  Unravelling the nature of doping-induced transition between a Mott insulator
and a weakly correlated metal is crucial to understanding novel emergent phases
in strongly correlated materials. For this purpose, we study the evolution of
spectral properties upon doping Mott insulating states, by utilizing the
cluster perturbation theory on the Hubbard and t-J-like models. Specifically, a
quasi-free dispersion crossing the Fermi level develops with small doping, and
it eventually evolves into the most dominant feature at high doping levels.
Although this dispersion is related to the free electron hopping, our study
shows that this spectral feature is in fact influenced inherently by both
electron-electron correlation and spin exchange interaction: the correlation
destroys coherence, while the coupling between spin and mobile charge restores
it in the photoemission spectrum. Due to the persistent impact of correlations
and spin physics, the onset of gaps or the high-energy anomaly in the spectral
functions can be expected in doped Mott insulators.
