Normal state properties
Introducing holes into the parent compound, one finds experimentally that the antiferromagnetic order is destroyed by a small density, followed by the development of superconductivity, with the maximum superconducting transition temperature, $T_c$, occurring for $x\approx 0.16$.
The normal (to be specific -- non-antiferromagnetic and non-superconducting) state properties deviate strongly from the conventional behaviors of simple metals and semiconductors.
Below optimal doping:
- The charge carrier concentration measured by optical conductivity and Hall effect differs from the predictions of the non-interacting electronic band theory
- The quasiparticles are not well defined (spectral functions measured by photo-emission are strongly broadened), except below $T_c$
- Existence of "Pseudogap" regime, characterized by strong magnetic fluctuations and suppressed electronic density of states
- Possibility of complex magnetically and charge ordered phases ("stripes")
- broad temperature range of linear-T behavior of resistivity
In contrast, for larger carrier concentrations, and in particular above the optimal doping, the applicability of conventional band theory seems to improve.