OPTICAL STUDY OF HIGH-T-C SUPERCONDUCTORS - SOLVED AND REMAINING PROBLEMS

Optical spectroscopy, which is the simplest tool for probing the carri er motion in materials, has made a big contribution to the research of high-T-c superconductors (HTSC). I summarize the main results of opti cal spectroscopy which have revealed the characteristic nature of HTSC , such as (i) the doping dependence of the normal state spectra and th e electronic phase diagram, (ii) far-infrared studies of the supercond ucting gap and Josephson plasma, (iii) spin-gap (pseudogap) related pr oblems, and (iv) pair-breaking effects. Although all of these issues g ive rich information about the electronic states of HTSC, they have no t been fully understood yet. Some of the key experimental facts that g ive constraints on theories are indicated.