Comparison of local structure measurements from c-axis polarized XAFS between a film and a single crystal of YBCO as a function of temperature

TL;DR

This study compares local structure measurements from c-axis polarized XAFS in YBCO films and single crystals, revealing differences in axial oxygen distribution and temperature-dependent bond correlations near Tc.

Contribution

It provides the first direct comparison of local structural features between YBCO films and single crystals using polarized XAFS, highlighting the absence of axial oxygen splitting in films.

Findings

No evidence of axial oxygen splitting in YBCO films.

Temperature-dependent bond broadening shows unique behavior near Tc.

O(4) position becomes less correlated with Cu near Tc.

Abstract

We have performed fluorescence x-ray absorption fine-structure (XAFS) measurements from 20-200 K on a 5000 \AA, c-axis film of YBa_2Cu_3O_{7-\delta} (YBCO) on MgO (Tc=89 K) using photons polarized perpendicular to the film. These data are compared to YBCO data from a single crystal and from a film on LaAlO_3 with the same Tc. The main difference between the single crystal and the film data is that while the single crystal data is well described by a two-site axial oxygen [O(4)] distribution, we see no evidence for such a distribution in either thin film sample. We place an upper limit on the size ofan axial oxygen splitting for the film on MgO at < 0.09 \AA. Therefore, the magnitude of the splitting is not directly related to Tc. Fits to the temperature dependent data from the YBCO film on MgO indicate that all bonds show a smooth change of their broadening factor \sigma, except the Cu-O(4) bonds, which show an increase in \sigma in the vicinity of Tc, followed by a decrease of the same magnitude. Such a feature does not occur in diffraction measurements. Since XAFS measurements of \sigma include any correlation between the atoms in a given bond, we conclude that the O(4) position becomes less correlated with the Cu positions near Tc. Correlation measurements of these and several further near-neighbors are also reported.