Nonperturbative effective-field theory for two-leg antiferromagnetic spin ladders - art. no. 144408

We study the long wavelength limit of a spin-1/2 Heisenberg antiferromagnet ic two-leg ladder, treating the interchain coupling in a nonperturbative wa y. We perform a mean field analysis and then include the fluctuations in an exact way. This allows for a discussion of the phase diagram of the system and provides an effective-field theory for the low-energy excitations. The coset fermionic Lagrangian obtained corresponds to a perturbed SU(4)(1)/U( 1) conformal field theory (CFT). This effective theory is naturally embedde d in a SU(2)(2) X Z(2) CFT, where perturbations are easily identified in te rms of conformal operators in the two sectors. Crossed and zigzag ladders a re also discussed using the same approach.