Evidence of two species of carriers from the far-infrared reflectivity ofBi2Sr2CuO6

An optical “pseudogap” is often assumed to open at low T in the “anomalous Drude” absorption, which models the optical conductivity $\sigma \left(\omega \right)\propto {\omega }^{-1}$ of high-$T_c$ superconductors by a linewidth $\Gamma \approx {10}^3{\mathrm{cm}}^{-1}$ varying with $\omega .$ In the $\sigma \left(\omega \right)$ of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CuO}}_6$ measured down to 10 ${\mathrm{cm}}^{-1},$ we have resolved, instead, two components separated by a deep minimum: (i) a normal Drude term with $\Gamma =35\mathrm{}{\mathrm{cm}}^{-1}$ at 30 K, in very good agreement with transport data; (ii) a strong band peaked in the far infrared (FIR), likely due to bound charges, whose tail exhibits the ${\omega }^{-1}$ dependence. As the FIR peak softens for $\overrightarrow{T}0,$ it opens a pseudogaplike depression in $\sigma \left(\omega \right)$ accordingly to ordinary sum rules.