Low-energy optics of Sr1-xCaxRuO3

The pseudo-cubic perovskite ruthenates SrRuO3 and CaRuO3 have recently attracted interest due to their unconventional electronic properties. For both materials, non-Fermi liquid behavior has been reported in previous optical studies at infrared frequencies. In addition to these two pure compounds, the doping series Sr1-xCaxRuO3 offers a rich phase diagram: going from the itinerant ferromagnet SrRuO3 to the paramagnet CaRuO3, indications for a quantum phase transition at x∼0.8 have been found. Using THz spectroscopy at frequencies between 5 cm-1 and 45 cm-1, we have studied thin-film samples of the Sr1-xCaxRuO3 system, which were prepared by metalorganic aerosol deposition. From transmission and phase measurements we have determined the frequency-dependent conductivity for a set of temperatures between 5 K and 300 K, and we discuss it in the framework of the extended Drude model with frequency-dependent relaxation rate and effective mass. While for pure SrRuO3 as well as for doped systems approaching the quantum phase transition we find conventional metallic Drude behavior, CaRuO3 exhibits highly unusual optical properties which we compare to results of dc measurements on these thin films, which also revealed temperature ranges with non-Fermi liquid behavior.