Correlations Between the Cu-O Bonding States and the Superconductivity - an Xps Investigation
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CORRELATIONS BETWEEN THE Cu-O BONDING STATES AND THE SUPERCONDUCTIVITY - AN XPS INVESTIGATION S.C.Han*.**, D.Z.Liu***, X.M.Xie*, Z.L.Wu*, and G.C.Huth** * Shanghai Institute of Metallurgy, Chinese Academy of Sciences, Shanghai 200050, China Xsirius Superconductivity, Inc., 7590 E. Gray Road, Suite 103, Scottsdale, AZ 85260 Shanghai Institute of Testing Technology, Shanghai, China
ABSTRACT Auger electron spectroscopy (AES) and core-level x-ray photoelectron spectroscopy (XPS) have been used to study the compositional and electronic-state variations from the contaminated surface layer to the inner region of YBa 2Cu 30x and Bi2 (Sr,Ca)n+lCunO 2n,4 compounds. The results showed that the carbon-rich contamination layer in BSCCO is thin and easier to be than that of YBCO. This stable surface removed by Ar÷ sputtering, indicating a much more 2 9 layer is oxygen deficient and contains higher Cu + satellites ( 2,23d final states) than in the bulk materials. Line-shape analysis suggests three-Gaussian features for both Cu 2p3/2 and 0 ls lines. The 529 eV signal is observed in both YBCO and BSCCO 0 Is spectra.
INTRODUCTION It is generally acknowledged that the novel high-T, superconductivity discovered by Bednorz and Miller 1 is unlikely due to the conventional BCS electron-phonon coupling. In the recent three years, several non-BCS pairing mechanisms 2-5 have been proposed and being evaluated by various experiments. While photoemission spectroscopy is considered one of the most direct tools for studying the electronic structure and providing evidence for the establishment of theory of superconductivity, 6-11 it has been hampered by the difficulty in the preparation of high-quality surfaces in these copper oxides. The surfaces of these compounds are not stable. They react easily with the atmosphere with a carbon-rich contamination layer formed on the surface. This is very undesirable if one wants to learn something about the electronic states of the bulk materials from the spectroscopic results. Nevertheless, recent progress has shown that reliable information about the electronic states could be obtained from at 20 K photoemission spectroscopy of (Eu,Y)Ba 2 Cu 3 O7 single crystals cleaved in situ 12 13 which found evidence of both strong O 2p and Cu 3Jcharacters at the Fermi edge. In this work, we used core-level x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) to probe the compositional and electronic-state variations from.the contaminated surface layer to the inner region of the copper oxides. We systematically compared various samples of YBa2 Cu 30x (x=6.5-6.9), and Bi 2 (Sr,Ca)n+lCunO 2n+4 (n=2-3). Our results confirm that the surfaces of BSCCO compounds are much more stable than that of YBCO.
EXPERIMENTAL
"TheYBCO and BSCCO samples were prepared by the standard solid-state reaction method. The electrical resistance-temperature (R-T) transitions of BSCCO and YBCO samples YB2 are shown in figs.1 and 2. BSCCO sample A was essentially of the 85K superconducting phase and sample G of the 105K
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