Existence of massive singlet excitations in an antiferromagnetic alternating chain with S=1/2

The one-dimensional Heisenberg model with alternating antiferromagnetic bon d and spin-1/2 has been studied by the Monte Carlo method. The thermodynami c characteristics, spin-spin, and four spin-correlation function have been calculated. From several maxima of the correlation radius of the four spin- correlation function and sharp slopes of the dimer order parameter, a longi tudinal component of the total spin square as a function of temperature and magnetic field, the existence of two mass singlet excitation branches with Delta S-z=0 and one triplet with Delta S-z=1 is suggested. Critical temper atures and fields related to singlet filled bands have been estimated. From these results low-temperature (T<T-SP) properties of spin-Peierls (SP) com pounds CuGeO3 and NaV2O5 have been explained.