Thermodynamics of the kagome-lattice Heisenberg antiferromagnet with arbitrary spin $S$

Abstract: We use a second-order rotational invariant Green's function method (RGM) and the high-temperature expansion (HTE) to calculate the thermodynamic properties, of the kagome-lattice spin-$S$ Heisenberg antiferromagnet with nearest-neighbor exchange $J$. While the HTE yields accurate results down to temperatures of about $T/S(S+1) \sim J$, the RGM provides data for arbitrary $T \ge 0$. For the ground state we use the RGM data to analyze the $S$-dependence of the excitation spectrum, the excitation velocity, the uniform susceptibility, the spin-spin correlation functions, the correlation length, and the structure factor. We found that the so-called $\sqrt{3}\times\sqrt{3}$ ordering is more pronounced than the $q=0$ ordering for all values of $S$. In the extreme quantum case $S=1/2$ the zero-temperature correlation length is only of the order of the nearest-neighbor separation. Then we study the temperature dependence of several physical quantities for spin quantum numbers $S=1/2,1,\dots,7/2$. As increasing $S$ the typical maximum in the specific heat and in the uniform susceptibility are shifted towards lower values of $T/S(S+1)$ and the height of the maximum is growing. The structure factor ${\cal S}(\mathbf{q})$ exhibits two maxima at magnetic wave vectors $\mathbf{q}={\mathbf{Q}_i}, i=0,1,$ corresponding to the $q=0$ and $\sqrt{3}\times\sqrt{3}$ state. We find that the $\sqrt{3}\times \sqrt{3}$ short-range order is more pronounced than the $q=0$ short-range order for all temperatures $T \ge 0$. For the spin-spin correlation functions, the correlation lengths and the structure factors, we find a finite low-temperature region $0 \le T < T^*\approx a/S(S+1)$, $a \approx 0.2$, where these quantities are almost independent of $T$.