  Coulomb interaction is of central importance in localized energy levels
(bound states) or electronic flat bands and could result in many exotic quantum
phases, such as magnetic, superconducting, and topological phases in graphene
systems1-14. In graphene monolayer, the relativistic massless Dirac fermion
nature of the charge carriers enables us to realize unprecedented quasibound
states, which are trapped temporarily via whispering-gallery modes (WGMs) with
the lifetime (trapping time) of ~ 10 fs, in circular graphene quantum dots
(GQDs)15-20. Here we show that Coulomb interaction still plays a dominating
role in determining the electronic properties of the temporarily-confined
quasibound states. Our scanning tunneling microscopy (STM) and spectroscopy
(STS) measurements demonstrate that the discrete quasibound state in a GQD will
split into two peaks when it is partially filled. The energy separation of the
two split peaks increases linear with inverse effective radius of the GQDs,
indicating that the splitting arises from the Coulomb interaction. Moreover, we
show that the real space distribution of the two split states separates in
different regions of the GQD to reduce the Coulomb interaction, leading to the
breaking of the WGM of the quasibound states.
