Pre-pairing and polarization of doped charge carriers in unconventional superconductors

Abstract

Based on first principles calculations, we show in hole-doped La2-xSrxCuO4, the Sr dopants prefer to form dimers, rather than be isolated as generally believed. Same trend applies to other superconductors such as Na-doped Ca2CuO2Cl2, and electron type Nd2-xCexCuO4. The dimer structure makes the two doped holes (electrons) bound up and form a singlet which is also polarized. Another type of dopant in cuprates is oxygen which behaves similarly. So, we propose that, for all oxide high TC superconductors, the holes (electrons) are pre-paired (Z-pairs) during the preparation of the superconductor. The spontaneously formed Z-pairs are conceptually different from not only the Cooper pairs, but also the Zhang-Rice singlets. The attraction force in a Z-pair originates basically from the strong antiferromagnetic exchange energy. We also found that the on-site Coulomb repulsion (U) is no longer a constant due to doping. Around a doping site, U can reduce its value discontinuously, which plays a key role in stabilizing the Z-pairs against isolated holes (electrons). A Z-pair is a p-wave like localized state which breaks the symmetry of the CuO2 plain and attracts each other along the direction perpendicular to the polarization. The Z-pairs may be popular in all of the unconventional superconductors and the periodic assembly can result in the emergence of coherence states and superconductivity.