Ground state of a quantum mixed spin- – –1–1 chain with uniaxial single-ion-type anisotropy

Abstract Effect of the uniaxial single-ion-type anisotropy for the S =1 spins on the ground state of a quantum mixed spin- 1 2 – 1 2 –1–1 chain is discussed. It is found that, when the ground state in the case of no anisotropy is of the Haldane-phase character, the anisotropy causes a second-order phase transition accompanying the disappearance of the string order.