Unconventional Pairs Glued by Conventional Phonons in Cuprate Superconductors
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It has gone almost unquestioned that superexchange in the t−J (or Hubbard) model, and not phonons, is responsible for the unconventional (“d-wave”) pairing symmetry of cuprate superconductors. However, a number of advanced numerical studies have not found superconductivity in the Hubbard (or t−J) model. On the other hand, compelling experimental evidence for a strong electron–phonon interaction (EPI) has currently arrived. Here I briefly review some phonon-mediated unconventional pairing mechanisms. In particular the anisotropy of sound velocity makes the phonon-mediated attraction of electrons non-local in space providing unconventional Cooper pairs with a non-zero orbital momentum already in the framework of the conventional BCS theory with weak EPI. In the opposite limit of strong EPI rotational symmetry breaking appears as a result of a reduced Coulomb repulsion between unconventional bipolarons. Using the variational Monte Carlo method we have found that a relatively weak finite-range EPI induces a d-wave BCS state also in doped Mott–Hubbard insulators or strongly-correlated metals. These results tell us that poorly screened EPI with conventional phonons is responsible for the unconventional pairing in cuprate superconductors.
KeywordsElectron–phonon interaction Sound speed anisotropy Pairing symmetry Bipolarons Cuprates
PACS71.38.-k 74.40.+k 72.15.Jf 74.72.-h 74.25.Fy
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