Evidence of two distinct charge carriers in underdoped high Tc cuprates

We present results on heavily underdoped Y_1-xCa_xBa_2Cu_3O_6+y which provide the evidence that the doping mechanism (cation substitution or oxygen loading) directly determines whether the corresponding injected mobile holes contribute to superconductivity or only to the normal metallic properties. We argue that this hole tagging calls for a subtler description of the correlated bands than the usual one. We also map in great detail the underdoped superconducting phase diagram T_c vs. hole doping which shows that the number of mobile holes is not the critical parameter for the superconductivity.