Two-particle spin-singlet excitations in coupled spin-1/2 antiferromagnetic alternating chains

The spectrum of the two-particle spin-singlet (DeltaS(z) = 0, +/-1) excitat ions of a weakly coupled antiferromagnetic spin-1/2 alternating (J(1) +/- d elta) chain is calculated using a mean-field approximation for the intercha in exchange (J(2)) by the quantum Monte Carlo method. The bandwidth change of these excitations, the mass gaps in the singlet-singlet excitation spect rum, the top boundaries of the bands and the velocities of these excitation s are estimated as functions of the alternating bond delta. The temperature s at which the singlet gaps close are determined.