Role of Cu-O chains in the charge transfer mechanism in YBa 2Cu 3O 6+ x

Abstract

We present in this work a numerical analysis of energetically preferable configurations of -Cu-O- chains, which appears in oxygen deficient Cu(1) planes of YBa 2Cu 3O 6+ x . The “reduced” energies of finite chain fragments, corresponding to their fermion degrees of freedom, are analyzed. The main intra-chain contributions are Kondo-like and Coulomb interactions. Using this approach we explain: (1) that the oxygen vacancies are occupied randomly at small x leading to predominance of very short Cu 2+-O 2--Cu 2+ fragments; (2) at 0.7 > x > 0.55 oxygen-poor chains are filled with oxygen atoms mainly incorporated into short fragments; (3) at x increasing from ≈0.4 to ≈0.55 the -Cu-O- chain fragments increase their lengths significantly; (4) at x≈0.8 and above, oxygen ions are arranged in long chain fragments whereas oxygen vacancies within the chains are few and distributed randomly. The hole transfer into CuO 2 layers is strongly connected with the mechanism of oxygen filling in Cu(1) planes. In addition to the quantum mechanics of chain fragments we describe their ensemble by means of the traditional methods of statistical physics. The calculations of the transferred carrier concentration as well as the monovalent copper amount versus x are presented in figures which look similar to the experimental ones.