Synopsis

# Surface Protection

Physics 6, s28
Surface spectroscopy shows that a material long known as a Kondo insulator also exhibits the metallic surface states of a topological insulator.

Single localized spins in a material can enlist nearby electrons to form a protective shield. This “Kondo interaction” causes an increase in electron scattering from the isolated spins at low temperature due to a hybridization of localized and conducting electrons. If the localized spins are arrayed in a periodic lattice, this effect can result in the transformation of a metal at room temperature into a Kondo insulator at low temperatures. Recent predictions suggest that one of the canonical Kondo insulators—samarium boride (SmB $6$)—might also show signs of another effect that has reached celebrity status in condensed matter: topologically protected surface states. In a paper in Physical Review X, Xiaohang Zhang of the University of Maryland, College Park, and colleagues report experiments showing that (SmB $6$) indeed shows conducting behavior at surfaces, much like that expected from topological insulators.

Protected surface-conducting states occur in topological insulators because the electrons’ spin and momentum are locked together, making backscattering only possible with an energetically unfavorable spin flip. Zhang et al. use point-contact spectroscopy to track the evolution of the bulk electron energy states in (SmB $6$) as a function of temperature. They find that below $100$ K, the spectra show the beginnings of the Kondo interaction between samarium ions and the electrons, while around $30$ K the formation of the insulating Kondo gap from inter-ion interactions appears. Below $10$ K, according to the authors, the bulk material remains in this condition, whereas the observed low-temperature conductivity can only be explained by the formation of metallic surface states.

The experimental discovery of predicted surface states shows the emergence of some intriguing exotic properties of (SmB $6$). What is needed next is an investigation of the detailed nature of its topologically protected surface conduction. – David Voss

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