Electrically tunable coplanar transmission line resonators using YBa ₂Cu₃O_7-x/SrTiO₃ bilayers

We have prepared electrically tunable microwave resonators incorporating superconducting YBa₂Cu₃O_7-x (YBCO) and paraelectric SrTiO₃ (STO) layers on LaAlO₃ substrates. The top YBCO layer for each sample was patterned into a 8 mm long coplanar transmission line with a 40 μm gap and a 20 μm center line width. The microwave transmission through the coplanar transmission line exhibits resonances corresponding to standing microwaves along the coplanar transmission line. These resonances are modulated by applying a bias voltage between the center line and the ground planes. Samples with a 0.5 μm thick (2 μm thick) bottom STO layer show, for a resonance at around 8 GHz (5 GHz), a frequency modulation of about 4% (24%) and a quality factor Q of about 200 (50) under 100 V bias at 80 K.