Remarkable Relationship between Structure and Physical Properties in the Superconducting Pyrochlore Oxides

Abstract

Pyrochlore oxides have the general chemical formula A2B2O7 or A2B2O6O', where A is a larger cation and B is a smaller transition metal cation. The ideal pyrochlore structure contains a tetrahedral network called the pyrochlore lattice, which has been studied extensively in order to elucidate the effect of geometrical frustration on the properties of localized spin and itinerant electron systems. Recently, superconductivity was found in Cd2Re2O7 at Tc= 1.0 K for the first time in the family of pyrochlore oxides. Although the mechanism of the superconductivity appears to be conventional, it exhibits unique successive structural phase transitions at high temperatures accompanied by large changes in physical properties. Very recently, new pyrochlore oxides AOs2O6 with A = K, Rb, and Cs were discovered, which show superconductivity at 9.6 K, 6.3 K, and 3.3 K, respectively. Although the nature of superconductivity on these β-pyrochlore superconductors is not clear at the moment, we believe that interesting physics is involved on the basis of electron correlations on the pyrochlore lattice.