Infrared Nano-imaging and nano-spectroscopy of phase transition in Ca$_{\mathrm{2}}$RuO$_{\mathrm{4}}$

We investigated the electric-field-induced insulator-to-metal transition (IMT) in bulk single crystal Ca$_{\mathrm{2}}$RuO$_{\mathrm{4}}$. Ca$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ is a 4$d$ electron transition metal oxide which has an IMT above 358 K. By applying a DC voltage above the threshold of 40 V/cm, a discontinuous conductivity jump can be readily observed with a dramatic change of electron and phonon signatures at the infrared frequency range. With infrared nano-imaging and nano-spectroscopy enabled by the scattering-type scanning near-field microscope (s-SNOM) techniques, this phase transition and the propagation of insulator-metal phase boundary are revealed with 10 nm resolution. We will discuss different stages of the phase transition and the nano-textures observed during the experiment.