Tunability and loss tangent of ferroelectric SrTiO₃ films in YBa₂Cu₃O_y superconducting resonator

Using a high-temperature superconducting resonator, measurements of the dielectric properties of ferroelectric SrTiO₃ films at microwave frequency (∼5 GHz) are performed. The variation in dielectric constant, ε, of SrTiO₃ film under the influence of an applied dc voltage, U_b, at liquid-nitrogen temperature affects the value of the capacitance, C, and loss tangent, tan δ, of the SrTiO₃ ferroelectric capacitor. In order to evaluate the values of C and tan δ of the SrTiO₃ capacitor, we used the YBa₂Cu₃O_y resonator which has a planar ferroelectric capacitor in the break of the microstrip line. The couplings of each microwave input and output port are designed in the resonator, including the bias circuits of the zero-field points. SrTiO₃ ferroelectric capacitors with values of the capacitance of about 0.5 pF and tan δ less than 0.01 successfully control the tunability of the resonance frequency in the resonator with a factor of ∼1.5.