  In a magnetic field few-layer graphene supports, at the lowest Landau level,
a multiplet of zero-mode levels nearly degenerate in orbitals as well as in
spins and valleys. Those pseudo-zero-mode (PZM) levels are generally sensitive
to interactions and external perturbations, and have a crossing among
themselves or with other higher Landau levels when an external field is swept
over a certain range. A close study is made of how such PZM levels evolve when
they are gradually brought from empty to filled levels under many-body
interactions. It is pointed out that the level spectra generally avoid a
crossing via orbital level mixing and that orbital mixing is governed by a
non-Abelian Berry phase that derives from an approximate degeneracy and
interactions. A look is also taken into evolution/crossing of many-body ground
states with increasing external bias in bilayer graphene.

Mesoscale and Nanoscale Physics