Alternating Heisenberg Spin-1/2 Chains in a Transverse Magnetic Field

The ground-state phase diagram of alternating spin-1/2 chains with anisotropic ferromagnetic coupling under the influence of a symmetry-breaking transverse magnetic field is studied using the exact diagonalization technique. In the limit where the antiferromagnetic coupling is dominant, we have identified two Ising-type quantum phase transitions. We have calculated two critical fields h_c1 and h_c2, corresponding to the transition between different magnetic phases of the system. It is found that the intermediate state (h_c1<h<h_c2) is gapful, and describes the stripe-antiferromagnetic phase.