Title: High temperature thermodynamics of the honeycomb-lattice Kitaev-Heisenberg model: A high temperature series expansion study,
Abstract: 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
exachange coupling $J$ (with $|J|\le |K|$) 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 $10$-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 Na$_2$IrO$_3$ are consistent with a dominant {\it
antiferromagnetic} Kitaev exchange constant of about $300-400$ $K$.