Nonlinear sigma model study of a frustrated spin ladder

A model of a two-leg spin-S ladder with two additional frustrating diagonal exchange couplings J(D), J'(D) is studied within the framework of the nonlinear sigma model approach. The phase diagram has a rich structure and contains 2S gapless phase boundaries which split off the boundary to the fully saturated ferromagnetic phase when J(D) and J'(D) become different. For the S = 1/2 case, the phase boundary is identified as separating two topologically distinct Haldane-type phases as discussed recently by Kim and co-workers (Kim E H, Fath G, Solyom J and Scalapino D J 1999e-print cond-mat/9910023).