Forefront for the quest of mechanism of high-temperature superconductivity

Unraveling the mechanism of high-Tc superconductivity in copper oxide materials continues to be one of the most challenging problems in physics. This challenging problem leads to a marked development of various spectroscopies which make it possible to obtain detailed electronic information in both real and momentum spaces. Now, a target toward the goal is to elucidate the unknown 'pseudogap phase' that competes and sometimes coexists with a superconducting phase. The research forefront for the quest of the high-Tc mechanism is reported in this article on the basis of the recent results of advanced spectroscopies. The central issues are the electronic state of the pseudogap phase and its role in the high-Tc superconductivity. (author)