Charge fluctuations and an ionic–covalent transition in La2–xSrxCuO4

Abstract

The search for the mechanism responsible for high-T_c superconductivity in the copper oxides begins with a consideration of their distinctive structures, normal-state properties, and superconductive properties. All have intergrowth structures, and some consequences of this situation are enumerated. The normal-state properties are discussed with reference to the La_2–xSr_xCuO₄ system in particular. Transport data are presented that provide evidence for a transition from small polarons to itinerant electrons with increasing x and, in the range of compositions 0.035 ≲x≲ 0.125 where this transition occurs, the onset of dynamic charge fluctuations in the normal state below a T_ρ > T_c; T_ρ exhibits a maximum value near 150 K in a narrow interval about x≈ 0.075. These charge fluctuations appear to separate domains containing antiferromagnetic spin fluctuations from domains containing weakly correlated electrons within which there could be a formation of disordered large bipolarons. X-Ray photoelectron spectroscopy (XPS) data are cited and interpreted in terms of a transition from more ionic to more covalent Cu–O bonding on passing from the antiferromagnetic to the superconductive compositions. A critical temperature T_c proportional to the hole concentration p≈x in the oxidized CuO₂ sheets is interpreted to be a consequence of a small superconductive coherence length and a two-dimensional conduction band. A change in the character of the condensation to the superconductive state in the interval 0.12 < x < 0.15 takes place; in the range x≤ 0.12, condensation occurs from a state containing dynamic charge fluctuation within which large bipolarons may be formed, whereas for x≥ 0.15 condensation of Cooper pairs occurs directly from the normal state. It is noted that these observations fulfil two important conditions for our ‘correlation bag’ model of superconductivity to be viable: (1) stabilization of charge fluctuations by co-operative, dynamic atomic displacements and (2) an on-site electron–electron Coulomb repulsion that decreases sensitively with increased screening via an increase in the covalent character of the Cu–O bond with increasing hole concentration and decreasing mean Cu–O bond length.