Band Structure and Terahertz Optical Conductivity of Transition Metal Oxides: Theory and Application to CaRuO$_3$

Density functional plus dynamical mean field calculations are used to show that in transition metal oxides, rotational and tilting (GdFeO$_3$-type) distortions of the ideal cubic perovskite structure produce a multiplicity of low-energy optical transitions which affect the conductivity down to frequencies of the order of $1$ or $2$~mV (terahertz regime), mimicking non-Fermi-liquid effects even in systems with a strictly Fermi-liquid self-energy. For CaRuO$_3$, a material whose measured electromagnetic response in the terahertz frequency regime has been interpreted as evidence for non-Fermi-liquid physics, the combination of these band structure effects and a renormalized Fermi-liquid self-energy accounts for the low frequency optical response which had previously been regarded as a signature of exotic physics. Signatures of deviations from Fermi-liquid behavior at higher frequencies ($\sim 100$~meV) are discussed.