Perturbation theory by flow equations: dimerized and frustrated S=1/2 chain
Institut für Theoretische Physik, Universität zu Köln,
Zülpicher Straße 77, Köln 50937, Germany
Revised: 7 June 1999
Published online: 15 January 2000
The flow equation method (Wegner, 1994) is used as continuous unitary transformation to construct perturbatively effective Hamiltonians. The method is illustrated in detail for dimerized and frustrated antiferromagnetic S=1/2 chains. The effective Hamiltonians conserve the number of elementary excitations which are S=1 magnons for the dimerized chains. The sectors of different number of excitations are clearly separated. Easy-to-use results for the gap, the dispersion and the ground state energies of the chains are provided.
PACS: 75.10.Jm – Quantized spin models / 02.30.Mv – Approximations and expansions / 03.65.-w – Quantum mechanics
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2000