Terahertz conductivity ofSr1−xCaxRuO3

We investigate the optical conductivity of ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_x{\mathrm{RuO}}₃$ across the ferromagnetic to paramagnetic transition that occurs at $x=0.8$. The thin films were grown by metalorganic aerosol deposition with $0\ensuremath{\le}x\ensuremath{\le}1$ onto ${\mathrm{NdGaO}}₃$ substrates. We performed terahertz frequency domain spectroscopy in a frequency range from 3 to $40 {\mathrm{cm}}^{\ensuremath{-}1}$ (100 GHz to 1.4 THz) and at temperatures ranging from 5 to 300 K, measuring transmittivity and phase shift through the films. From this we obtained the real and imaginary parts of the optical conductivity. The end-members, ferromagnetic ${\mathrm{SrRuO}}₃$ and paramagnetic ${\mathrm{CaRuO}}₃$, show a strongly frequency dependent metallic response at temperatures below 20 K. Due to the high quality of these samples we can access pronounced intrinsic electronic contributions to the optical scattering rate, which at 1.4 THz exceeds the residual scattering rate by more than a factor of three. Deviations from a Drude response start at about 0.7 THz for both end-members in a remarkably similar way. For the intermediate members a higher residual scattering originating in the compositional disorder leads to a featureless optical response instead. The relevance of low-lying interband transitions is addressed by a calculation of the optical conductivity within density functional theory in the local-density approximation.