Variation of the performance of YBa₂Cu₃O_7- radio frequency superconducting quantum interference device with bicrystal grain boundary angle

High transition temperature (high-T-c) superconducting quantum interference devices (SQUIDs) based on bicrystal grain boundary junctions have been utilized in a wide range of applications. To explore the effect of the bicrystal grain boundary angle on the property of the high-T-c SQUID, we fabricated and characterized YBa2Cu3O7-delta (YBCO) radio frequency (rf) SQUIDs using [001]-tilt grain boundary junctions on SrTiO3 (STO) bicrystal substrates with three different misorientation angles of theta = 18 degrees, 24 degrees, and 30 degrees. Unlike the device of theta = 18 degrees showing only the triangular waveform of the voltage-flux (V - Phi) characteristics, for device of theta = 24. and 30 degrees, the shape of the V - Phi curve can be tuned from approximately triangular into nearly rectangular by adjusting the working point of the device. It suggests that, as theta increases from 18 degrees to 30 degrees, the operation of the device may change from the hysteretic mode into the nonhysteretic mode. This is backed by electrical transport measurements of YBCO bicrystal junctions on separate STO substrates, which help to reveal that lying behind the indicated changeover of the SQUID operation mode is the exponential decrease of the junction critical current density J(c) as theta. increases. The white noise of the device, which decreases with increasing flux-to-voltage transfer coefficient partial derivative V/partial derivative Phi, shows a large drop as theta. increases to 24 degrees. For both 24 degrees and 30 degrees devices, the white field noise at 77 K is about 90 fT/Hz1/2 at respective optimum working points. For device of 30 degrees with nearly rectangular V - Phi characteristics showing the largest partial derivative V/partial derivative Phi among the fabricated samples, the white noise region extends below 10 Hz and a field noise of 145 fT/Hz1/2 at 1 Hz is shown, demonstrating the potential of obtaining high-performance bicrystal YBCO rf SQUIDs with this grain boundary angle.