2022-08-12T06:07:22Zhttps://niigata-u.repo.nii.ac.jp/oaioai:niigata-u.repo.nii.ac.jp:000016472021-03-01T20:55:53ZFirst- and Second Order Phase Transitions in the Holstein-Hubbard ModeFirst- and Second Order Phase Transitions in the Holstein-Hubbard ModeKoller, W.4787Meyer, D.4788Ono, Y.62Hewson, A. C.4790We investigate metal-insulator transitions in the half-filled Holstein-Hubbard model as a function of the on-site electron-electron interaction U and the electron-phonon coupling g. We use several different numerical methods to calculate the phase diagram, the results of which are in excellent agreement. When the electron-electron interaction U is dominant the transition is to a Mott-insulator; when the electron-phonon interaction dominates, the transition is to a localised bipolaronic state. In the former case, the transition is always found to be second order. This is in contrast to the transition to the bipolaronic state, which is clearly first order for larger values of U. We also present results for the quasiparticle weight and the double-occupancy as function of U and g.journal articleIOP Publishing2004-05application/pdfEurophysics Letters466559564Europhysics LettersAA1066812702955075https://niigata-u.repo.nii.ac.jp/record/1647/files/1014.pdfjpninfo:doi/10.1209/epl/i2003-10228-6