Tuning of Y-Ba-Cu-O ring resonators by Sr-Ti-O thin films

Tunable YBa₂Cu₃O_7-δ (YBCO) ring resonators were produced to study the nonlinear dielectric properties of SrTiO₃ (STO) thin films. Epitaxial STO/YBCO double layers were deposited by sputtering on LaAlO₃ substrates and patterned to coplanar ring resonators. By applying an electric field of 100kV/cm the resonant frequency could be shifted by 5% from 5.13 to 5.38 GHz and the unloaded quality factor, Q₀, of the resonator increased from 195 to 394. The dielectric properties sensitively depended on the deposition temperature of the STO films which was varied from 710 to 810°C. With increasing deposition temperature the relative permittivity, ε_r, at 77 K raised from 234 to 424. Annealing the film deposited at 810°C in 1 bar oxygen for 2 hours at 1200°C resulted in an additional increase to ε_r=613. The highest tunability with a 70% reduction of the permittivity at 120 kV/cm was also reached in the annealed film. Although the lowest loss tangent, tanδ, of 0.006 at 77 K was found for the film deposited at 710°C, the annealed film had the best tunability to loss ratio. The measured temperature and electric field dependence of the permittivity and loss tangent could be described by a phenomenological model which elucidates the loss mechanisms in the STO films.