Spatially Resolved Nano-Scale Characterization of Electronic States in Bi-layer Ruthenates by STM/STS

We have performed low temperature scanning tunneling microscopy/spectroscopy (STM/STS) on bi-layer ruthenates AE$_3 $Ru$_2$O$_7$ (AE = Ca, Sr). Sr$_3$Ru$_2$O$_7$ is a paramagnetic metal very close to the itinerant ferromagnet, while Ca$_3$Ru$_2 $O$_7$ is a poor conductor (2-dimensional metal with antiferromagnetic order). Atomic lattice images were obtained in both systems for the first time. In Sr$_3$Ru$_2$O$_7$, STM image is independent of tip-sample bias voltage from -700 mV to +700 mV. On the contrary, in Ca$_3$Ru$_2$O$_7$, different atomic sites and complicated internal structures within the unit cell are observed depending on the bias voltage. We speculate that the specific orbital states of ruthenium and/or oxygen are responsible for the STM image at each bias voltage.