Magnetic-field-effect three-terminal device based on YBa₂Cu ₃O_7-δ grain boundary junctions

Three-terminal YBa₂Cu₃O_7-δ thin film devices based on a parallel array of grain boundary Josephson junctions were fabricated. The multiloop superconducting device is controlled by the magnetic field generated by a current I_ctrl, which modulates the critical current I_c through the Josephson junction array. For these devices a current gain ranging between 2 at 77 K and 5 at 30 K was obtained. The spatial homogeneity of the device properties was studied by low-temperature scanning electron microscopy and by the determination of the supercurrent correlation function. Comparing the measured and the theoretically expected supercurrent correlation function shows that the device characteristics agree well with those expected from the device geometry.