National High Magnetic Field Laboratory, Florida State University and the Center for Emergent Superconductivity Florida State University • University of Florida • Los Alamos National Laboratory

Samarium hexaboride (SmB6), a well-known Kondo insulator in which the insulating bulk arises from strong electron correlations, has recently attracted great attention owing to its possibly topological nature, thereby harboring protected surface states. Although there is strong evidence for this, corroborative spectroscopic evidence is still lacking; unlike in the weakly correlated counterpart, e.g., Bi2Se3. We report planar tunneling spectroscopy results obtained on the (001) and (011) crystal faces that reveal the linear density of states (DOS) as expected for Dirac cones. Our spectroscopic results also reveal the bulk gap, whose signature arises from spin excitons in the bulk interacting with the surface states. Above ~ 4 K, the thermal population of the spin excitons keeps the surface state from being protected.