X-ray photoemission spectroscopy has been used to investigate the depth dependent crystal structures and chemical compositions of sequentially chemical-etched YBa2Cu3O7-d (YBCO) ultrathin film superconductors. In the near-interface region the crystal structure is severely oxygen deficient and of tetragonal symmetry. We consider this a revelation of retarded oxygen diffusion into the O(1) sites during the post-deposition annealing in the presence of interface mismatch strain. Near the free surface, the oxygen-deficiency was much alleviated because of the partial strain relaxation and the crystal symmetry becomes orthorhombic. Compared with as-deposited films of equivalent thickness, which is less oxygen deficient and shows superconducting transition, the stripped-down near-interface layer exhibits no sign of superconductivity.
@article{arxiv.cond-mat/0603767,
title = {Depth evolution of YBa2Cu3O7-d ultrathin films probed by X-ray photoemission spectroscopy},
author = {H. W. Seo and Q. Y. Chen and Paul van der Heide and Wei-Kan Chu},
journal= {arXiv preprint arXiv:cond-mat/0603767},
year = {2007}
}