▎ 摘　　要

Coulomb interaction is of central importance in localized energy levels (bound states) or electronic flat bands and can result in many exotic quantum phases. In a 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), in circular graphene quantum dots (GQDs). Here we show that Coulomb interaction still plays a dominating role in determining the electronic properties of the temporarily confined quasibound states with the lifetime (trapping time) of similar to 10 fs. Our scanning tunneling microscopy and spectroscopy 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 linearly 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.