Phase Diagram of the Holstein-Hubbard Two-Leg Ladder
  Using a functional renormalization group method, we obtain the phase diagram of the two-leg ladder system within the Holstein-Hubbard model, which includes both electron-electron and electron-phonon interactions. Our renormalization group technique allows us to analyze the problem for both weak and strong electron-phonon coupling. We show that, in contrast results from conventional weak coupling studies, electron-phonon interactions can dominate electron-electron interactions because of retardation effects. 
Ka-Ming Tam, S.-W. Tsai, D. K. Campbell, A. H. Castro Neto