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Bulk single crystals of superconducting YBCO (YBa 2 Cu 3 O 7 ), grown with incorporation of different additives like BaF 2 , B 2 O 3 and Bi 2 O 3 , have been investigated to gain an insight into the problem of combating T c degradation due to aluminium contamination from the crucible material. Electron probe microanalysis (EPMA) along with T c values of the above crystals, shows that, with BaF 2 introduced as additive, the contamination due to aluminium is significantly reduced, which improves crystal growth and superconductivity. The crystals have been subjected to high resolution imaging of planes and chains using STM, while the ensuing changes in the conductance spectra of chains due to Al contamination were studied by scanning tunnelling spectroscopy (STS). The contamination of CuO chains is corroborated by chains becoming quasi-insulating on account of substitutional disorder, and also by the increase in their surface roughness at the atomic level. The effect of additives is discussed and the observed T c variation is linked with the relative changes in the adulteration and disordering of CuO chains. The results are in general accord with the role of interlayer coupling between CuO 2 planes and other effects that are known to control the T c of layered cuprates. |
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