Double-peak specific heat feature in frustrated antiferromagnetic clusters

We study the nature of a double-peak specific heat structure in kagome clusters. A cluster containing 12 spins is considered thoroughly by means of numerical diagonalization. On base of analysis of exact spectra simple models are proposed to explain the nature of the low-$T$ peak at $T_l<\Delta$ ($\Delta$ is the spin gap) in this case and in those of larger clusters studied so far. Using these models we show that the rapid increase in density of states just above the spin gap gives rise to the peak, and clarify the weak magnetic field sensitivity of the peak. Spin susceptibility and entropy are considered as well. Our approach could be appropriate for other frustrated antiferromagnetic systems.