Evidence of two distinct charge carriers in underdoped high T_c cuprates

We present the results on heavily underdoped Y(1-x)Ca(x)Ba(2)Cu(3)O(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 high-temperature transport. We argue that this hole tagging is a signature of the complexities of single-hole doping in Mott insulators, and it 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 total number of mobile holes is not the driving parameter for superconductivity.