  Many-body localization (MBL) is currently a hot issue of interacting systems,
in which quantum mechanics overcomes thermalization of statistical mechanics.
Like Anderson localization of non-interacting electrons, disorders are usually
crucial in engineering the quantum interference in MBL. For translation
invariant systems, however, the breakdown of eigenstate thermalization
hypothesis due to a \emph{pure} many-body quantum effect is still unclear. Here
we demonstrate a possible MBL phenomenon without disorder, which emerges in a
lightly doped Hubbard model with very strong interaction. By means of density
matrix renormalization group numerical calculation on a two-leg ladder, we show
that whereas a single hole can induce a very heavy Nagaoka polaron, two or more
holes will form bound pair/droplets which are all localized excitations with
flat bands at low energy densities. Consequently, MBL eigenstates of finite
energy density can be constructed as composed of these localized droplets
spatially separated. We further identify the underlying mechanism for this MBL
as due to a novel `Berry phase' of the doped Mott insulator, and show that by
turning off this Berry phase either by increasing the anisotropy of the model
or by hand, an eigenstate transition from the MBL to a conventional
quasiparticle phase can be realized.
