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Samarium hexaboride (SmB$_6$), a Kondo insulator with mixed valence, hasrecently attracted much attention as a possible host for correlated topologicalsurface states. Here, we use a combination of x-ray absorption andreflectometry techniques, backed up with a theoretical model for the resonant$M_{4,5}$ absorption edge of Sm and photoemission data, to establish laterallyaveraged chemical and valence depth profiles at the surface of SmB$_6$.  We show that upon cleaving, the highly polar (001) surface of SmB$_6$undergoes substantial chemical and valence reconstruction, resulting in borontermination and a Sm$^{3+}$ dominated sub-surface region. Whereas at roomtemperature, the reconstruction occurs on a time scale of less than two hours,it takes about 24 hours below 50 K. The boron termination is eventuallyestablished, irrespective of the initial termination. Our findings reconcileearlier depth resolved photoemission and scanning tunneling spectroscopystudies performed at different temperatures and are important for bettercontrol of polarity and, as a consequence, surface states in this system.

Chemical and valence reconstruction at the surface of SmB6 revealed with resonant soft x-ray reflectometry