Fig. 1: Temperature-vs-doping phase diagram of the superconducting cuprates.

In the red region encompassed between the pseudogap temperature T^* and the upturn temperature T_up of the resistance, above the superconducting critical temperature T_c, in particular close to the optimally doped regime (e.g. at hole doping p ≈ 0.17), these compounds display a strange-metal behaviour. This is revealed in the experimental resistance R data by the presence of a linear temperature dependence, displayed as a red thick solid line in the R(T) curves above the phase diagram. In the underdoped regime (e.g. at p ≈ 0.11), below T^* (blue region) a downturn from the linear-in-T resistance is observed, since additional mechanisms lead to deviations from the strange-metal regime. In the overdoped regime (e.g. at p ≈ 0.21), below T_up (yellow region) the upturn from the linear-in-T resistance is due to the setting in of the Fermi-liquid regime. Recent Resonant X-Ray Scattering experiments¹² showed that also the charge order phenomenon is widespread in the phase diagram. In particular, short-ranged dynamical charge density fluctuations (sketched by red waves highlighted in the red circle, and observed in the striped area) populate the strange-metal region, while in the underdoped region, below the onset temperature T_CDW, they coexist with the usual longer-ranged charge density waves (sketched by blue waves in the blue circle, and observed in the wavy area). T_N is the Néel temperature. The data of the R(T) curves are taken from Refs. ^12,40.