In-plane optical response of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CuO}}_6$

We report on infrared reflectivity measurements of the ab-plane response of superconducting ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CuO}}_6$ single crystals. The frequency-dependent conductivity has a maximum near 500 ${\mathrm{cm}}^{\mathrm{-}1}$ at room temperature, which shifts to lower frequency and merges with a Drude peak below 100 K. We attribute the unusual behavior of the midinfrared conductivity to low-frequency transitions between electronic bands of mainly BiO character near the M-bar point. The linear temperature dependence of the low-frequency resistivity can be followed down to approximately 40 K where it saturates.