Generalized nonlinear sigma model approach to alternating spin chains and ladders
Service de Physique Théorique, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
Published online: 15 March 2000
We generalize the nonlinear sigma model treatment of quantum spin chains to cases including ferromagnetic bonds. When these bonds are strong enough, the classical ground state is no longer the standard Néel order and we present an extension of the known formalism to deal with this situation. We study the alternating ferromagnetic-antiferromagnetic spin chain introduced by Hida. The smooth crossover between decoupled dimers and the Haldane phase is semi-quantitatively reproduced. We study also a spin ladder with diagonal exchange couplings that interpolates between the gapped phase of the two-leg spin ladder and the Haldane phase. Here again we show that there is a good agreement between DMRG data and our analytical results.
PACS: 75.10.Jm – Quantized spin models / 75.20.Hr – Local moment in compounds and alloys; Kondo effect, valence fluctuations, heavy fermions
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2000