Origin and roles of a strong electron-phonon interaction in cuprate oxide superconductors

Abstract

A strong electron-phonon interaction arises from the modulation of the superexchange interaction by lattice vibrations. It is responsible for the softening of the half-breathing modes around (±π∕a,0) and (0,±π∕a) in the two-dimensional Brillouin zone, with a being the lattice constant of CuO2 planes, as is studied in Phys. Rev. B 70, 184514 (2004). Provided that antiferromagnetic spin fluctuations are developed around Q=(±3π∕4a,±π∕a) and (±π∕a,±3π∕4a), the electron-phonon interaction can also cause the softening of Cu−O bond-stretching modes around 2Q, or around (±π∕2a,0) and (0,±π∕2a). The softening around 2Q is accompanied by the development of charge fluctuations corresponding to the so called 4a-period stripe or (4a×4a)-period checkerboard state. However, an observation that the 4a-period modulating part or the 2Q part of the density of states is almost symmetric with respect to the chemical potential contradicts a scenario in which the stabilization of a single-2Q or double-2Q charge-density wave following the complete softening of the 2Q bond stretching modes is responsible for the ordered stripe or checkerboard state. It is proposed that the stripe or checkerboard state is simply a single-Q or double-Q spin-density wave, whose second-harmonic effects can explain the observed almost symmetric 2Q part of the density of states. The strong electron-phonon interaction can play no or only a minor role in the occurrence of dγ-wave superconductivity in cuprate oxides.

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