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)—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) 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