Excitation spectrum of doped two-leg ladders: A field theory analysis
  We apply quantum field theory to study the excitation spectrum of doped two-leg ladders. It follows from our analysis that throughout most of the phase diagram the spectrum consists of degenerate quartets of kinks and anti-kinks and a multiplet of vector particles split according to the symmetry of the problem as 3 + 2 +1. This basic picture experiences corrections when one moves through the phase diagram. In some regions the splitting may become very small and in others it is so large that some multiplets are pushed in the continuum and become unstable. At second order transition lines masses of certain particles vanish. Very close to the first order transition line additional generations of particles emerge. Strong interactions in some sectors may generate additional bound states (like breathers) in the asymmetric charge sector. We briefly describe the properties of various correlation functions in different phases. 
D. Controzzi and A. M. Tsvelik