Ground state in proximity to a possible Kitaev spin liquid: The undistorted honeycomb iridate

We report the results of our study of a recently synthesized honeycomb iridate . Single-crystal adopts a honeycomb lattice noticeably without distortions and stacking disorder inherently existent in its sister compound . The oxidation state of the Ir ion is a mixed valence state resulting from a majority ion and a minority ion. is a Mott insulator likely with a predominant pseudospin state. It exhibits an effective moment of and a Curie-Weiss temperature of −19 K but with no discernible long-range order above 1 K. The physical behavior below 1 K features two prominent anomalies at and in both the heat capacity and AC magnetic susceptibility. Intermediate between and lies a pronounced temperature linearity of the heat capacity with a large slope of , a feature expected for highly correlated metals but not at all for insulators. These results along with a comparison drawn with the honeycomb lattices and point to an exotic ground state in proximity to a possible Kitaev spin liquid.