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Because of high permittivity of SrTiO/sub 3/ (STO) substrate, YBCO high-Tc SQUIDs have distributed impedance. Based on the simulation by the analytical method to calculate the I-V characteristic, we have designed and fabricated YBCO dc SQUIDs that have a transmission-line-like geometry of thin parallel strips with a wide slit between them. The geometrical parameters were determined to give high modulation voltage. The fabricated SQUIDs had a white noise of (0.5-1.0)/spl times/10 /spl Phi//sub 0//Hz/sup 1/2/. We also designed a direct coupling magnetometer, in which the transmission line SQUID was located within the hole of the pickup coil. On a small substrate area of 5 mm/spl times/10 mm, the fabricated magnetometer had an effective area of 0.22 mm/sup 2/. The field noise of about 100 fT/Hz/sup 1/2/ was obtained in the white noise region.
Date of Publication: June 1999