Non-adiabatic electron-lattice interactions in the copper-oxide superconductors

Abstract

Transport measurements distinguish the superconductive phase from the parent antiferromagnetic and the overdoped metallic phases in La_2−xSr_xCuO₄. For small x, holes in the CuO₂ sheets form non-adiabatic polarons containing, experimentally, five Cu atoms. Within a polaron, a first-order contraction of the mean Cu-O bond length collapses the Hubbard U and transfers spectral weight from the Hubbard bands to the gap between them. In the superconductive phase above T_c, condensation into a polaron liquid produces an ɛ(k) dispersion curve, and dynamic pseudo Jahn-Teller deformations within the polaron give a Seebeck enhancement with T_max≈140 K. The polarons order pair-wise into 〈110〉 stripes alternating with parent-phase stripes; below T_c, the stripes are perfectly ordered. In the overdoped phase, the parent phase is squeezed out; the vibronic states become homogeneous.