  Kondo insulating materials lie outside the usual dichotomy of weakly versus
correlated - band versus Mott - insulators. They are metallic at high
temperatures but resemble band insulators at low temperatures because of the
opening of an interaction induced band gap. The first discovered Kondo
insulator (KI) SmB$_6$ has been predicted to form a topological KI (TKI) which
mimics a topological insulator at low temperatures. However, since its
discovery thermodynamic and transport anomalies have been observed that have
defied a theoretical explanation. Enigmatic signatures of collective modes
inside the charge gap are seen in specific heat, thermal transport and quantum
oscillation experiments in strong magnetic fields. Here, we show that TKIs are
susceptible to the formation of excitons and magneto-excitons. These charge
neutral composite particles can account for long-standing anomalies in SmB$_6$
which is crucial for the identification of bulk topological signatures.

Strongly Correlated Electrons