Pentavalent iridium pyrochlore Cd_2Ir_2O_7: A prototype material system for competing crystalline field and spin-orbit coupling

摘要

A new pyrochlore oxide Cd_2Ir_2O_7 with an Ir~(5+) charge state was prepared by high-pressure techniques. Although strong spin-orbit coupling (SOC) dominates the electronic states in most iridates so that a SOC-Mott state is proposed in Sr_2IrO_4 in the assumption of an undistorted IrO_6 octahedral crystalline field, the strongly distorted one in the current Cd_2Ir_2O_7 exhibits a competing interaction with the SOC. Unexpected from a strong SOC limit, Cd_2Ir_2O_7 deviates from a nonmagnetic and insulating J=0 ground state. It displays short-range ferromagnetic correlations and metallic electrical transport properties. First-principles calculations well reproduce the experimental observation, revealing the large mixture between the j_(eff) =1/2 and j_(eff) = 3/2 bands near the Fermi surface due to the significant distortion of IrO_6 octahedra. This work sheds light on the critical role of a noncubic crystalline field in electronic properties which has been ignored in past studies of 5d-electron systems.