  The number of electronic bands is usually considered invariant regardless of
the electron density in a band picture. However, in interacting systems, the
spectral-weight distribution generally changes depending on the electron
density, and electronic states can even emerge or disappear as the electron
density changes. Here, to clarify how electronic states emerge and become
dominant as the electron density changes, the spectral function of the Hubbard
ladder with strong repulsion and strong intrarung hopping is studied using the
non-Abelian dynamical density-matrix renormalization-group method. A mode
emerging in the low-electron-density limit gains spectral weight as the
electron density increases and governs the dimer Mott physics at
quarter-filling. In contrast, the antibonding band, which is dominant in the
low-electron-density regime, loses spectral weight and disappears at the Mott
transition at half-filling, exhibiting the momentum-shifted magnetic dispersion
relation in the small-doping limit. This paper identifies the origin of the
electronic states responsible for the Mott transition and brings a new
perspective to electronic bands by revealing the overall nature of electronic
states over a wide energy and electron-density regime.
