SO(N) symmetries in the two--chain model of correlated fermions

Abstract

The two-chain model of correlated fermions, appropriate e.g. for doped spin ladders or carbon nanotubes, is investigated in the weak-interaction limit. It is shown that the model scales to situations with high internal symmetries, generically SO(6), whereas on critical lines there is SO(5) or SO(3) symmetry. Consequences for the level spectroscopy of the model are discussed. In particular, the ``pi-mode'' is shown to be sixfold degenerate and contains both spin and charge excitations.