Renormalized excitonic method in terms of block excitations:: Application to spin lattices -: art. no. 224412

Dividing the lattice into blocks with a singlet ground state and knowing the exact low-energy spectrum of the blocks and of dimers (or trimers) of blocks, it is possible to approach the lowest part of the lattice spectrum through an excitonic type effective model. The potentialities of the method are illustrated on the one-dimensional (1D) frustrated chain and the 1/5-depleted square and the plaquette 2D lattices. The method correctly locates the phase transitions between gapped and nongapped phases.