  The cuprate superconductors distinguish themselves from the conventional
superconductors in that a small variation in the carrier doping can
significantly change the superconducting transition temperature (T_c), giving
rise to a superconducting dome where a pseudogap (ref. 1,2) emerges in the
underdoped region and a Fermi liquid appears in the overdoped region. Thus a
systematic study of the properties over a wide doping range is critical for
understanding the superconducting mechanism. Here, we report a new technique to
continuously dope the surface of Bi2Sr2CaCu2O8+x through an ozone/vacuum
annealing method. Using in-situ ARPES, we obtain precise quantities of energy
gaps and the coherent spectral weight over a wide range of doping. We discover
that the d-wave component of the quasiparticle gap is linearly proportional to
the Nernst temperature that is the onset of superconducting vortices (ref. 3),
strongly suggesting that the emergence of superconducting pairing is
concomitant with the onset of free vortices, with direct implications for the
onset of superconducting phase coherence at T_c and the nature of the pseudogap
phenomena.
