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Jeff=1/2 Mott spin-orbit insulating state close to the cubic limit in Ca4IrO6

The Jeff=1/2 state is manifested in systems with large cubic crystal field splitting and spin-orbit coupling that are comparable to the on-site Coulomb interaction, U. 5d transition metal oxides host parameters in this regime and strong evidence for this state in Sr2IrO4, and additional iridates, has been presented. All the candidates, however, deviate from the cubic crystal field required to provide an unmixed canonical Jeff=1/2 state, impacting the development of a robust model of this novel insulating and magnetic state. We present results that not only show Ca4IrO6 hosts the state, but furthermore uniquely resides close to the ideal case required for an unmixed Jeff=1/2 state.