Appearance of Universal Metallic Dispersion in a Doped Mott Insulator

Under strong electronic correlations the parent compounds of all cuprate high-temperature superconductors assume the so-called Mott-Hubbard insulating state. By what routes these insulators accomplish the miraculous transformation into superconductors with the addition of electrons or holes is a question of intense current interest. In this study we consider the classic superconductor La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) over the wide doping range of $x=0.03-0.30$, delineating how the electronic spectrum evolves with doping for binding energies extending to several hundred meV's. Our analysis indicates that this Mott insulator contains `nascent' or `preformed' metallic states, which develop finite spectral weight with doping, but otherwise undergo relatively little change in dispersion over a wide doping range. Our findings challenge existing theoretical scenarios for cuprates. Work supported in part by the USDOE.