Valence Characterization of Surface and Subsurface Region in SmB6
  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. 
P. Lutz, M. Thees, T. R. F. Peixoto, B. Y. Kang, B. K. Cho, Chul-Hee   Min, and F. Reinert
cond-mat.str-el