Linked Temperature Evolution of the bulk gap and helical topological surface states in SmB$_{6}$*

The paradigm mixedvalent insulator SmB$_{6}$ with a temperature dependent bulk gap has recently become the first paradigm example of a strongly correlated topological insulator with f-d band inversion and with experimental evidences for in-gap surface states and surface transport. In this work temperature- and polarization-dependent angle-resolved photoemission on cleaved \textless 100\textgreater surfaces of SmB$_{6}$ quantifies the T-evolution of (i) the Sm 4f state coherence, (ii) the X-point f-conduction band energy and many-body gap destabilization, and (iii) the intimately connected fate of topologically protected in-gap states. DFT and DFT$+$DMFT calculations confirm early theory [1] that hybridization between boron 2p and Sm 4f states provides crucial assistance in the full opening of the many-body f-d gap. Also a dimensional crossover above 100K from 3D bulk d-band states crossing E$_{\mathrm{F}}$ at high T to low T 2D in-gap surface states is shown to coincide with the development of a circular dichroism signature of in-gap state helicity. \\[4pt] [1] R.M. Martin and J. W. Allen, J. Appl. Phys. 50, 7561 (1979).