Rattling Good Superconductor: β-Pyrochlore Oxides AOs₂O₆

Abstract

In this review, we present a summary of experimental and theoretical investigations on the rattling and superconductivity of the β-pyrochlore osmium oxides AOs 2 O 6 (A = Cs, Rb, and K), after these oxides were found in 2004. First, we provide the background of our research, a brief history of rattling, and the characteristics of cage compounds thus far studied in terms of rattling. Then we show the structural properties of β-pyrochlore oxides, focusing on the rattling of an A ion in an oversized cage made of oxide ions. After the chemical trends of the normal and superconducting states are summarized, the role of rattling, which is a local, essentially anharmonic oscillation with a large atomic excursion, is discussed. Furthermore, two recent topics, i.e., an isomorphic structural transition and peculiar behavior under high pressure, are addressed. We conclude that rattling vibrations markedly affect the structural and electronic properties of β-pyrochlore oxides and even induce or enhance superconductivity through strong electron–rattler interactions. The vital role of anharmonicity is emphasized from a theoretical point of view on the basis of the strong-coupling theory.