In-plane ( $\mathrm{ab}$-plane) and out-of-plane ( $c$-axis) optical spectra were investigated at various temperatures for ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ which is isostructural with the cuprate superconductor ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ and becomes superconducting below $1\mathrm{K}$. The ratio of the in-plane to out-of-plane low-energy spectral weight of this compound is approximately ${10}^2$, an anisotropy comparable to that of the cuprate. The temperature dependence of the $c$-axis spectrum suggests coherent motion of conduction carriers with strongly renormalized mass and scattering rate along the $c$ axis at low temperatures.

• Received 17 July 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.126