  Topologically protected chiral states at a mass-inverted quantum dot in
graphene are studied by analyzing both tight-binding and kernal polynomial
method calculations. The mass-inverted quantum dot is introduced by considering
a heterojunction between two different mass domains, which is similar to the
domain wall in bilayer graphene. The numerical results show emergent metallic
channels across the mass gap when the signs of the mass terms are opposite. The
eigenstates of the metallic channels are revealed to be doubly
degenerate---each state propagates along opposite directions, maintaining the
time-reversal symmetry of graphene. The robustness of the metallic channels is
further examined, concluding with the fact that chiral states are secured
unless atomic vacancies form near the domain wall. Such chiral states
circulating along the topological defects may pave a novel route to engineering
topological states based on graphene.
