Metallic Ferroelectricity and Superconductivity in the Transition Metal Oxide Pyrochlore Cd₂Re₂O₇

Single crystalline samples of transition metal oxide Cd₂Re₂O₇, the first superconductor among pyrochlore oxides with general formula A₂B₂O₇ and the first example of “ferroelectric metal” theoretically proposed by P.W. Anderson and E.I. Blount back in 1960s, have been grown using vapor transport and carefully characterized, regarding its chemical stoichiometry and structure. In addition to the superconductivity transition, two normal state phase transitions and the anomalous high temperature phase have been comprehensively investigated, by means of transport, thermodynamic and spectroscopy techniques. The system undergoes a subtle continuous cubic-to-tetragonal phase transition at T*~ 200 K, a weak first-order tetragonal-to-tetragonal phase transition at T’~ 120 K. Theoretical calculation indicates the primary lattice instability comes from a two-fold degenerate soften phonon mode with E_u symmetry, and the motions of the oxygen octahedron coordinating Re dominate the energetics of the 200 K transition. Consistently, structural study suggests a staggered distortion of the Re-Re bond in the Re tetrahedral network, such that finite local distortions do not produce appreciable change in lattice constant. The complex and subtle change in normal state physical properties can be explained within a new type of ionic fluctuation model with metallic “ferroelectric” nature. Although the low temperature structure is yet fully determined, the 3-fold symmetry and inversion symmetry are clearly lost upon the onset of superconductivity at T_c~1.0 K. The superconductivity is likely of BCS type-II, but exotic in its T_c-penetration depth relation (i.e., “within a Uemura band”).