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

The ground state phase diagram of the alternating spin-1/2 chains with anisotropic ferromagnetic coupling under the influence of a symmetry breaking transverse magnetic field is studied. We have used 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 $h_{c_1}$ and $h_{c_2}$, corresponding to the transition between different magnetic phases of the system. It is found that the intermediate state ($h_{c_1}<h<h_{c_2}$) is gapful, describing the stripe-antiferromagnetic phase.