A possible explanation of gap features in high-T c cuprates : Tunneling density of states and infrared conductivity

Abstract

Within the Eliashberg theory, we have studied the paramagnetic pair-breaking effect on gap features in the tunneling density of states and infrared optical responses in high- T c cuprates. Especially, the dependence of this effect on the shape of α 2 F(ω) is examined by using two types of α 2 F(ω), each of which is supposed to be relevant to Bi2212 and YBCO, respectively. The feature of gap-filling and the reduction of both T c and the gap edge due to the pair-breaking effect are shown to depend significantly on the shape of α 2 F(ω). From these analyses, a possible explanation of the discrepancy between the gap ratios estimated from tunneling measurements and infrared measurements is proposed. Calculations of the optical responses with the inclusion of both paramagnetic and normal impurities of various concentrations indicate that infrared responses in these materials can be described well by the Eliashberg theory including both kinds of impurities of certain concentrations.