Anisotropic optical conductivity of Sr4Ru3O10

The optical conductivity $\ensuremath{\sigma}(\ensuremath{\omega})$ of Sr${}_{4}$Ru${}_{3}$O${}_{10}$ has been studied by means of reflectivity measurements between 12 and 300 K both in the $ab$ planes (from 40 to 28 000 cm${}^{\ensuremath{-}1}$) and along the $c$ axis (from 40 to 19 000 cm${}^{\ensuremath{-}1}$). In the far and mid infrared, ${\ensuremath{\sigma}}_{ab}(\ensuremath{\omega})$ results from a superposition of a Drude term, with a plasma frequency which increases below the ferromagnetic transition at ${T}_{c}=105$ K, and of two bands in the infrared. One is peaked at about 400 cm${}^{\ensuremath{-}1}$ for $Tl$ 300 K, but softens to 300 cm${}^{\ensuremath{-}1}$ at room temperature. The other one is peaked at 1300 cm${}^{\ensuremath{-}1}$ and, below ${T}_{c}$, provides spectral weight to the Drude term, suggesting that it has a magnetic origin. Along the $c$ axis, ${\ensuremath{\sigma}}_{c}(\ensuremath{\omega})$ is instead that of a good insulator, with phonon peaks whose frequencies ${\ensuremath{\omega}}_{\mathrm{ph}}$ regularly increase as $T$ decreases. For the mode at the highest frequency, however, which can be assigned to the apical oxygens of the RuO${}_{6}$ octahedra, both ${\ensuremath{\omega}}_{\mathrm{ph}}$ and the intensity ${I}_{\mathrm{ph}}$ have an anomalous behavior below ${T}_{c}$.