Twenty years of extensive research have yet to produce a general consensus on the origin of high temperature superconductivity. Recently, mounting evidence, e.g. from the in-plane isotope shift studies and other experiments has led us to a re-examination of the role of phonons. In a novel approach , we find that the interaction responsible for pairing in cuprates depends on the stability of the Cu-O-Cu bond in the CuO 2 plane. The new model is based on the observation that the charge carrier motion along the in-plane Cu-O-Cu bond must be non-linearly modulated by the oxygen vibrational degrees of freedom, enabling d-wave pairing mediated by an anharmonic two-(local) phonon process. This fluctuating bond pairing mechanism, decoupled from the large on-site Coulomb repulsion, is qualitatively different from the conventional one-phonon BCS pairing interaction. Our model provides a natural explanation of several salient features of high-temperature superconductors, including the recently observed electron-pockets and nanoscale inhomogeneity.
 D.M. Newns and C.C. Tsuei, Nature Physics 3 , 184 (2007).