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MPIPKS:
Superconductivity and magnetism
Abstract:
We use a secondorder rotational invariant Green'sfunction method (RGM) and the hightemperature expansion (HTE) to calculate the thermodynamic properties of the kagomelattice spinS Heisenberg antiferromagnet with nearestneighbor exchange J. While the HTE yields accurate results down to temperatures of about T/S(S + 1) similar to J, the RGM provides data for arbitrary T >= 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 spinspin correlation functions, the correlation length, and the structure factor. We found that the socalled root 3 x root 3 ordering is more pronounced than the q = 0 ordering for all values of S. In the extreme quantum case S = 1/2, the zerotemperature correlation length is only of the order of the nearestneighbor separation. Then we study the temperature dependence of several physical quantities for spin quantum numbers S = 1/2,1.....7/2. As S increases, the typical maximum in the specific heat and that in the uniform susceptibility are shifted toward lower values of T/S(S + 1), and the height of the maximum is growing. The structure factor 8(q) exhibits two maxima at magnetic wave vectors q = Q(i), i = 0,1, corresponding to the q = 0 and root 3 x root 3 state. We find that the root 3 x root 3 shortrange order is more pronounced than the q = 0 shortrange order for all temperatures T >= 0. For the spinspin correlation functions, the correlation lengths, and the structure factors, we find a finite lowtemperature region 0 <= T < T* approximate to a/S(S + 1), a approximate to 0.2, where these quantities are almost independent of T.