  We use novel STM techniques in concert to study the doping dependence of
electronic structure in Bi-2212. At all dopings, the low energy states are
relatively homogenous except for dispersive density-of-states modulations whose
properties are used to elucidate the momentum-space characteristics of
quasi-particles. The superconductive coherence-peaks, ubiquitous in
near-optimal tunneling spectra, are destroyed with strong underdoping. A new
spectral type, likely characteristic of the zero temperature pseudogap regime,
appears in these samples. Exclusively in regions exhibiting this new spectrum,
we find a quasi periodic modulations in dI/dV as well as in topograph with
Q(2pi/4.5a,0) and (0,2pi/4.5a)+-15%. This is consistent with the existence of a
local charge density modulation at these wave vectors. Surprisingly, this state
coexists harmoniously with the low energy nodal quasi-particles. We discuss the
relevance of these findings to the cuprate phase diagram and to the
relationship between the pseudogap and superconductivity.
