The spin-1/2 Heisenberg spin ladder with bond alternation

The spin-1/2 Heisenberg spin ladder model is studied by via a perturbation calculation and via bosonization. We mainly focus on the case of ferromagnetic interchain coupling in relation to the Haldane problem. Using the GKO coset construction of the conformal field theory, we bosonize the system in terms of the SU(2) (spin) sector and a residual Ising sector. The interchain interaction generically drives the system to strong-coupling massive phases (the Haldane phase or the dimerized phase). However, it is suggested that the criticality appears for particular choices of the couplings.