Heisenberg Spin-One Chain in Staggered Magnetic Field : A Density Matrix Renormalization Group Study

Using the density matrix renormalization group technique, we calculate numerically the low energy excitation spectrum and magnetization curve of the spin-1 antiferromagnetic chain in a staggered magnetic field, which is expected to describe the physics of $R_2 Ba Ni O_5 (R \neq Y) $ family below the N\'eel temperature of the magnetic rare-earth ($R$) sublattice. These results are valid in the entire range of the staggered field, and agree with those given by the non-linear $\sigma$ model study for small fields, but differ from the latter for large fields. They are consistent with the available experimental data. The correlation functions for this model are also calculated. The transverse correlations display the anticipated exponential decay with shorter correlation length, while the longitudinal correlations show explicitly the induced staggered magnetization.