Narrow Carrier Concentration Range of Superconductivity and Critical Point of Pseudogap Formation Temperature in Pb-Substituted Bi<SUB>2</SUB>Sr<SUB>2</SUB>CuO<SUB>6+δ</SUB>

We have determined the precise <I>T</I>–<I>p</I> phase diagram of Pb-substituted Bi<SUB>2</SUB>Sr<SUB>2</SUB>CuO<SUB>6+δ</SUB> by using the hole concentration <I>p</I> estimated from the Hall coefficient, and found that the superconducting region of Pb-substituted Bi<SUB>2</SUB>Sr<SUB>2</SUB>CuO<SUB>6+δ</SUB> is rather narrow as compared with other Bi-based high-<I>T</I><SUB>c</SUB> cuprates. In Bi-based single-layer high-<I>T</I><SUB>c</SUB> cuprates, the width of the superconducting region becomes more narrow as the maximum <I>T</I><SUB>c</SUB> (<I>T</I><SUB>c,max</SUB>) gets smaller. This is different from other high-<I>T</I><SUB>c</SUB> superconductors, in which <I>T</I><SUB>c</SUB>⁄<I>T</I><SUB>c,max</SUB> is described by a universal function of <I>p</I>. Furthermore, we have found that the <I>p</I> dependence of the pseudogap formation temperature <I>T</I><SUP>*</SUP> projects to zero at the critical point <I>p</I><SUB>cr</SUB>∼0.205–0.21, which is outside the superconducting region of Pb-substituted Bi<SUB>2</SUB>Sr<SUB>2</SUB>CuO<SUB>6+δ</SUB>. Taking into account that <I>p</I><SUB>cr</SUB> of high-<I>T</I><SUB>c</SUB> cuprates is commonly located around <I>p</I>∼0.19–0.21, it is concluded that whether <I>p</I><SUB>cr</SUB> is inside or outside the superconducting region simply depends on the width of the superconducting region.