Measurements of phase shifts in YBCO transmission lines for evaluation of kinetic inductance change

Abstract

We fabricated a coplanar waveguide (CPW) using high temperature superconductor YBa2Cu3O7-δ (YBCO) and evaluated its characteristics for the realization of a superconducting parametric amplifier. A YBCO thin film with a film thickness of 200 nm was formed by the PLD method using a YAG laser on a 10 × 10 mm MgO substrate. The YBCO thin films were processed into the CPW shape with a line width of 8 μm, a gap width of 8 μm and a line length of 5 mm by photolithography and sputter etching. Using a network analyzer, phase differences of microwave were measured at dc current bias from 0 mA to 14 mA. From the measurement results, it was found that the phase difference decreased with increasing temperature. In addition, it was found that the phase shifts greatly varied near the superconducting transition temperature at each current bias. We observed a maximum phase difference Δθ of -0.08 rad at a bias current of 14.0 mA, which is the closest to the transition temperature. A parametric gain was estimated to be 0.15 dB from the measured maximum phase difference.