A new triangular system:: NaxCoO2

This is a review of the newly discovered triangular system, NaxCoO(2). If water molecules are intercalated to this system, it shows superconductivity. Its electron density is more- than- half- filling in the Co t(2g) 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.