Tunability of resonant frequencies in a superconducting microwave resonator by using SrTiO ₃ ferroelectric films

An applied de voltage varies the dielectric constant of ferroelectric SrTiO ₃ films. A tuning mechanism for superconducting microwave resonators was realized by using the variation in the dielectric constant of SrTiO ₃ films. In order to estimate the values of the capacitance, C ₁ and the loss tangent, tan δ, of SrTiO ₃ ferroelectric capacitors, we used high-temperature superconducting microwave resonators which were composed of two ports, two poles, and dc bias circuits at the zero-field points. SrTiO ₃ ferroelectric capacitors successfully controlled the resonant frequency of the resonator. Resonant frequencies of 3.98 GHz and 4.20 GHz were measured at bias voltages of 0 V and 50 V which correspond to capacitance values of 0.94 pF and 0.7 pF, respectively. The values of the loss tangent, tan δ _eff, obtained in this measurements, were about 0.01.