Low-energy properties of spin-1/2 two-leg antiferromagnetic Heisenberg ladders with ferromagnetic diagonal coupling - art. no. 012401

Using the density matrix renormalization group method, we have obtained the ground-state phase diagram of a spin-1/2 two-leg antiferromagnetic Heisenb erg ladder with a ferromagnetic diagonal interaction J(x). The boundary bet ween the two phases found in the system, the Haldane phase and the singlet phase, lies in the frustrated area of the parameter space, i.e. both the ru ng coupling J(perpendicular to) and the diagonal coupling J(perpendicular t o) are ferromagnetic. The spin gap is found to be insensitive to the change of the ferromagnetic diagonal coupling strength when the ratio of the rung coupling constant J(perpendicular to) to the leg coupling constant J(paral lel to) is 0.5, in agreement wit the analysis of recent experimental spin s usceptibility data.