  The massless Dirac fermions and the ease to introduce spatial and magnetic
confinement in graphene provide us unprecedented opportunity to explore
confined relativistic matter in this condensed-matter system. Here we report
the interplay between the confinement induced by external electric fields and
magnetic fields of the massless Dirac fermions in graphene. When the magnetic
length lB is larger than the characteristic length of the confined electric
potential lV, the spatial confinement dominates and a relatively small critical
magnetic field splits the spatial-confinement-induced atomic-like shell states
by switching on a pi Berry phase of the quasiparticles. When the lB becomes
smaller than the lV, the transition from spatial confinement to magnetic
confinement occurs and the atomic-like shell states condense into Landau levels
(LLs) of the Fock-Darwin states in graphene. Our experiment demonstrates that
the spatial confinement dramatically changes the energy spacing between the LLs
and generates large electron-hole asymmetry of the energy spacing between the
LLs. These results shed light on puzzling observations in previous experiments,
which hitherto remained unaddressed.
