Far-infrared reflectivity of Bi_2Sr_2CuO_6: the "anomalous Drude" model   and optical pseudogap revisited
  An optical "pseudogap" is usually assumed to open at low T in the "anomalous Drude" absorption, which models the optical conductivity sigma(omega) (proportional to omega-1) of HCTS by a linewidth Gamma (on the order of 1000 cm-1) varying with omega. In the sigma(omega) of BSCO, measured down to 10 cm-1, we have resolved instead: i) a normal Drude term with Gamma=35 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 decreasing T, it opens a pseudogap-like depression in sigma(omega) accordingly to ordinary sum rules. 
S. Lupi, P. Calvani, M. Capizzi (Dipartimento di Fisica, Universita La   Sapienza, Roma, Italy), P. Roy (LURE, Universite' P...