms. To test the proposed phases, the present study explores the possibility of achieving the highly ordered free flux flow (FFF) phase in optimally doped Ba(Fe1-0.92Co0.08)2As2. This may well be the first time such a measurement is reported for iron arsenides, which are also known for strong pinning. Results and preliminary analyses are discussed.
    10/2010;
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    ABSTRACT: Results of recent measurements on two very clean, single-crystal samples of the A15 superconductor V3Si are presented. Magnetization and transport data already confirmed the ``clean'' quality of both samples, as manifested by: (i) high residual resistivity ratio, (ii) very low critical current densities, and (iii) a ``peak'' effect in the field dependence of critical current. The (H,T) phase line for this peak effect is shifted in the slightly ``dirtier'' sample, which consequently also has higher critical current density Jc(H). High-current Lorentz forces are applied on mixed-state vortices in order to induce the highly ordered free flux flow (FFF) phase, using the same methods as in previous work. A traditional model by Bardeen and Stephen (BS) predicts a simple field dependence of flux flow resistivity rhof(H), presuming a field-independent flux core size. A model by Kogan and Zelezhina (KZ) takes core size into account, and predict a clear deviation from BS. In this study, rhof(H) is confirmed to be consistent with predictions of KZ, as will be discussed.
    10/2010;
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    ABSTRACT: Preliminary results are presented on a comparison between two very clean, single-crystal samples of the A15 superconductor V3Si. Three independent measurements on the same samples -- namely: (i) resistivity versus temperature, (ii) magnetic susceptibility, and (iii) nuclear magnetic resonance -- confirm that (a) both samples have a high residual resistivity ratio and (b) the Martensitic transformation is manifest in one sample, but suppressed in the other. This provides the opportunity to study how the Martensitic transformation causes the magnetoresistivity of V3Si to deviate from Kohler's Rule, adding more detailed information to results obtained previously. Results and analysis are discussed.
    10/2010;
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    ABSTRACT: We have used a cryogenic four-probe scanning tunneling microscope and scanning electron microscope (SEM) to investigate local electrical transport properties on the surface of optimally-doped superconducting Ba(Fe1-xCox)2As2 single crystal. For the regions that showed uniform SEM contrast, a superconducting transition has been observed with a very sharp transition width of 0.2 K. On the other hand, superconducting transition temperature was found to vary with transition widths up to 3 K in the non-uniform SEM contrast region. The wavelength dispersive x-ray spectroscopy reveals that dopant (Co) concentration remains a fixed value in the uniform region, but changes gradually in the non-uniform region. We will show that local superconductivity is connected with local chemical-compositional change.
    03/2010;
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    ABSTRACT: Studying the temperature and field-orientation dependence of critical current density Jc gives insight into vortex pinning. The orientation dependence of Jc in YBa2Cu3O˜7, containing both correlated disorder and localized `isotropic' defects, was determined at temperatures T = 77 K to 5 K in magnetic fields up to mu0H = 6 T. The inductive measurements ensured very low dissipation levels, with electric fields E ˜ 10-10 V/cm in the highly textured YBCO thin layers. The study revealed systematic changes in the orientation dependent Jc. At high temperatures, Jc exhibited a peak for H || c-axis; with decreasing T, the Jc became almost independent of angle and at still lower temperatures, it exhibited a minimum for H || c-axis. These findings are interpreted in terms of a competition between different types of pinning sites in a complicated pinning landscape. Research at ORNL sponsored by Div. Materials Sciences and Engineering, USDOE.
    03/2010;
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    ABSTRACT: We have investigated the electrical and magnetic properties of several parent compounds of Fe-based superconductors in a wide temperature range. In addition to the well-known transitions (one structural transition at Ts and one spin-density-wave (SDW) transition at TM), all investigated parent compounds (BaFe2As2,SrFe2As2, LaFeAsO, FeTe) show unusual features in both magnetic susceptibility and electrical resistivity at high temperatures (T). Above Ts, their magnetic susceptibility increases more or less linearly with increasing temperature up to at least 700 K. On the other hand, the electrical resistivity exhibits non-monotonic temperature dependence, revealing non-metallic character at high temperatures. The implication of these results will be discussed.
    03/2010;
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    ABSTRACT: Pinning of vortices by second phase BaSnO3 ``columnar defects'' has been studied in thick, c-axis oriented films of YBa2Cu3O˜7, deposited epitaxially on (100) LaAlO3. The dependence of the critical current density Jc on orientation of the magnetic field was determined for temperatures T = 77-5 K in applied magnetic fields mu0H = 0-6 T. The contact-free inductive methods insured measurements at very low dissipation levels, with electric field E ˜ 10-10 V/cm. Results of this study will be compared and contrasted with findings for YBCO materials with a more complex pinning landscape from multiple defect types. Research at ORNL sponsored by Div. Materials Sciences and Engineering, USDOE. Work by CVV supported by AFOSR and Air Force Research Lab.
    03/2010;
  • ChemInform 01/2010; 25(52).
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    ABSTRACT: We have investigated the electronic, magnetic, and optical properties of two Fe-based superconductors and related parent compounds via three powerful techniques: scanning tunneling microscopy/spectroscopy (STM/S), high-temperature vibrating sample magnetometry, and optical transmission spectroscopy (OTS). Below the superconducting transition temperature T c ∼ 48 K, the STM/S of polycrystalline NdFeAsO 0.86 F 0.14 reveals a single-gap feature. The quantitative fitting of STS data results in Bardeen–Cooper–Schrieffer-like temperature dependence of the energy gap (T), with 2(0)/k B T c ∼ 4.3. The tunneling spectra of BaFe 2 As 2 single crystals show no evidence for the opening of a gap below the magnetic/structural transition temperature T MS ∼ 140 K. This transition also had little impact on the transmission spectra in the wavelength range between 400 and 2400 nm. But its effect on the magnetic properties is dramatic, as reflected in the unusual magnetic susceptibility over a wide temperature range.
    Superconductor Science and Technology 01/2010; 23(23):54005-54005. · 2.76 Impact Factor
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    ABSTRACT: The dependence of the critical current density (Jc) on the orientation of an applied magnetic field was studied for a prototype (Gd,Y)Ba2Cu3O7−x (GdYBCO) coated conductor fabricated by MOCVD on an IBAD-MgO template. Additional rare-earth cations (Y and Gd) and Zr were incorporated into the superconducting film to form (Y,Gd)2O3 and BaZrO3 nanoparticles extended nearly parallel to the a–b planes and to the c-axis, respectively, to enhance the flux pinning. In-field measurement of Jc was carried out with electrical current flowing either along or perpendicular to the longitudinal axis of the tape, while a maximum Lorentz force configuration was always maintained. Details in the angular dependence of Jc were related to the unique structure of the film, specifically the tilt in the GdYBCO lattice and the tilts in the extended (Y,Gd)2O3 and BaZrO3 nanoparticles. XRD and TEM were used to study the structure of the coated conductor. The effect of the misalignment between the external field H and the internal field B on the angular dependence of Jc is discussed.
    Physica C Superconductivity 12/2009; · 0.72 Impact Factor
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    ABSTRACT: The co-existence of two different ordering schemes in the conduction electrons of superconductors -- charge density waves (CDWs) and superconductivity, particularly in the motion of quanta of magnetic flux -- is studied in high-quality samples of the compound, NbSe2. Of specific interest is the magnetic-field dependence of transport critical current density Jc and that of the Lorentz-driven free flux flow (FFF) resistivity associated with the ordered motion of vortices when the CDWs are switched on and off -- as effected by doping with Ta. The CDW phase is manifested as a broadening of NMR peaks and as a ``knee'' in the temperature dependence of normal-state resistivity. While both doped and non-doped samples show similar field dependence in FFF resistivity, Jc(H) reveals a surprising double peak, probably due to sample inhomogeneity combined with the effects of crystal anisotropy.
    11/2009;
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    ABSTRACT: The normal-state transport magnetoresistivity rho(H) in high-quality samples of A15 compound V3Si is studied. Contrary to the results of a previous study by Zotos et al. (Sol. State Comm. 50 (5), 1984, p. 453) which found that rho ˜ H^2 over a wide range of temperatures, the result of the current study is closer to rho ˜ H^1.5 at higher temperatures. However, there seems to be also a deviation from this form at lower temperatures, thus showing a slight temperature dependence for rho(H) -- in other words, a temperature-dependent deviation from the traditional Kohler's Rule, deltarho/rho0 = f(H/rho0). Possible reasons for this are discussed.
    11/2009;
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    ABSTRACT: Two 3-m long, single-phase cables have been fabricated by Ultera from second generation (2G) superconductor wire supplied by American Superconductor. The first cable was made with two layers of 2G tape conductor and had a critical current of 5,750 A while the second cable had four layers and a critical current of 8,500 A. AC loss was measured for both cables at ac currents of up to 4 kA<sub>rms</sub>. Ultera performed initial fault current studies of both cables in Denmark with limited currents in the range from 9.1 to 44 kA. Results from these tests will provide a basis for a 25-m long, three-phase, prototype cable to be tested at ORNL early next year and a 300-m long, fault current limiting, superconducting cable to be installed in a Consolidated Edison substation in New York City.
    IEEE Transactions on Applied Superconductivity 07/2009; · 1.20 Impact Factor
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    ABSTRACT: Previously, it has been well established that in an IBAD architecture for coated conductors, (1) LaMnO<sub>3</sub> (LMO) buffer layers are structurally and chemically compatible with an underlying homo-epitaxial MgO layer and (2) high current density YBCO films can be grown on these LMO templates. In the present work, the homo-epi MgO layer has been successfully eliminated and a LMO cap layer was grown directly on the IBAD-MgO template. The performance of the LMO/IBAD-MgO samples has been qualified by depositing 1 mum-thick YBCO coatings by pulsed laser deposition. Electrical transport measurements of YBCO films on the standard (with homo-epi MgO) and simplified (without homo-epi MgO) IBAD architectures were carried out. The angular dependence of critical current density ( J <sub>c</sub>) is similar for both IBAD architectures. XRD measurements indicate good, c-axis aligned YBCO films. Transmission electron microscopy (TEM) images reveal that microstructures of YBCO/LMO/IBAD-MgO and YBCO/LMO/homo-epi MgO/IBAD-MgO templates are similar. These results demonstrate the strong potential of using LMO as a single cap layer directly on IBAD-MgO for the development of a simplified IBAD architecture.
    IEEE Transactions on Applied Superconductivity 07/2009; · 1.20 Impact Factor

Publication Stats

4k Citations
569.02 Total Impact Points

Institutions

  • 1981–2012
    • Oak Ridge National Laboratory
      • • Chemical Sciences Division
      • • Materials Science and Technology Division
      • • Solid State Division
      • • Physics Division
      Oak Ridge, Florida, United States
  • 1978–2010
    • University of Tennessee
      • Department of Physics & Astronomy
      Knoxville, Tennessee, United States
  • 2008
    • Kyoto University
      • Department of Physics II
      Kyoto, Kyoto-fu, Japan
  • 1977–2007
    • The University of Tennessee Medical Center at Knoxville
      Knoxville, Tennessee, United States
  • 2000–2003
    • The University of Warwick
      • Department of Physics
      Coventry, ENG, United Kingdom
  • 2002
    • University of Alberta
      • Department of Physics
      Edmonton, Alberta, Canada
  • 1992–2001
    • Universidad de Talca
      • Faculty of Engineering
      Talca, Maule, Chile
  • 1995
    • Los Alamos National Laboratory
      • Superconductivity Technology Center
      Los Alamos, California, United States
  • 1982
    • Iowa State University
      • Ames Laboratory
      Ames, IA, United States