Evidence of two species of carriers from the far-infrared reflectivity of Bi2Sr2CuO6

An optical "pseudogap" is often assumed to open at low T in the "anomalous Drude?' absorption, which models the optical conductivity sigma(omega)propo rtional to omega (-1) of high-T-c superconductors by a linewidth Gamma appr oximate to 10(3) cm(-1) varying with omega. In the sigma(omega) of Bi2Sr2Cu O6 measured down to 10 cm(-1), we have resolved, instead, two components se parated by a deep minimum: (i) a normal Drude term with Gamma = 35 cm(-1) a t 30 K, in very good agreement with transport data; (ii) a strong band peak ed in the far infrared (FIR), likely due to bound charges, whose tail exhib its the omega (-t) dependence. As the FIR peak softens for T-->0 it opens a pseudogaplike depression in a(w) accordingly to ordinary sum rules.