Superconductor Cobaltites

Since the discovery of high transition-temperature (high Tc ) superconductivity in layered copper oxides [1], they have held the pole position in the race for achieving higher Tc [2]. In this race many different compounds which contain copper oxide layers has been examined. Although there are reports of Tc s up to 185K for very rare phases of complex compounds like (Sn1.0Pb0.5In0.5)Ba4Tm5Cu7O20+[3] the highest Tc record for a single phase material belongs to Hg0.8T l0.2Ba2Ca2Cu3O8+δ [4]. Even though the highest Tc reported is higher than the coldest temperature ever measured on earth it is still far below the room temperature. As a result of the success of layered copper oxides, many researchers have searched for similar behavior in other layered metal oxides. Most of the attempts involving 3d-transition metals, such as nickel and cobalt have failed. But Takada et. al. [5] have reported that NaxCoO2 · yH2O (x ∼0.35,y ∼1.3) is a superconductor with a Tc of about 5K. Despite its low Tc this material has attracted considerable attention because of the similarities and contrasts between it and copper oxides.