Hole states inCuO2planes and Cu-O chains ofYBa2

Order and disorder in the ensemble of Cu-O chain fragments in oxygen-deficient planes ofYBa2Cu3<

“…The function of the CF ensemble to control the YBCO behavior is closely connected with the charge transfer which could be conditionally subdivided into two parts: the intrachain charge redistribution and that one from chains onto CuO 2 planes. The former is the oxydation of monovalent Cu ions, forming the CFs, to the bivalent state According to this process almost all oxygen atoms in not very short Cu-O CFs would be subjected to transformation into the monovalent state where it not for the hole transfer to outside: Around 30% of oxygen atoms in chains persist their monovalency, the rest oxygen constituents appear to be in the bivalent state [19]. It is noteworthy, that the distribution of oxygen valencies is neither frozen, nor ordered in any charge density superstructure, but due to strong p-d hybridization oxygen holes form strongly correlated systems of finite lengths as discussed in [20].…”

“…A short resumé of the mechanism proposed in [20] is as follows: Existence of the CF, involving n+1 Cu 2+ and n O atoms, m of which are O 2− and n−m atoms are O − , is consistent with m−1 holes transferred from the fragment. In the limit of very long CFs (n ≫ 1), ie, the case investigated in Ref [19], the free energy per one oxygen site of the fragment, f (n, m), depends practically of m/n and has a minimum around ξ ≈ 0.70. It allows to represent the free energy in the following form:…”

“…Nevertheless, the lattice-gas approach is reasonable for the orthorhombic side of the phase diagram and it can be phenomenologically generalized to incorporate the fermionic degrees of freedom into the effective interaction of oxygen atoms within CFs. Generalization is based on the result obtained in the soft X-ray absorption spectroscopy investigation of untwinned YBa 2 Cu 3 O 7 and reported in [19]: The concentration of oxygen holes in the Cu-O chains has been estimated around 30%. This experimental estimate is essential for applying the charge transfer mechanism discussed theoretically in [20] and partly based on the experimental investigations [21,22] of the monovalent Cu amount in YBa 2 Cu 3 O 6+x .…”

Oxygen nonstoichiometry and valence of copper in the Cu-1222 superconductor

“…(In the case of the HPO samples, O K-edge data were not considered due to Ag oxide inclusions in these samples in not welldefined amounts.) The pre-edge peak seen for all the samples about $528.1 eV is the well-known signature of hole states in the p-type doped CuO 2 planes in high-T c superconductors [20,[25][26][27][28]31,[34][35][36]. We can distinguish another pre-edge peak at $527.3 eV partly overlapping with this peak.…”

A two-phase charge-density real-space-pairing model of high-Tc superconductivity

“…Con®rmation that the charge carriers in the normal state are holes is provided by Hall-effect measurements, and in the superconducting state the charge carriers are hole pairs (Gough et al, 1987). Calculations (Temmerman et al, 1988;Oles & Grzelka, 1991) show that it requires less energy to create a hole on an oxygen site (so that its valency is effectively O À ) than on a copper site, and experiments (Krol et al, 1992;Nu È cker et al, 1996;Alloul et al, 1989) con®rm that the holes are on oxygen sites and that they occur not only in the CuO 2 planes but on some other oxygen sites throughout the structure.…”