Quasimolecular electronic structure of the spin-liquid candidate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi mathvariant="normal">Ba</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">InIr</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>9</mml:mn></mml:msub></mml:math>
Alessandro Revelli, M. Moretti Sala, G. Monaco, M. Magnaterra, Jan Attig, L. Peterlini, T. Dey, Alexander A. Tsirlin, P. Gegenwart, T. Fröhlich, M. Braden, Christoph P. Grams, J. Hemberger, P. Becker, P. H. M. van Loosdrecht, D. I. Khomskiǐ, Jeroen van den Brink, Maria Hermanns, M. Grüninger
Abstract
The mixed-valent iridate ${\mathrm{Ba}}_{3}{\mathrm{InIr}}_{2}{\mathrm{O}}_{9}$ has been discussed as a promising candidate for quantum spin-liquid behavior. The compound exhibits ${\mathrm{Ir}}^{4.5+}$ ions in face-sharing ${\mathrm{IrO}}_{6}$ octahedra forming ${\mathrm{Ir}}_{2}{\mathrm{O}}_{9}$ dimers with three ${t}_{2g}$ holes per dimer. Our results establish ${\mathrm{Ba}}_{3}{\mathrm{InIr}}_{2}{\mathrm{O}}_{9}$ as a cluster Mott insulator. Strong intradimer hopping delocalizes the three ${t}_{2g}$ holes in quasimolecular dimer states while interdimer charge fluctuations are suppressed by Coulomb repulsion. The magnetism of ${\mathrm{Ba}}_{3}{\mathrm{InIr}}_{2}{\mathrm{O}}_{9}$ emerges from spin-orbit entangled quasimolecular moments with yet unexplored interactions, opening up a new route to unconventional magnetic properties of $5d$ compounds. Using single-crystal x-ray diffraction we find the monoclinic space group $C2/c$ already at room temperature. Dielectric spectroscopy shows insulating behavior. Resonant inelastic x-ray scattering reveals a rich excitation spectrum below 1.5 eV with a sinusoidal dynamical structure factor that unambiguously demonstrates the quasimolecular character of the electronic states. Below 0.3 eV, we observe a series of excitations. According to exact diagonalization calculations, such low-energy excitations reflect the proximity of ${\mathrm{Ba}}_{3}{\mathrm{InIr}}_{2}{\mathrm{O}}_{9}$ to a hopping-induced phase transition based on the condensation of a quasimolecular spin-orbit exciton. The dimer ground state roughly hosts two holes in a bonding $j=\frac{1}{2}$ orbital and the third hole in a bonding $j=\frac{3}{2}$ orbital.