Interpretation of Ir L-edge isotropic x-ray absorption spectra across the pressure-induced dimerization transition in hyperhoneycombβ-Li2IrO3

The extended nature of atomic 5d orbitals, together with a relatively short ???3 ?? Ir-Ir distance across edgeshared octahedra in honeycomb iridate lattices, leads to a tendency to disrupt the local, spin-orbit-entangled jeff = 12 moments of Ir4+ ions in favor of dimerization and the formation of molecular orbitals, especially upon lattice compression. The sensitivity of Ir L-edge spectroscopy to both spin-orbit entanglement in jeff states and the quenching of orbital degrees of freedom in dimerized states results in a peculiar evolution of x-ray absorption spectra across dimerization transitions, including energy shifts in the opposite direction for L3 and L2 leading absorption edges and substantial changes in their isotropic branching ratio. We present a theoretical description of the evolution of 5d electronic states, and related x-ray absorption spectra, in going from the single ion to the dimerized limit. The calculations reproduce the experimental results for hyper-honeycomb ??-Li2IrO3 [L. Veiga et al., Phys. Rev. B 100, 064104 (2019); 96, 140402(R) (2017)] and shed light into the weakening of the coupling between spin and orbital degrees of freedom as the strength of dimerization increases. The results provide a basis for the interpretation of L-edge x-ray absorption spectra in 5d systems where competition between the formation of local jeff states and molecular orbitals is at play.