Effects of Low Magnetic Fields on Gold-YBCO Contacts
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EFFECTS OF LOW MAGNETIC FIELDS ON GOLD-YBCO CONTACTS YONHUA TZENG AND MITCHELL A. BELSER Department of Electrical Engineering Auburn University, Alabama 36849 ABSTRACT The effects of low magnetic field (0-40 Gauss) on electrical properties of gold-YBCO contacts are investigated. The contact resistance increases significantly with applied magnetic fields. For high magnetic field or when high current is flowing through the contact, the contact resistance decays after removing the magnetic field to a finite value greater than that is measured before the contact is exposed to the magnetic field and stays there for long time as long as the sample is kept below the superconductivity critical temperature. INTRODUCTION Most practical applications of high temperature superconductors require the connection of the superconductors through low resistivity contacts [1-5) with external normal metal circuits. Talvacchio has reviewed the requirements and properties of metal-superconductor interfaces for bulk and thin film materials [6]. Little has been reported concerning the effect of magnetic fields on the electrical properties of metal-YBCO interface. Wieck reported the use of a two-point four-wire method for characterizing gold-YBCO contacts under the influence of medium to high magnetic fields (7]. In this work we report the use of a three-point, four-wire technique, which excludes the resistance of the bulk superconductor under magnetic fields, for determining current-voltage (I-V) curves, contact resistance, and the recovery of the contact resistance after the magnetic field is turned off as a function of the applied magnetic fields for gold-YBCO contacts. EXPERIMENTAL PROCEDURES Gold contacts to YBCO are fabricated by placing gold foils (about 0.1 mm x 2 mm) onto superconductor powder and pressing the powder into disks. The YBCO disks are sintered in oxyqen at 950C for 6 hours and then slowly cooled to 450C, at which temperature the sample is kept for 2 hours before being taken out of the furnace (4]. Current is passed between the center contact and one neighboring contact and the voltage is measured between the same center contact and the third for the gold-YBCO contacts as a The I-V curves contact. function of applied magnetic fields are obtained by applying magnetic fields up to 40 G perpendicular to the contact area. The recovery of the contact resistance is measured using a Keithly 195A digital multimeter to record the voltage across the contact as a function of time after the magnetic field is removed.
Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
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Current-voltage characteristics of a gold-YBCO contact as a function of applied magnetic field. These four curves correspond to 0, 1, 10, and 30 G.
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Contact resistance as a function of applied magnetic field.
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