Polarization dependence of EXAFS and XANES spectra for superconducting Y-Ba-Cu-O films

Abstract

XANES and EXAFS x-ray absorption spectroscopy is widely used in new ceramics showing high-temperature superconductivity [1-6]. XANES provides conclusions on the charge states of the atoms, while EXAFS not only enables one to determine interatomic distances and coordination numbers but also to examine the temperature dependence of the Debye-Waller factors for various atomic pairs [1-3]. The XANES data represent certain difficulties in analysis, and conflicting conclusions are sometimes drawn from the same data. For example, in [4, 5], the CuK-XANES spectra were used to identify the presence of Cu(3+) in YBa2Cu30?_6, while in [6], a mixture of Cu(2+) and Cu(l+) was deduced. There are also ambiguities in analyzing the EXAFS data. It has been shown [i] that the Cu-O pairs in the ab planes in YBa2Cus07_ 6 give a weak temperature dependence for the Debye-q4aller factor (i.e., the bond is the strongest in the system) without any anomalies. On the other hand, in [2], there was an anomalous increase in the Debye-Waller factor below T C for the Cu-O pairs in which the oxygen atoms lie between the CuO= layers and the CuO chains. In [3], an anomalous increase was also found for analogous pairs of atoms, but at 170 K. Those data were used in the latter two papers to discuss the role of the soft made in the high-temperature superconductivity mechanism. There are difficulties in analyzing the temperature dependence of the DebyeWaller factor because there is a set of similar Cu-O distances in the YBa2Cu3Ov_ 6 system, which are not resolved in the radial atomic distribution. The Debye-Waller factors have been determined for the various Cu-O pairs by fitting with a large number of parameters, which introduces a certain ambiguity. Some problems can be avoided in analyzing the temperature dependence of the Debye-Waller factors for the Y-Ba-Cu-O system by the use of the EXAFS polarization dependence. When the polarization vector is parallel to the axis, only the oxygen atoms that link the CuO 2 layers and CuO chains appear in the radial atomic distribution. The polarization dependence in XANES has been determined for copper atoms in flat square complexes and has been analyzed in [8]. There are two types of copper atom in YBa2CU~07_6: those in Cu02 layers and those in CuO chains. The copper atoms in the chains have an oxygen-atom environment similar to a planar square (at high oxygen contents); those in the layers have a pyramidal environment, which can be considered as a planar square one since the fifth vertex oxygen atom is at a considerable distance from the copper atom [12]. The more informative polarization form of CuI<-XANES can be analyzed on thebasis of the available data [8] and calculations [7, 9].