Spin- and angle-resolved photoemission on the first topological Kondo Insulator: SmB$_{6}$*

The concept of a topological Kondo insulator (TKI) has been brought forward as a new class of topological insulators in which non-trivial surface states appear in the bulk Kondo band gap at low temperatures due to the strong spin-orbit coupling. Theoretical considerations have shown that SmB$_{6}$ is a promising candidate for the first realization of TKIs. In this contribution we will present comprehensive studies of the bulk and surface electronic structures of SmB$_{6}$. Using high-resolution angle-resolved photoemission spectroscopy (ARPES) we revealed that the two dimensional surface states reside within a bulk Kondo gap and form three Fermi surfaces in the surface Brillouin zone [1]. The odd number of surface bands crossing the Fermi level fulfills the necessary condition of topologically nontrivial surface states and is in good agreement with the theoretical prediction. Applying spin-resolved ARPES to SmB$_{6}$, we show that the energy bands of the surface states around the X bar points are spin-polarized. The spins of the surface states are locked to crystal momentum and the spin-helical structure fulfills the requirement of time-reversal symmetry [2]. Our results prove that SmB$_{6}$ is the first realization of strongly correlated topological Kondo insulator. We will also show the evolution of the bulk electronic structure from a metallic state at high temperatures to a Kondo insulating phase, and how the non-trivial states appear in the system [3]. \\[4pt] [1] N. Xu \textit{et al}., Surface and bulk electronic structure of the strongly correlated system SmB$_{6}$ and implications for a topological Kondo insulator, Phys. Rev. B \textbf{88}, 121102 (rapid, editor's suggestion) (2013)\\[0pt] [2] N. Xu \textit{et al}., Direct observation of the spin texture in SmB$_{6}$ as evidence of the topological Kondo insulator, Nature Comms. \textbf{5}, 4566 (2014).\\[0pt] [3] N. Xu \textit{et al}., Exotic Kondo cross-over in a wide tempersture region in the toplogical Kondo insulator SmB$_{6}$ revealed by high-resolution ARPES, \quad Phys. Rev. B \textbf{90}, 085148 (2014)