Spin dynamics of a tetrahedral cluster magnet
  We study the magnetism of a lattice of coupled tetrahedral spin-1/2 clusters which might be of relevance to the tellurate compounds Cu2Te2O5X2, with X=Cl, Br. Using the flow equation method we perform a series expansion in terms of the inter-tetrahedral exchange couplings starting from the quadrumer limit. Results will be given for the magnetic instabilities of the quadrumer phase and the dispersion of elementary triplet excitations. In limiting cases of our model of one- or two dimensional character we show our results to be consistent with findings on previously investigated decoupled tetrahedral chains and the Heisenberg model on the 1/5-depleted square lattice. 
Wolfram Brenig