Condensed Matter Seminars

High-temperature superconductivity in an oxide containing quasi-two-dimensional copper-oxygen planes was observed by Bednorz and Muller in 1986. Recently, several high-temperature superconductors without copper or oxygen have been discovered including MgB₂ with a T_c of 39K and electric field doped C₆₀ with a T_c as high as 117K. Infrared spectroscopy has emerged as one of the most powerful experimental tools for the study of correlated electron systems and for high-T_c superconductors in particular. This talk will be focused on the infrared studies of two representative high-T_c superconductors: MgB₂ (T_c = 39.6 K) and optimally doped Bi₂Sr₂CaCu₂O_8+¦Ä (T_c = 91.5 K). Effects of electron-boson coupling are observed in optical conductivities for both systems and their significance with respect to superconductivity will be discussed. In general, having a small free carrier plasma frequency (< 3 eV) seems to be an universal characteristic shared by almost all high-temperature superconductors with a T_c > 30 K, which means that the issue of reduced screening should be treated carefully in all of these systems.