Emergence of a coherent in-gap state in SmB$_{6}$ Kondo insulator revealed by scanning tunneling spectroscopy

SmB$_{6}$ is a Kondo insulator that exhibits transport anomalies at temperatures below 5 K, where resistivity saturates instead of diverging. It has long been ascribed to in-gap states which become coherent at low temperatures. Recently, a host of theoretical and experimental studies suggest that SmB$_{6}$ may be a topological Kondo insulator with topological protected metallic surface states. In this talk we present STM studies of the (001) surface of cleaved SmB$_{6}$ single crystal. We have observed four different kinds of surface morphologies with similar \textit{dI/dV} spectra. Variable temperature \textit{dI/dV} spectroscopy up to 60 K reveals a gap-like density of state suppression around the Fermi level, which is due to the hybridization between the itinerant Sm 5$d$ band and localized Sm 4$f$ band. At temperatures below 40 K, a sharp coherence peak emerges within the hybridization gap near the lower gap edge. We propose that the in-gap resonance state is due to a collective excitation in magnetic origin that is specific to the mixed valent Kondo insulator. Implications of these results to the electronic structure evolution and transport anomaly in SmB$_{6}$ will be discussed.