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

Underdoped high-T-c superconductors are frequently characterised by a tempe rature, T*, below which the normal-state pseudogap opens. Two different "ph ase diagrams" based on the doping (p) dependence of T* are currently consid ered: one where T* falls to zero at a critical doping state and the other w here T* merges with T-c in the overdoped region. By examining the temperatu re dependence of the NMR Knight shift and relaxation rate, entropy, resisti vity, infrared conductivity, Raman scattering, ARPES and tunnelling data, i t is concluded that the second scenario is not at all supported. Neither ca n 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 re served.