SO(5)-SYMMETRICAL DESCRIPTION OF THE LOW-ENERGY SECTOR OF A LADDER SYSTEM

We study a system of two Tomonaga-Luttinger models coupled by a small transverse hopping (a two-chain ladder). We use Abelian and non-Abelia n bosonization to show that the strong coupling regime at low energies can be described by an SO(5)(1) Wess-Zumino-Witten model (or equivale ntly five massless Majorana fermions) deformed by symmetry-breaking te rms that nonetheless leave the theory critical at T= 0. The SO(5) curr ents of the theory comprise the charge and spin currents and linear co mbinations of the so-called pi operators [S.C. Zhang, Science 275, 108 9 (1997)], which are local in terms both of the original fermions and those of the effective theory. Using bosonization we obtain the asympt otic behavior of all correlation functions. We find that the five-comp onent ''superspin'' vector has power-law correlations at T=0; other fe rmion bilinears have exponentially decaying correlations and the corre sponding tendencies are suppressed. Conformal field theory also allows us to obtain the energies, quantum numbers, and degeneracies of the l ow-lying states and fit them into deformed SO(5) multiplets.