High-temperature thermodynamics of the honeycomb-lattice Kitaev-Heisenberg model: A high-temperature series expansion study.

We develop high-temperature series expansions for the thermodynamic properties of the honeycomb-lattice Kitaev-Heisenberg model. Numerical results for uniform susceptibility, heat capacity, and entropy as a function of temperature for different values of the Kitaev coupling K and Heisenberg exchange coupling J (with |J|≤|K|) are presented. These expansions show good convergence down to a temperature of a fraction of K and in some cases down to T=K/10. In the Kitaev exchange dominated regime, the inverse susceptibility has a nearly linear temperature dependence over a wide temperature range. However, we show that already at temperatures ten times the Curie-Weiss temperature, the effective Curie-Weiss constant estimated from the data can be off by a factor of 2. We find that the magnitude of the heat-capacity maximum at the short-range-order peak, is substantially smaller for small J/K than for J of order or larger than K. We suggest that this itself represents a simple marker for the relative importance of the Kitaev terms in these systems. Somewhat surprisingly, both heat-capacity and susceptibility data on Na2IrO3 are consistent with a dominant antiferromagnetic Kitaev exchange constant of about 300-400K. [ABSTRACT FROM AUTHOR]