Universal properties of cuprate superconductors: Evidence and   Implications
  To provide an understanding of the universal thermodynamic properties of cuprate superconductors, emerging from the empirical correlations and phase diagrams, we analyze them in terms of the scaling theory of finite temperature and quantum critical phenomena. The universal features are traced back to the existence of quantum critical lines, representing the end lines of the finite temperature transition surface. At the respective quantum critical lines two dimensional superconductor to insulator (2D-QSI) transitions and three dimensional superconductor to normal state (3D-QSN) transitions occur. The flow to this quantum critical points is tuned by doping, substitution and anisotropy. It is shown that the empirical correlations, like the dependence of $T_{c}$on dopant and substitution concentration, the dependence of $T_{c}$ on zero temperature in-plane penetration depth, etc., as well as the anomalous isotope effect on $T_{c}$ and penetration depth, reflect the universal properties associated with the flow to these quantum critical points and of the crossover from one to the other. 
T. Schneider