  Coupled quantum dots (QDs), usually referred to as artificial molecules, are
important not only in exploring fundamental physics of coupled quantum objects,
but also in realizing advanced QD devices. However, previous studies have been
limited to artificial molecules with nonrelativistic fermions. Here, we show
that relativistic artificial molecules can be realized when two circular
graphene QDs are coupled to each other. Using scanning tunneling microscopy
(STM) and spectroscopy (STS), we observe the formation of bonding and
antibonding states of the relativistic artificial molecule and directly
visualize these states of the two coupled graphene QDs. The formation of the
relativistic molecular states strongly alters distributions of massless Dirac
fermions confined in the graphene QDs. Because of the relativistic nature of
the molecular states, our experiment demonstrates that the degeneracy of
different angular-momentum states in the relativistic artificial molecule can
be further lifted by external magnetic fields. Then, both the bonding and
antibonding states are split into two peaks.
