Surface structure of SmB$_6$ investigated by STM and HAXPES*

The intermediate-valence compound SmB$_6$ is typically considered a ``Kondo insulator'' albeit the concept of the Kondo effect does, in principle, not hold for an intermediate-valence material. Nonetheless, the hybridization between conduction electrons and the strongly interacting Sm $f$-electrons results in a gap at the Fermi energy and hence, an insulating ground state arises at temperatures below about 40 K. Recently, SmB$_6$ has become of enormous topical interest because it is a candidate material for hosting topologically protected surface states. The intermediate valence of Sm in SmB$_6$ was confirmed by HAXPES measurements down to 5 K. Such measurements conducted at high photon energies for improved probing depth are of importance in view of valency limits given for strong topological insulators [1]. Scanning tunneling microscopy (STM) has the unique capability of providing combined topographic and spectroscopic information. We conducted STM on numerous samples cleaved {\it in situ} at around 20 K [2]. Cleavage along the \{001\} plane of the cubic structure through breaking inter-octahedral B-B bonds gives rise to polar surfaces. In result, we found disordered chain-like as well as ordered ($2 \times 1$) surface reconstructions. Occasionally, we also observed patches of non-reconstructed surface areas of both, Sm and B termination. On such areas, we found indications for the Kondo effect being at play. Also, for non-reconstructed surface areas of some ten nanometers in size the d$I$/d$V$-curves can be well described by a Fano resonance. Thus, the hybridization picture typically considered for this material could be fully confirmed. All types of surfaces, reconstructed and non-reconstructed, displayed a finite zero-bias conductance of considerable magnitude. This finding, in spite of different surface topologies, confirms the robustness of the metallic states and is in line with the proposal of SmB$_6$ being a topological insulator.\\[4pt] [1] V. Alexandrov {\it et al.}, Phys. Rev. Lett. {\bf 111} (2013) 226403.\\[0pt] [2] S. R\"o\ss ler {\it et al.}, Proc. Natl. Acad. Sci. USA {\bf 111} (2014) 4798.