Planar GHz Resonators on SrTiO3: Suppressed Losses at Temperatures below 1 K

Abstract

The complex dielectric constant $\hat{\epsilon }={\epsilon }_1+i{\epsilon }_2$ of SrTiO${}_3$ reaches high values ${\epsilon }_1\approx 2\ast 10^4$ at cryogenic temperatures, while the dielectric losses (${\epsilon }_2$) are much stronger than for other crystalline dielectrics. SrTiO${}_3$ is a common substrate for oxide thin films, like the superconducting LaAlO${}_3$/SrTiO${}_3$ system, but the large ${\epsilon }_1$ and ${\epsilon }_2$ restrict high-frequency quantum devices on SrTiO${}_3$. Here we present superconducting coplanar Nb resonators on SrTiO${}_3$, which we successfully operate in a distant-flip-chip geometry at frequencies that exceed 1 GHz. We find a pronounced and unexpected increase in resonator quality factor $Q$ at temperatures below 1 K, reaching up to $Q\approx 800$. We attribute this to substantial changes of the dielectric losses in SrTiO${}_3$ at mK temperatures, and we also detect non-monotonous changes in the temperature-dependent ${\epsilon }_1$. These findings challenge our present understanding of the dielectric properties of SrTiO${}_3$ and at the same time demonstrate that cryogenic high-frequency devices on SrTiO${}_3$ are more feasible than previously assumed.