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:

1. The charge carrier concentration measured by optical conductivity and Hall effect differs from the predictions of the non-interacting electronic band theory
2. The quasiparticles are not well defined (spectral functions measured by photo-emission are strongly broadened), except below $T_c$
3. Existence of "Pseudogap" regime, characterized by strong magnetic fluctuations and suppressed electronic density of states
4. Possibility of complex magnetically and charge ordered phases ("stripes")
5. 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.