Phase transitions in staggered spin ladders

Abstract

We map spin ladders with nl legs and couplings J′ across all rungs and J(1 ± γ) along the legs, staggered in both directions, to a sigma model. Setting its θ = (2m+1\)π (where it is known to be gapless), we locate the critical curves in the γ versus J′/J plane at each nl, and spin S. The phase diagram is rich and has some surprises: When two gapped chains are suitably coupled, the combination becomes gapless. With nl, γ, and J′/J to control, the prospects for experimentally observing any one of these equivalent transitions are enhanced. We interpret our results in the framework of the resonating valence bond description of ladders. © 1996 The American Physical Society.