The doping dependence of T* - what is the real high-T-c phase diagram?

Underdoped high-T-c superconductors are frequently characterised by a temperature, T*, below which the normal-state pseudogap opens. Two different phase diagrams based on the doping (p) dependence of T* are currently considered: one where T* falls to zero at a critical doping state and the other where T* merges with T-c in the overdoped region. By examining the temperature dependence of the NMR Knight shift and relaxation rate, entropy, resistivity, infrared conductivity, Raman scattering, ARPES and tunnelling data, it is concluded that the second scenario is not at all supported. Neither can one distinguish a small and a large pseudogap as is often done. T* is an energy scale, which falls abruptly to zero at p = 0.19. Colour figures are available at cond-mat/0005063. (C) 2001 Elsevier Science B.V. All rights reserved.