Investigation of the magnetic properties of chains with alternating ferro-and antiferromagnetic exchange interactions in the Heisenberg model with spin S=1/2

The quantum Monte Carlo method is used to calculate the susceptibility and pairwise spin-spin correlation functions of chains with alternating ferro ( K)- and antiferromagnetic (J)-exchange interactions within the Heisenberg m odel with spin S=1/2. From the susceptibility, the energy gap between the g round state and excited triplet states is determined for arbitrary ratios K /J. The value of the gap coincides with the Haldane gap for spin S=1 when K /J > 1.25. (C) 1999 American Institute of Physics. [S1063-7834(99)02409-0].