We present the results on heavily underdoped ${\text{Y}}_{1-x}{\text{Ca}}_x{\text{Ba}}_2{\text{Cu}}_3{\text{O}}_{6+y}$, 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.

• Received 7 May 2008

DOI:https://doi.org/10.1103/PhysRevB.77.224511