Phase diagram of the Holstein-Hubbard two-leg ladder using a functional renormalization-group method

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 electronphonon couplings. We show that, in contrast to results from conventional weak-coupling studies, electronphonon interactions can dominate electron-electron interactions, and retardation effects play an important role on the phase diagram.