  In relativistic physics, both atomic collapse in heavy nucleus and Hawking
radiation in black hole are predicted to occur through Klein tunneling process
that couples particles and antiparticles. Recently, atomic collapse states
(ACSs) were explicitly realized in graphene because of its relativistic Dirac
excitation with large fine structure constant. However, essential role of the
Klein tunneling on the ACSs remains elusive in experiment. Here we
systematically study the quasibound states in elliptical graphene quantum dots
(GQDs). Bonding and antibonding molecular collapse states formed by two coupled
ACSs are observed in the elliptical GQDs. Our experiments, supported by
theoretical calculations, indicate that the antibonding state of the ACSs will
change into a Klein-tunneling-induced quasibound state, revealing deep
connection between the ACSs and the Klein tunneling.
