  Samarium hexaboride (SmB$_6$), a well-known Kondo insulator in which the
insulating bulk arises from strong electron correlations, has recently
attracted great attention owing to increasing evidence for its topological
nature, thereby harboring protected surface states. However, corroborative
spectroscopic evidence is still lacking, unlike in the weakly correlated
counterparts, including Bi$_2$Se$_3$. Here we report results from planar
tunneling that unveil the detailed spectroscopic properties of SmB$_6$. The
tunneling conductance obtained on the (001) and (011) single crystal surfaces
reveal linear density of states as expected for two and one Dirac cone(s),
respectively. Quite remarkably, it is found that these topological states are
not protected completely within the bulk hybridization gap. A phenomenological
model of the tunneling process invoking interaction of the surface states with
bulk excitations (spin excitons), as predicted by a recent theory, provides a
consistent explanation for all of the observed features. Our spectroscopic
study supports and explains the proposed picture of the incompletely protected
surface states in this topological Kondo insulator SmB$_6$.
