Conductive Polymer Switch for Controlling Superconductivity
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757 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society
EXPERIMENTAL Thin films of YBa2Cu3O7-8 (-200 to 5000 A in thickness) were first deposited onto single crystalline MgO (100) substrates using the pulsed laser ablation method. 8 The films of YBa2Cu307-8 were characterized by 4-point probe conductivity, critical current, scanning electron microscopy and x-ray powder diffraction measurements. To explore the polymer/superconductor interactions, microbridges (-3 mm long and -100 g.tm wide) were then created 9 on the YBa2Cu307-8 film by scribing the superconductor layer with a diamond tip controlled by a micromanipulator or by using a laser etching technique. In certain cases, films of YBa2Cu3O7-8 were produced which exhibited poor morphologies as well as a large number of defects. Films of this type tended to be more sensitive to the influence of the conductive polymer layers. Better defined weak links were created by depositing the superconductor over the natural step edges that exist on the surfaces of cleaved and unpolished MgO (100) substrates. Working electrodes were fashioned from freshly deposited YBa2Cu3O7-8 thin films. Polypyrrole was then electrochemically polymerized on the microbridge surface. Polymerization reactions were completed by using both voltammetric and potential step techniques from solutions of 0.25 M Et4NBF4 in neat pyrrole for the preparation of the sandwich structures. Using such methods, the potential of the superconductor electrode was raised to values in the range of 0.7 to 1.0 V vs. SCE. Electrochemical doping was performed using monomer free solutions of 0.1M Et4NBF4 in acetonitrile. Oxidative doping of the polymer into the conductive form was accomplished by raising the potential of the superconductor electrode to values of - 0.7 V vs. SCE. The maximum room temperature conductivities of such doped polymer films were found to be - 50 Q-1 cm- 1 for polypyrrole, close to values reported previously. 10 RESULTS AND DISCUSSION One of the most important and interesting features associated with the conductive polymer/high-Tc superconductor structures involves the study of the polymer/superconductor electronic interactions which occur at temperatures below Tc. Several sets of resistance vs. temperature data were acquired for the cases of the bare microbridge, the bridge coated with the doped polymer and the bridge coated with the neutral polymer. The resulting Tc (mid) are presented in Table I for various thickness values of the superconductor. When thick films (i.e.>1000 A) of YBa2Cu3O7_6 are employed, very little change in the properties of the superconductor is noted as the polymer is cycled between its neutral and oxidized forms. In each case, the transition temperatures for the polymer coated films are very close to those obtained for the uncoated structure. For the thinnest superconductor element composed of a - 200.A film of YBa 2 Cu 3O7-8, Tc (onset) is close to 75 K for both the uncoated bridge as well as for the structure coated with the neutral polymer. On the
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