Heavy-Ion Damage to Magnesium Diboride Films: Electrical Transport-Current Characterization
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Heavy-Ion Damage to Magnesium Diboride Films: Electrical Transport-Current Characterization H. R. Kerchner, C. Cantoni, M. Paranthaman, D. K. Christen, H. M. Christen, and J. R. Thompson, Oak Ridge National Laboratory, Oak Ridge, TN, and D. J. Miller, Argonne National Laboratory, Argonne, IL.
ABSTRACT The use of magnesium diboride in superconducting magnets, transmission lines, or other large-scale applications depends on the transport-current characteristics of this material in magnetic field, and how they compare to the properties of conventional and high-temperature superconductors. Thin films of boron grown on sapphire substrates during electron-beam evaporation were exposed to Mg vapor to produce 0.5-µm thick layers of the metallic compound MgB2. Four-terminal measurements of their voltagecurrent relations, E(J), were carried out before and after exposure to Bf=1-T and higher doses of 1-Gev U ions. These doses lowered critical temperatures Tc≈39 K less than 0.1 degree, raised the normal-state resistivity, and reduced the loss-free critical current density, Jc. Higher doses added little. The reduction of current densities was greater in the presence of applied magnetic field greater than 0.1 T.
INTRODUCTION The relatively new superconducting material MgB2 [1] shows characteristics that give it encouraging prospects for many practical future applications. This compound is intrinsically stable and not sensitive to environmental exposures as are some hightemperature superconducting compounds such as YBa2Cu3Ox. Early studies [2] indicated that, unlike the higher temperature superconductors YBa2Cu3Ox and Bi2Sr2Ca2Cu3Ox, MgB2 grain boundaries permit excellent intergranular electrical current flow. These features are common to both bulk ceramic material and to thin films. On the other hand, the naturally strong electrical anisotropy limits bulk, intergranular current flow. Like classical low temperature superconductors, crystalline defects can interact with mixed-state vortices, pinning them and reducing their lossy motion while they are under a driving force arising from electrical transport current. Such defects can be introduced in a controlled way by exposing the material to ionizing radiation. Our films were irradiated by 1-Gev U ions at Argonne National Laboratory. One expects such radiation to produce badly damaged or amorphous tracks through the crystalline film. For the high-Tc compounds such badly damaged regions locally reduce the superconducting order parameter. Low temperature superconductors cannot change their order parameters over such short ranges and have no such local reduction. With a low-temperature coherence length of 3-6 nm, MgB2 is expected to behave similarly to the high-Tc materials. Consistent with an earlier proton irradiation study, [3] we found damage to reduce loss-free critical currents at significant magnetic field.
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FILM GROWTH Thin films have been grown by several techniques.[ 4-9 ] The films of this report were produced by evaporating boron, depositing it amorphously on sa
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