Low-Noise Serial High- Superconducting Quantum Interfer

The fabrication of high-T_c 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-T_c SQUIDs for biomagnetic applications. Epitaxial high-T_c 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_PP ≥ 15 μV. The challenges and perspectives of the high-T_c serial SQUID array for biomagnetic applications are addressed and discussed.