FIGURE 1. Phase diagram of high-temperature superconductors.

a, Schematic doping dependence of the antiferromagnetic (T_N) and superconducting (T_c) transition temperatures and the pseudogap crossover temperature T* in YBCO. The vertical lines at p = 0.1 and p = 0.25 mark the positions of copper oxide materials discussed in the text: ortho-II ordered YBa₂Cu₃O_6.5 and Na-CCOC, located well into the underdoped region, and Tl-2201, well into the overdoped region, respectively. b, c, Distribution of ARPES spectral intensity in one quadrant of the Brillouin zone, measured (b), on Na-CCOC at p = 0.1, and (c), on Tl-2201 at p = 0.25 (reproduced from ref. 6 and ref. 4, with permissions from K. M. Shen and A. Damascelli, respectively). These respectively reveal a truncated Fermi surface made of 'Fermi arcs' at p = 0.10, and a large, roughly cylindrical and continuous Fermi surface at p = 0.25. The red ellipse in b encloses an area A_k that corresponds to the frequency F of quantum oscillations measured in YBCO.