In-plane optical response of Bi2Sr2CuO6

We report on infrared reflectivity measurements of the $ab$-plane response of superconducting Bi$_2$Sr$_2$CuO$_6$ single crystals. The frequency dependent conductivity has a maximum near 700 cm$^{-1}$ at room temperature, which shifts to lower frequency and merges with a Drude-peak below 100 K. We attribute the unusual behaviour of the mid-infrared conductivity to low frequency transitions between electronic bands of mainly BiO character near the $\overline{M}$ point. The linear temperature dependence of the low-frequency resistivity can be followed down to approximately 40 K where it saturates.