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

The complex dielectric constant $\hat{\epsilon} = \epsilon_1 + i \epsilon_2$ of SrTiO$_3$ reaches high values $\epsilon_1 \approx 2*10^{4}$ 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.