ELEMENTARY EXCITATIONS OF S=1 ANTIFERROMAGNETIC HEISENBERG-CHAINS WITH BOND ALTERNATION

A world-line quantum Monte Carlo method is employed to study the elementary excitations of S=1 antiferromagnetic Heisenberg chains with bond alternation described by the Hamiltonian, scrH=J tsumi [1-(-1)iδ]SiSi+1. Imaginary-time correlation functions of this system at low-enough temperatures exhibit almost single-exponential decay and thus the lowest excitation spectrum can be successfully extracted from them. As δ increases from 0 to 1, the system encounters a continuous phase transition at δ0.25, where the energy(E)-momentum(q) relation is fitted well to the form E(q)=(v2 sin2q+Δ2)1/2 with the effective light velocity v=2.46±0.08 and the energy gap Δ which vanishes in the long chain limit. The central charge c at the massless critical point is estimated to be 1.02±0.09, which suggests the universality class of the Gaussian model. © 1995 The American Physical Society.