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

Single crystals of Bi2M4Co2Oy 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 far 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 approximate to 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 Bi2Ba3CO2Oy can be completely suppressed by Pb-doping, and (Bi,Pb)(2)Ba]Co2Oy is metallic dawn to 30mK. The magnetic susceptibility data did not show any evidence of ordering, and the magnetic moment/Co atom was found to be approximate to1 muB. Resistivity measurements carried out up to 20 GPa have shown that the samples become more semiconducting with the increase of pressure.