Narrow Carrier Concentration Range of Superconductivity and Critical Point of Pseudogap Formation Temperature in Pb-Substituted Bi2Sr2CuO6+δ

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We have determined the precise $T$–$ p$ phase diagram of Pb-substituted Bi2Sr2CuO6+δ by using the hole concentration $ p$ estimated from the Hall coefficient, and found that the superconducting region of Pb-substituted Bi2Sr2CuO6+δ is rather narrow as compared with other Bi-based high-$T_{\text{c}}$ cuprates. In Bi-based single-layer high-$T_{\text{c}}$ cuprates, the width of the superconducting region becomes more narrow as the maximum $T_{\text{c}}$ ($T_{\text{c,max}}$) gets smaller. This is different from other high-$T_{\text{c}}$ superconductors, in which $T_{\text{c}}/T_{\text{c,max}}$ is described by a universal function of $ p$. Furthermore, we have found that the $ p$ dependence of the pseudogap formation temperature $T^{*}$ projects to zero at the critical point $ p_{\text{cr}} \sim 0.205--0.21$, which is outside the superconducting region of Pb-substituted Bi2Sr2CuO6+δ. Taking into account that $ p_{\text{cr}}$ of high-$T_{\text{c}}$ cuprates is commonly located around $ p \sim 0.19--0.21$, it is concluded that whether $ p_{\text{cr}}$ is inside or outside the superconducting region simply depends on the width of the superconducting region.
2009-08-15