Suppression of the metal to semiconductor transition in bismuth cobaltates: Can cobaltates superconduct?

Single crystals of Bi₂M₃Co₂O_y phases with M=Ca, Sr, and Ba were synthesized by the flux method. These compounds are analogous to Bi-2212 superconducting systems with cobalt replacing copper. Due to its strong chemical and structural similarity with the Bi-2212 superconductors, this system provides a unique opportunity for determining which electronic features are essential for superconductivity in the cuprates. The undoped Ca- and Sr-analogues are semiconducting while the Ba-analogue has a metal to semiconductor transition at ≈80K. In order to change the carrier concentration of these phases, new Pb-substituted single crystals were prepared by a similar method. Resistivity measurements have shown that Pb-doping induces a clear increase in the metallic character of the samples for the Sr- and Ba-analogues. The metal to semiconductor transition in Bi₂Ba₃CO₂O_y, can be completely suppressed by Pb-doping, and (Bi,Pb)₂Ba₃Co₂O_y, is metallic down to 30mK. The magnetic susceptibility data did not show any evidence of ordering, and the magnetic moment/Co atom was found to be ≈1μB Resistivity measurements carried out up to 20 GPa have shown that the samples become more semiconducting with the increase of pressure.