A new triangular system: NaxCoO2

This is a review of the newly discovered triangular system, NaxCoO2. If water molecules are intercalated to this system, it shows superconductivity. Its electron density is more-than-half-filling in the Co t2g band, so this material is considered to be an electron-doped system. The present situation of experiments on superconductivity as well as magnetism is reviewed. From a theoretical point of view, the results of a local density approximation (LDA) band calculation and the obtained Fermi surfaces are discussed. With regards to possible mechanisms of superconductivity, recent results in a multi-band model are summarized with an emphasis on the effects of trigonal crystal-field splitting and spin–orbit coupling. Finally, alternative mechanisms using a single-band model are also reviewed.