Integer-spin Heisenberg chains in a staggered magnetic field. A nonlinear sigma-model approach

We present here a nonlinear sigma-model (NL sigmaM) study of a spin-1 antif erromagnetic Heisenberg chain in an external commensurate staggered magneti c field. We find, already at the mean-field level, excellent agreement with recent and very accurate Density Matrix Renormalization Group (DMRG) studi es, and that up to the highest values of the field for which a comparison i s possible, for the staggered magnetization and the transverse spin gap. Qu alitative but not quantitative agreement is found between the NL sigmaM pre dictions for the longitudinal spin gap and the DMRG results. The origin of the discrepancies is traced and discussed. Our results allow for extensions to higher-spin chains that have not yet been studied numerically, and the predictions for a spin-a chain are presented and discussed. Comparison is a lso made with previous theoretical approaches.