Abstract
We report the results of our study of a recently synthesized honeycomb iridate NaxIrO3 (0.60≤x≤0.80). Single-crystal NaxIrO3 adopts a honeycomb lattice noticeably without distortions and stacking disorder inherently existent in its sister compound Na2IrO3. The oxidation state of the Ir ion is a mixed valence state resulting from a majority Ir5+(5d4) ion and a minority Ir6+(5d3) ion. NaxIrO3 is a Mott insulator likely with a predominant pseudospin =1 state. It exhibits an effective moment of 1.1μB/Ir 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 Th=0.9K and Tl=0.12K in both the heat capacity and AC magnetic susceptibility. Intermediate between Th and Tl lies a pronounced temperature linearity of the heat capacity with a large slope of 77mJ/moleK2, a feature expected for highly correlated metals but not at all for insulators. These results along with a comparison drawn with the honeycomb lattices Na2IrO3 and (Na0.2Li0.8)2IrO3 point to an exotic ground state in proximity to a possible Kitaev spin liquid.
