High pressure studies of high Tc superconductors in A–Cu–O system: The role of apical oxygen

We address the role of apical oxygen that plays the bridges linking charge reservoir and CuO2 conducting layers for high Tc superconductors (HTS). The presentation will introduce our recent work on high pressure synthesis of novel superconductors in the A–Cu–O simple system by tuning apical oxygen content and arrangement (A represents alkaline earth element). The topic will focus on 214 type compounds highlighting apical oxygen as a novel doping mechanism in oxychloride HTS, or dopant at a partially occupied apex, especially the unusual enhancement of Tc up to 95 K in a monolayered HTS by improving ordering state.