ON THE INCOMMENSURATE PHASE IN MODULATED HEISENBERG-CHAINS

Using the density matrix renormalization group method (DMRG) we calcul ate the magnetization of frustrated S = 1/2 Heisenberg chains for vari ous modulation patterns of the nearest neighbour coupling: commensurat e, incommensurate with sinusoidal modulation and incommensurate with s olitonic modulation. We focus on the order of the phase transition fro m the commensurate dimerized phase (D) to the incommensurate phase (I) . It is shown that the order of the phase transition depends sensitive ly on the model. For the solitonic model in particular, a k-dependent elastic energy modifies the order of the transition. Furthermore, we c alculate gaps in the incommensurate phase in adiabatic approximation.