Far-infrared composite-medium study of sintered ${\mathrm{La}}_2$Ni${\mathrm{O}}_4$ and ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}\mathrm{Cu}{\mathrm{O}}_{4-y}$

The infrared and far-infrared properties of sintered ${\mathrm{La}}_2$Ni${\mathrm{O}}_4$ and ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}\mathrm{Cu}{\mathrm{O}}_{4-y}$ are modeled accurately within the effective-medium approximation. The electrodynamic properties of ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}\mathrm{Cu}{\mathrm{O}}_{4-y}$ grains are highly anisotropic with the strong 0.5-eV electronic transition polarized along the low-conductivity crystal direction. The apparent superconducting gap frequency for the sintered material depends crucially on the grain geometry. Our experimental data are fit with needlelike crystallites and $\frac{2\Delta }{k_B{T}_c}=2.6\mathrm{}$.