Abstract

Most of the recently discovered superconductors, like all cuprates, ${\mathrm{MgB}}_2$, or ${\mathrm{Na}}_x{\mathrm{CoO}}_2\bullet n{\mathrm{H}}_2\mathrm{O}$, are uniaxial crystals with metallic ab planes orthogonal to an insulating, or sometimes poorly conducting, c axis. The reflectivity $R_{\mathit{ab}}\left(\omega \right)$ provides, through the optical conductivity ${\sigma }_{\mathit{ab}}\left(\omega \right)$, valuable information on the low-energy electrodynamics of these intriguing materials. However, in the far infrared, $R_{\mathit{ab}}\left(\omega \right)\simeq 1$ and a slight contamination from the c-axis response or an unsuitable choice of the extrapolation to $\omega =0$ may lead to large errors on ${\sigma }_{\mathit{ab}}\left(\omega \right)$. These issues are quantitatively discussed here with particular regard to ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_{4+y}$, one of the most-studied high-$T_c$ materials.

© 2005 Optical Society of America

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