Several Important Problems in YBaCuO and Its Doped Systems
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SEVERAL IMPORTANT PROBLEMS IN YBaCuO AND ITS DOPED SYSTEMS H.Zhang, S.Q.Feng, Q.R.Feng, X.Zhu and Z.Z.Gan Department of Physics, Peking University, Beijing 100871, P.R.China
Abstract The orthorhombic-tetragonal transition, relationship between oxygen content and Tc, electronic state of Cu and Tc, lattice energy and Tc in YBCO and its doped systems (Cu doped by Sn, Al, Fe, Zn, Ni; Ba by Sr, Ca; Y by Pr,Gd,Dy, respectively.) have been studied systematically. The experiments show that the 0-T transition may not be the predominant factor governing superconductivity; there is no regular relationship between superconductivity and the oxygen content. In order to fully understand superconductivity, other defects must be considered; the valence of Cu has a strong correlation with oxygen content, but has no direct relationship with the Tc value. The correlation among the electronic states of Ba, Cu and 0 are discovered, which is helpful to know the electfonic behavior of this material. Finally, we show that the lattice energy has a close relationship with the magnitude of Tc. As the lattice energy is decreased, the Tc is always depressed without any exception. The authors suggest that the lattice energy may be a probable predominant factor to superconductivity.
Three problems about the correlation between structure and property Since the identification of the orthorhombic 123 phase in YBa2Cu 3 07 _y super-conductorLL,2], much work has been done to reveal the origin of superconductivity from the crystallographic structure point of view. (1) Previous studies[ 3,4] have shown that as the structure experiences orthorhombic to tetragonal transition, the sample loses superconductivity; there is also a similar relationship among some substituted systems[5,6]. Therefore, the orthorhombic to tetragonal transition was considered to be one of the predominant factors governing superconductivity. (2) Since the orthorhombic to tetragonal transition occurs at y=0.5, and the variation of oxygen takes place in Cu-O chain along the b direction, so the oxygen content and Cu-O chain were also considered to be the predominant factors in superconductivity. (3) When y value is less than 0.5, Cu must appear as a partial Cu 3 + valence state, in order to be consistent with the need of charge balance. It was shown[ 3,4] that the higher the Cu 3+ /Cu 2 + ratio , the higher the Tc. Therefore it was suggested that the existence of Cu 3+ seem to predominate superconductivity. Many works have been done to demonstrate the importance of the factors suggested above. Here Mat. Res. Soc. Symp. Proc. Vol. 209. Q1991 Materials Research Society
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we will present our experimental results and compare these with the results in the literature.
Influence of orthorhombic-tetragonal transition on superconductivity Xiao et al[ 7] used Ga to substitute partially for Cu, and discovered that a small amount of doping Ga(6%) induces an orthorhombic to tetragonal transition, but the values of Tc are as high as 81K in the tetragonal phase. This result indicates that the
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