R. Wheeler

The Ohio State University, Columbus, OH, United States

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Publications (26)80.72 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: This article surveys the formation and effects of “tailored” defects, having controlled numbers and several different morphologies, in high temperature superconductors. Defects can affect the equilibrium properties, such as the superconducting length scales ξ (the coherence length) and λ (the London penetration depth). Very importantly, defects provide vortex pinning that supports the conduction of a macroscopic current density. The article introduces these topics and illustrates them with specific examples.
    01/2007: pages 321-335;
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    ABSTRACT: Strong vortex pinning by fission-induced uniformly splayed columnar tracks in anisotropic mercury cuprates is demonstrated to result from (re)scaling of the pinning landscape by a large superconducting anisotropy. The effective ``narrowing'' of the splay distribution restores variable range vortex hopping (VRH) motion expected for nearly parallel pins. VRH emerges as a distinctive peak in the vortex creep rate ( ~12% at low fields at T/Tc~0.5) of the most anisotropic HgBa2Ca2Cu3O 8+delta, a peak well described by a glassy dynamics with the characteristic exponent mu~1/3.
    Physical Review Letters 11/1998; 81(18):3948-3951. DOI:10.1103/PhysRevLett.81.3948 · 7.51 Impact Factor
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    ABSTRACT: For the successful application of high-temperature copper oxide superconductors, the problem of the ease of motion of magnetic vortices (quantized flux lines) within the material must be solved. The motion results in finite electrical resistance which prevents the desired loss-free conduction of current. We demonstrate a solution to this problem by anchoring the vortices with crystallographic defects induced by fission of mercury atoms in a mercury/copper oxide superconductor.
    Nature 09/1997; 389(6648). DOI:10.1038/38415 · 41.46 Impact Factor
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    ABSTRACT: We demonstrate a method by which we expand the useful range of cuprate superconductors to above 100 K and enhance persistent currents by orders of magnitude in fields of several Tesla — namely fission of Hg nuclei within Hg-cuprates with 0.8 GeV protons. The fission process allows “doping” these cuprates with strongly pinning splayed columnar defects. The technique could be technologically relevant, since it is not limited by the short penetration range of heavier particles (or ions) and could permit modification of larger superconducting objects, such as magnets.
    Physica C Superconductivity 08/1997; 282-287:375-378. DOI:10.1016/S0921-4534(97)00279-7 · 0.94 Impact Factor
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    ABSTRACT: Orders-of-magnitude enhancements of persistent currents J are reported in Y Ba 2 Cu 3 O 7Gammaffi with columnar defects arranged in a variety of splayed configurations. The largest J is obtained for a planar distribution P pl (Theta), with a splay angle Theta opt = Sigma5 ffi . A comparison of P pl (Theta) and a gaussian distribution PG (Theta), suggests that pinning by the latter is controlled by large-angle tails of the gaussian, which appear to enhance thermal creep rate. Numerical simulations confirm the existence of the regimes where vortex motion is promoted rather than suppressed by splay. Pinning of magnetic vortices in the mixed state of a type-II superconductor is unarguably optimized with linear defects [1]. In cuprate superconductors, an effective defect structure, consisting of a random array of nearly parallel columns of amorphized material, ¸50 Gamma 80 A in diameter, can be installed by the irradiation with swift (¸ GeV ) heavy ions, such as Pb, Sn, or A...
    Physical Review Letters 04/1996; 76(14). DOI:10.1103/PhysRevLett.76.2563 · 7.51 Impact Factor
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    ABSTRACT: We report large current enhancements inY Ba 2 Cu 3 O 7-σ with splayed columnar defects. An optimalrelative splay angle is Θ opt = 10° for the planar splay distribution. The vortex motion through gaussian splay appears to be controlled by the large-angle tails of the distribution. The regimes are observed where vortex motion isenhanced rather thansuppressed by splay.
    Czechoslovak Journal of Physics 03/1996; 46:1799-1800. DOI:10.1007/BF02563014 · 0.57 Impact Factor
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    ABSTRACT: Composite Bi<sub>2</sub>Sr<sub>2</sub>Ca<sub>1</sub>Cu<sub>2</sub>O<sub>8</sub>/Ag tapes were irradiated with 0.8 GeV protons to create splayed columnar defects in the superconductor. The resultant effective pinning of vortices leads to an enhanced persistent current density in the CuO planes, displacement of the irreversibility line to higher temperatures and magnetic fields, and a major reduction in the logarithmic time decay rate of the supercurrent density.
    IEEE Transactions on Applied Superconductivity 07/1995; 5(2-5):1876 - 1879. DOI:10.1109/77.402947 · 1.24 Impact Factor
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    ABSTRACT: Results show that the splay in the orientation of the columnar defects (produced by heavy ion irradiation) has a significant effect on dynamics of the vortices. 0.58 GeV ¹¹Sn{sup 30+} and 1.08 GeV ¹Au{sup 23+} ions were used to irradiated YBaCuO single crystals. At high temperatures, the larger splay of the Sn tracks (10°) results in a current density one order of magnitude larger and a creep rate on order of magnitude smaller than the Au irradiation (1°). This indicates that a considerable further improvement of the current carrying capacity of high temperature superconductors can still be obtained.
    Physica C Superconductivity 12/1994; s 235–240. DOI:10.2172/204571 · 0.94 Impact Factor
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    ABSTRACT: A strategy to boost the current-carrying capacity of cuprate superconductors beyond the levels attainable with parallel columnar defects was recently proposed. It consists of the enforcement of vortex entanglement by controlled splay of columns. We demonstrate the validity of this suggestion in YBa2Cu3O2 single crystals using the difference in splay naturally occurring in irradiations with two ions differing in mass and energy. The terminal dispersion of the columns produced by 0.58-GeV 116Sn30+ is about 10°, as compared with 1° for 1.08-GeV 197Au23+. At high temperatures, this large splay results in a persistent current density one order of magnitude larger and a creep rate one order of magnitude smaller.
    Physical review. B, Condensed matter 07/1994; 50(6). DOI:10.1103/PhysRevB.50.4102 · 3.66 Impact Factor
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    ABSTRACT: Composite tapes of the superconductor Bi2Sr2CaCu2O8 on silver were irradiated with energetic light ions (0.8 GeV protons), creating extended splayed tracks approximately 7 nm in diameter via fission of Bi nuclei. Magnetic hysteresis indicates large enhancements of persistent currents J, especially at high fields and temperatures, and substantial expansion of the irreversible regime. The technique may be suitable for large scale applications due to the long range (approximately half meter) of fast protons.
    Applied Physics Letters 07/1994; 64(24-64):3331 - 3333. DOI:10.1063/1.111269 · 3.30 Impact Factor
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    ABSTRACT: Zn and Co substitute for Cu in YBa2Cu3O7−δ at the plane and chain sites, respectively. We report a high resolution angle-resolved photoemission study on Zn and Co substituted YBa2Cu3O7−δ single crystals to determine the effects on electronic structure. The valence states near EF along Γ-S are virtually unaffected by the substitutions for samples that are superconducting. However, the spectral weight near EF disappears for a higher Co-doped, non-superconducting crystal. The results are compared with earlier studies on pure YBa2Cu3O7−δ with controlled oxygen stoichiometry. Measurement on Gd-123 crystals with reduced oxygen stoichiometries show gradual reduction of spectral weight for a band along Γ-Y(X), in contrast to the results for the bands along Γ-S. The differences are probably due to a different admix of Cu-O chain and plane characters in these bands. We also measured Gd 4f resonance spectra, which exhibit complex structure, indicating the presence of Gd atoms in chemically inequivalent (probably bulk and near surface) sites.
    Journal of Physics and Chemistry of Solids 10/1993; 54(10-54):1177-1184. DOI:10.1016/0022-3697(93)90163-L · 1.85 Impact Factor
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    ABSTRACT: Defect microstructures in three YBa 2 Cu 3 O 7-δ single crystals irradiated with different high energy heavy ions show dramatically different damage morphologies. For Sn and Au projectiles with total energies of 580 MeV and 1.0 GeV, respectively, highly aligned tracks are observed which extend for many microns in the crystals. However, irradiation with 1.4 GeV Br ions results in the formation of spherical damage regions widely separated along the ion path through the entire crystal. The continuity of damage along the ion path for the different projectiles varies with the magnitude of the electronic energy loss.
    Applied Physics Letters 10/1993; 63(11-63):1573 - 1575. DOI:10.1063/1.110753 · 3.30 Impact Factor
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    ABSTRACT: Irradiation of high-Tc superconducting crystals with low doses (1010-1011 ions/cm2) of GeV heavy ions produces a unique microstructure consisting of discrete amorphous columns which are only a few nm in diameter but tens of mum long. It has been found recently that this columnar microstructure causes larger increases in magnetization and critical current at high temperature and high magnetic field than other types of defects in these materials. This can be understood as a consequence of the effective pinning of magnetic vortex lines provided by the columnar defects. Measurements confirm that the pinning is strongest when the magnetic field is aligned with the ion tracks. Differences in the pinning in different materials can be related to differences in their anisotropy, which affects the structure of the vortices and their pinning at columnar defects.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 06/1993; 80:1143-1149. DOI:10.1016/0168-583X(93)90753-S · 1.12 Impact Factor
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    ABSTRACT: Silver-clad composite tapes of Bi(Pb)SrCaCuO-2223 were irradiated with 1 GeV Au23+ ions perpendicular to the tape plane, creating columnar tracks with 10 nm diameter. Magnetic hysteresis measurements show substantial enhancement of the superconducting current density at all temperatures and fields, indicating the potential for further performance improvement in the high-temperature superconducting wire technology.
    Physica C Superconductivity 04/1993; 208(1-2-208):137-142. DOI:10.1016/0921-4534(93)90680-O · 0.94 Impact Factor
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    ABSTRACT: Deliberately introducing defects by particle irradiation is an effective way to increase the flux pinning in high-critical-temperature superconductors, which is a requirement for technological applications of these materials. Proton irradiation generates a random distribution of point defects, which largely enhances the critical current in YBa2Cu3O7−x single crystals; but it is not effective in shifting the irreversibility line to higher magnetic fields. The aligned columnar defects created by high-energy heavy-ion irradiation generate even stronger vortex pinning, resulting in higher critical currents at high temperatures and fields and a large displacement of the irreversibility line to higher fields.
    JOM: the journal of the Minerals, Metals & Materials Society 10/1992; 44(10). DOI:10.1007/BF03223174 · 1.76 Impact Factor
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    ABSTRACT: Single crystals of R(1)Ba2Cu3O(7-delta), (R=Y, Eu, and Gd), have been irradiated with 0.4-1.0 MeV electrons in directions near the c-axis. An incident threshold electron energy for producing flux pinning defects has been found. In-situ TEM studies found no visible defects induced by electron irradiation. This means that point defects or small clusters (less than or equal to 20 A) are responsible for the extra pinning. A consistent interpretation of the data suggests that the most likely pinning defect is the displacement of a Cu atom from the CuO2 planes.
    MRS Online Proceeding Library 06/1992; 275. DOI:10.1557/PROC-275-741
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    ABSTRACT: An enhancement in Jc of YBa2Cu3O7-x single crystals in a magnetic field is observed after irradiation with 1-MeV electrons. Typically, a factor-of-2 increase in Jc is deduced from magnetic hysteresis loops at 10 K and 1 T with H∥c. This enhancement is about 1/2 of that produced by proton and neutron irradiations under similar measurement conditions. In situ transmission-electron-microscopy studies found no visible defects induced by electron irradiation, which means that point defects or small clusters (of size <2 nm) are responsible for the extra pinning. Annealing studies suggest that effective pinning centers for H∥c do not include oxygen vacancies in the Cu-O chains. Based on calculations of cross sections for displacements on the different sublattices, and in conjunction with the results of a Jc calculation by Kes, we suggest that the most likely pinning defect is the displacement of a Cu atom from the CuO2-plane sites.
    Physical review. B, Condensed matter 04/1992; 45(18). DOI:10.1103/PhysRevB.45.10677 · 3.66 Impact Factor
  • S. Yang · H. Claus · B. W. Veal · R. Wheeler · A. P. Paulikas · J. W. Downey ·
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    ABSTRACT: We have measured the superconducting transition temperature T(c) of oxygen deficient single crystals of YBa2Cu3Ox, where x = 7 - delta in the range 6.5 < x < 6.9, after the crystals were subjected to a series of quenching and aging experiments. A change in the nature of the equilibrium state was observed at temperatures above a concentration dependent threshold temperature T*. We associate T* with the onset of a transition from a vacancy ordered eow temperature (ortho II and/or ortho III) phase to a high temperature ortho I phase. The nature of the vacancy ordering below T* varies with x. We believe that, upon heating above T*, a signigicant concentration of chain fragments develops in normally vacant chains of the ordered structure. When the crystals were quenched from temperatures above T*, and subsequently permitted to anneal at 51-degrees-C, two relaxation processes were found. The two processes have relaxation times differing by about two orders of magnitude. The faster relaxation process is associated with changes in vacancy short-range disorder. The slower process is attributed to restoration of long-range chain vacancy order as misplaced fragments anneal out. T* is determined for different oxygen concentrations and compared to theoretical phase diagram calculations. Electron diffraction measurements on well annealed single crystals with stoichiometries x = 6.50 and 6.65 show that different chain ordered phases (primarily ortho II and ortho III, respectively) exist at each of these compositions. Chain vacancy order is also observed at temperatures above T*.
    Physica C Superconductivity 04/1992; 193(3-4):243-252. DOI:10.1016/0921-4534(92)90725-R · 0.94 Impact Factor
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    ABSTRACT: Recent studies of high Tc superconductors irradiated with high energy heavy ions have indicated that a defect structure is produced which is extremely effective in pinning magnetic flux lines. In attempting to develop models to account for these observations, it is imperative to have a complete characterization of the defects responsible for the property enhancements. Hence, the defect microstructure produced in single crystal YBCO by 580 MeV Sn30+ irradiation, recently demonstrated to provide highly effective flux pinning at high fields and temperatures, has been investigated by conventional transmission electron microscopy. This irradiation condition is shown to produce linear, yet segmented, tracks of damaged material, approximately aligned with the incident irradiation direction, throughout the entire crystal thickness of 22μm. The cross sectional density of damage tracks is in close agreement with the ion fluence. Two specific characteristics of the track morphology have been studied, namely, their continuity and angular spread. The continuity of the tracks does not appear to follow threshold-like behavior within the electronic loss range studied here and the angular distribution of the defect track directions is only approximated by the Monte Carlo calculations of TRIM.
    MRS Online Proceeding Library 12/1990; 235. DOI:10.1557/PROC-235-683
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    ABSTRACT: We report orders-of-magnitude enhancements of persistent currents J in Y Ba2 Cu3 O_7-delta with columnar defects arranged in a variety of splayed configurations. The largest J is obtained for a planar distribution of splay angles, P_pl(Theta), with an optimal splay angle Theta_opt = ± 5 ^circ. A comparison of P_pl(Theta) and a gaussian distribution, P_G(Theta), suggests that pinning by the latter is controlled by large-angle tails of the gaussian, which appear to enhance thermal creep rate by generating numerous intersections of splayed defects (tracks). The track crossings allow for easier vortex jumps from one track to another, giving rise to "zig-zag" configurations promoting vortex motion. Numerical simulations confirm the existence of the regimes where vortex motion is promoted rather than suppressed by splay.

Publication Stats

551 Citations
80.72 Total Impact Points


  • 1996-2007
    • The Ohio State University
      • Department of Materials Science and Engineering
      Columbus, OH, United States
  • 1995
    • University of Tennessee
      Knoxville, Tennessee, United States
  • 1990-1994
    • Argonne National Laboratory
      • Division of Materials Science
      Lemont, Illinois, United States