High tunability of the permittivity of YBa2Cu3O7−∂/SrTiO3 heterostructures on sapphire substrates

Abstract

The combination of a low loss, high-Tc superconductor and a polarizable dielectric looks interesting from the point of tunable, high Q filters for cellular communication. Epitaxial heterostructures of YBa2Cu3O7−∂/SrTiO3/YBa2Cu3O7−∂/CeO2 and SrTiO3/YBa2Cu3O7−∂/CeO2 were grown on sapphire substrates in this study. Superconducting transition temperatures of 88–90 K and a critical current density of about 2×106 A/cm2 at 77 K were determined for YBa2Cu3O7−∂ films on CeO2 buffered sapphire. An effective permittivity of 340 at 300 K was measured capacitively for the SrTiO3 layer between YBa2Cu3O7−∂ electrodes; it increased three times at 50 K. The dielectric constant of SrTiO3 was observed to decrease a factor of 2 as a dc bias voltage of ±2.5 V was applied between the electrodes. Much smaller nonlinearities and temperature dependences of the dielectric constant were noted if the top YBa2Cu3O7−∂ electrode was replaced by an Ag one.