Two-magnon bound states in an alternating Heisenberg chain

The behaviour of a singlet two-magnon bound state in an S=1/2 alternating Heisenberg antiferromagnetic chain is investigated by a quantum self-consistent theory with the help of the bosonization technique in the continuum limit approach. The singlet two-magnon bound state exists for any dimerization δ>0 at total momentum q=0. Its dispersion relation is given analytically in the vicinity of zero momentum. In the case of weak dimerization, the binding energy of the singlet two-magnon bound state monotonically increases with increasing dimerization parameter. Our results are consistent with recent experimental data of CuGeO3. (author). Letter-to-the-editor