Coupled cluster study of the dimer state of the spin-1/2 alternating Heisenberg chain

We investigate the properties of the spin-1/2 alternating Heisenberg chain using the coupled cluster method accompanied by exact diagonalization up to 24 sites. The ground-state energy per spin e and the spin gap Δ are calculated for a range of α. Our results show that the spin gap opens and the dimerized state dominates the properties of the ground state as α>1. We also study the approach of the ground-state energy per spin e0 to the uniform spin chain. The results show that the critical behaviour of e0 is well described by a power law with exponents 1.4307 and 1.4405 obtained by the coupled cluster method and exact diagonalization, respectively.