COMPETITION BETWEEN THE BOND ALTERNATION AND THE ANISOTROPY IN A SPIN-1/2 XY-CHAIN

We investigate the ground state of the alternating spin-1/2 XY chain with the anisotropy [formula omitted] where 0≤γ≤1 and 0≤δ≤1. From the exact results of the ground state energy calculation, the ground state is found to be either one of the following three states: the spin fluid state (γ=δ=0), the effective singlet pair state (ES state; δ>γ) and the Néel state (γ>δ). We also exactly calculate the asymptotic form of the longitudinal spin correlation 〈SizSjz〉 in the long distance limit. Both its functional form and the parameter dependence of the correlation length are quite consistent with our picture for the ES state and the Néel state. © 1990, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.