Low-noise serial high- Tc YBa₂Cu₃O_y superconducting quantum interference devices based on bicrystal junctions

The fabrication of high- Tc YBa₂Cu₃O_y (YBCO) superconducting quantum interference devices (SQUIDs) that produce low flux noise while maintaining a high fabrication yield continues to challenge scientists. In this paper, we report the characterization of low-noise serial high- Tc SQUIDs for biomagnetic applications. Epitaxial high- Tc YBCO thin films with a root-mean-square surface roughness less than 5 nm are optimized for the low-noise serial SQUID coupled to a common pickup coil to form magnetometers and gradiometers. Current-voltage curves, voltage-flux curves, and flux noise of SQUID devices are characterized. A smooth surface morphology across the bicrystal line is essential for SQUIDs to show good characteristics. The field sensitivity is improved by a factor of 2 1/2 with the designed serial SQUID array, which is coupled to a common pickup loop. This is demonstrated, and its fabrication yield is higher than 80% for devices with peak-to-peak voltage V-\rm PP\geq 15\ \mu\hbox{V}. The challenges and perspectives of the high- Tc serial SQUID array for biomagnetic applications are addressed and discussed.