Bond-Alternating Antiferromagnetic S=1/2 Heisenberg Ladder with Ferromagnetic Diagonal Coupling

Using the density matrix renormalization group method, we determine the phase diagram of a frustrated bond-alternating S=1/2 Heisenberg ladder with ferro-antiferromagnetic couplings at zero temperature. With the interactions between spins along the rungs set, we identify three spin-gapped phases (the Haldane phase, the singlet phase and the dimer phase) in the whole parameter range. The analysis of our data shows that two-leg spin bond-alternating ladders have a rich phase diagram if both rung and diagonal couplings are taken into account.