  We present the results of high-resolution valence-band photoemission
spectroscopic study of SmB6 which shows evidence for a V-shaped density of
states of surface origin within the bulk gap. The spectroscopy data is
interpreted in terms of the existence of heavy 4f surface states, which may be
useful in resolving the controversy concerning the disparate surface
Fermi-surface velocities observed in experiments. Most importantly, we find
that the temperature dependence of the valence-band spectrum indicates that a
small feature appears at a binding energy of about -9 meV at low temperatures.
We attribute this feature to a resonance caused by the spin-exciton scattering
in SmB6 which destroys the protection of surface states due to time-reversal
invariance and spin-momentum locking. The existence of a low-energy
spin-exciton may be responsible for the scattering which suppresses the
formation of coherent surface quasi-particles and the appearance of the
saturation of the resistivity to temperatures much lower than the coherence
temperature associated with the opening of the bulk gap.
