  Samarium hexaboride (SmB6), which lies in the mixed valence regime in the
Anderson model, has been predicted to possess topologically protected surface
states. The intensive investigations on SmB6 have brought up the long standing
questions about the discrepancy between the surface and bulk electronic
properties in rare-earth compounds in general. Here, we investigate and
eventually clarify this discrepancy in the particular case of SmB6 by the
photoemission core-level spectra. We focus on the change in both Sm and B
states depending on time, temperature, probing depth, and surface termination
on the cleaved (100) surface. Our spectra show that the unusual time-dependent
change in the Sm valence occurs within a period of hours, which is not related
to the adsorption of residual gases. Moreover, we observe a reduction of the
surface feature in the B and Sm states on the same timescale accompanied by the
formation of a subsurface region. Thus, it indicates the relatively slow charge
redistribution between the surface and subsurface regions. Our findings
demonstrate that the f states is strongly involved in the surface relaxation.
