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 nonl inear sigma model approach. The phase diagram has a rich structure and cont ains 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 topologic ally distinct Haldane-type phases as discussed recently by Kim and co-worke rs (Kim E H, Fath G, Solyom J and Scalapino D J 1999e-print cond-mat/991002 3).