<i>T</i>_C-<i>x</i>Phase Diagrams and Infrared Spectra Due to Axial Charge-Transfer Modes in La_2-<i>x</i>Sr_xCuO₄: a Composite-Boson Model

Abstract

T C - x phase diagrams for La 2- x Sr x CuO 4 are derived from a model where in-plane holes dimerize to form bosons via the polarization-induced pairing aided by nearby apical oxygens O(A). O(A) are off-centered `a la Toyozawa' by coupling to the E u bending mode of dual O(A)–Cu(P)–O(A) bonds which enhances their polarizability. Apex oxygens are also gates through which charge leaks from CuO 2 planes across Sr-containing La–O layers to effect the interlayer coupling. The latter is mainly controlled by coupling to the A 2 u stretching mode of the dual bond. The Cu(P)–O(A) covalent bonding is doping-dependent. It modulates the electronic energies and thereby the axial charge leak. Dome-shaped T C - x diagrams result from the interplay between the increasing hole concentration and decreasing gate opening as x grows. They compare well with experimental T C - x data reported for La 2- x Sr x CuO 4 .