Quasimolecular electronic structure of the trimer iridate Ba4NbIr3O12

The insulating mixed-valent Ir+3.66 compound Ba4⁢NbIr3⁢O12 hosts two holes per Ir3⁢O12 trimer unit. We address the electronic structure via resonant inelastic x-ray scattering (RIXS) at the Ir 𝐿3 edge and exact diagonalization. The holes occupy quasimolecular orbitals that are delocalized over a trimer. This gives rise to a rich intra-𝑡2⁢𝑔 excitation spectrum that extends from 0.5 eV to energies larger than 2 eV. Furthermore, it yields a strong modulation of the RIXS intensity as a function of the transferred momentum q. A clear fingerprint of the quasimolecular trimer character is the observation of two modulation periods, 2⁢𝜋/𝑑 and 2⁢𝜋/2⁢𝑑, where 𝑑 and 2⁢𝑑 denote the intratrimer Ir-Ir distances. We discuss how the specific modulation reflects the character of the wave function of an excited state. Our quantitative analysis shows that spin-orbit coupling 𝜆 of about 0.4 eV is decisive for the character of the electronic states, despite a large hopping 𝑡𝑎1⁢𝑔 of about 0.8 eV. The ground state of a single trimer is described very well by both holes occupying the bonding 𝑗= 1/2 orbital, forming a vanishing quasimolecular moment with 𝐽=0.