  In order to understand the origin of high-temperature superconductivity in
copper oxides, we must understand the normal state from which it emerges. Here,
we examine the evolution of the normal state electronic excitations with
temperature and carrier concentration in Bi2Sr2CaCu2O8 using angle-resolved
photoemission. In contrast to conventional superconductors, where there is a
single temperature scale Tc separating the normal from the superconducting
state, the high- temperature superconductors exhibit two additional temperature
scales. One is the pseudogap scale T*, below which electronic excitations
exhibit an energy gap. The second is the coherence scale Tcoh, below which
sharp spectral features appear due to increased lifetime of the excitations. We
find that T* and Tcoh are strongly doping dependent and cross each other near
optimal doping. Thus the highest superconducting Tc emerges from an unusual
normal state that is characterized by coherent excitations with an energy gap.

Superconductivity