Superconductivity at 31 K in Alkaline Metal—Doped Cobalt Oxides

By using a simple solid reaction method,we have fabricated alkaline metal doped cobalt oxides AnxCoO2 δ（An=Na,K).The magnetic measurement shows a superconducting-like diamagnetic signal at 31K based on a strong superparamagnetic signal.Below 31K,the magnetization hysteresis loops contain a strong rough linear superparamagnetic background and a superconducting hysteresis.The typical magnetization hysteresis loops for a type-Ⅲ superconductor are found.Preliminary resistive data also show a fast dropping of resistance below Tc-These give indication of superconductivity below 31K in AnxCoO2 δ(An=Na,K).