Dimensionality of localization in nonsuperconducting ${\mathrm{Bi}}_{2+\mathit{x}}$${\mathrm{Sr}}_{2\mathrm{-}\mathit{y}}$${\mathrm{CuO}}_{6+\mathrm{\delta }}$ crystals

Transport, magnetoconductivity, and tunneling were measured in nonsuperconducting ${\mathrm{Bi}}_{2+\mathrm{x}}$${\mathrm{Sr}}_{2\mathrm{-}\mathrm{y}}$${\mathrm{CuO}}_{6\pm \mathrm{\delta }}$ single crystals (x,y≤0.2), which show two-dimensional localization within the ${\mathrm{CuO}}_2$ planes. The weakening of disorder in the planes is found to be by itself insufficient for superconductivity. Negative magnetoconductivity, evidence for the onset of superconductive pairing, is observed when transport perpendicular to the planes indicates extended states in three dimensions.