Microstructures in Y-Ba-Cu-O Thin Films Investigated by XAFS Techniques
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MICROSTRUCTURES IN Y - Ba - Cu -0 THIN FILMS INVESTIGATED BY XAFS TECHNIQUES A. KROL*, Z. H. MING, C. S. LIN, Y. L. SO%, C. X. GU, Y. H. KAO*, E. NARUMI*-, D. T. SHAW", AND G. C. SMITH... *Department of Physics, State University of New York at Buffalo, Buffalo, NY 14260 York State Institute on Superconductivity, State University of New York at Buffalo, Buffalo,
"**New
NY 14260 ***Brookhaven National Laboratory, Upton, NY 11973
ABSTRACT Superconducting films of YBa 2 Cu 3 0- 6 obtained by laser ablation on MgO and ZrO 2 substrates were investigated by soft x-ray absorption fine structure technique (XAFS) at 0 K-edge. Local environment around oxygen atoms was probed and information on the distribution of the nearestneighbors was obtained. Much higher degree of local disorder than that in a bulk YBa 2 Cu 3 O 7 . 6 sample was observed. Superconducting films of YBa 2 Cu 3 0 7 6 (with 6=0.1 nominally) about 200 nm thick were obtained by laser ablation on MgO and Zr) 2 (YSZ) substrates. Detail for sample preparation was reported elsewhere [1]. Magnetization and transport data indicated the existence of superconductive transition at temperature above 77 K in three investigated samples. For comparison one nonsuperconducting film obtained by annealing in vacuum after ablation process was also investigated. X-ray diffraction showed that in these films the c axis was perpendicular to the surface with only small number of disoriented grains. Absorption spectra were obtained around the 0 K-edge by means of detection of 0 Ka soft-x-ray fluorescence, see Fig. 1.
3
t. M
2
492
53
574
615 656 697
738
779 820
86i 902
Photon energy (eV)
Fig. 1. X-ray absorption spectrum obtained for YBa 2 Cu 3 07. 6 film on MgO substrate.
Mat. Res. Soc. Symp. Proc. Vol. 209. 01991 Materials Research Society
874
In the fluorescence process 0 Ka photons in the superconductors investigated have a typical escape depth of the order of 200 nm. For this reason the fluorescence technique is really bulk sensitive and obtained spectra are free from spurious features caused by surface contamination and oxygen-depletion layers [2]. Experiment was performed at room temperature, with the electric field vector of x-rays perpendicular to the c-axis, at U15 beamline at NSLS (Brookhaven National Laboratory). A low pressure parallel-plate avalanche chamber in single-photon counting mode was used to measure the fluorescence yield [3]. The modulus of Fourier transform (not corrected for the phase shift) of the XAFS spectrum obtained with a YBa2Cu307A film on MgO substrate is shown in Fig. 2. Very similar spectra were obtained for all investigated films.
il 1.0
S6 -
9 8 7 5
4 3
2 I 0 7
6
8
10
R (A) Fig. 2. The modulus of Fourier transform of the x-ray absorption spectrum obtained for YBa 2 Cu 3 O. film on MgO substrate shown in Fig. 1.
6
Only one strong and broad peak is observed with maximum at - 1.4 A-' and a shoulder at - 2.1 AP. By using well characterized bulk YBa 2 Cu 3 0 7 .6 superconductor sample as a model compound we can ascribe this
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