  In 1928, P. Dirac proposed a new wave equation to describe relativistic
electrons. Shortly afterwards, O. Klein solved a simple potential step problem
for the Dirac equation and stumbled upon an apparent paradox - the potential
becomes transparent when the height is larger than the electron energy. For
massless particles, backscattering is completely forbidden in Klein tunneling,
leading to perfect transmission through any potential barrier. Recent advent of
condensed matter systems with Dirac-like excitations, such as graphene and
topological insulators (TIs), has opened the possibility of observing the Klein
tunneling experimentally. In the surface states of TIs, fermions are bound by
spin-momentum locking, and are thus immune to backscattering due to
time-reversal symmetry. Here we report the observation of perfect Andreev
reflection in point contact spectroscopy - a clear signature of Klein tunneling
and a manifestation of the underlying relativistic physics of a
proximity-induced superconducting state in a topological Kondo insulator.
