Numerical Study of the Spin-1 Antiferromagnetic Heisenberg Chain with a Spin-1/2 Impurity

Abstract

Low-lying excited states as well as the ground state of the spin-1 antiferromagnetic Heisenberg chain with a spin-1/2 impurity are studied by means of a method of numerical diagonalization. The isotropic nearest-neighbor exchange coupling plus the uniaxial single-ion-type anisotropy energy is assumed for the host-system Hamiltonian. The energy differences between the ground state and the low-lying excited states, which appear in the Haldane gap, are estimated in the thermodynamic limit. The results are used to analyze the electron-spin-resonance experimental data on Ni(C₂H₈N₂)₂NO₂(ClO₄), abbreviated NENP, containing a small amount of spin-1/2 Cu²⁺ impurities. It is found that in this system, the impurity-host (Cu²⁺--Ni²⁺) coupling is ferromagnetic and its magnitude is about 5 (Ni²⁺--Ni²⁺) coupling.