Longer-range lattice anisotropy strongly competing with spin-orbit interactions in pyrochlore iridates

Abstract

In the search for topological phases in correlated electron systems, materials with $5d$ transition-metal ions, in particular the iridium-based pyrochlores ${A}_{2}$Ir${}_{2}$O${}_{7}$, provide fertile grounds. Several topological states have been predicted but the actual realization of such states is believed to critically depend on the strength of local potentials arising from distortions of the IrO${}_{6}$ cages. We test this hypothesis by measuring with resonant inelastic x-ray scattering the electronic level splittings in the $A=\text{Y}$, Eu systems, which we show to agree very well with ab initio quantum chemistry electronic-structure calculations for the series of materials with $A=\text{Sm}$, Eu, Lu, and Y. We find, however, that the primary source for quenching the spin-orbit interaction is not a distortion of the IrO${}_{6}$ octahedra but longer-range lattice anisotropies which inevitably break the local cubic symmetry.