  Topological crystalline insulators (TCIs) are insulating electronic states
with nontrivial topology protected by crystalline symmetries. Recently, theory
has proposed new classes of TCIs protected by rotation symmetries \^C$_n$,
which have surface rotation anomaly evading the fermion doubling theorem, i.e.
n instead of 2n Dirac cones on the surface preserving the rotation symmetry.
Here, we report the first realization of the \^C$_2$ rotation anomaly in a
binary compound SrPb. Our first-principles calculations reveal two massless
Dirac fermions protected by the combination of time-reversal symmetry \^T and
\^C$_{2y}$ on the (010) surface. Using angle-resolved photoemission
spectroscopy, we identify two Dirac surface states inside the bulk band gap of
SrPb, confirming the \^C$_2$ rotation anomaly in the new classes of TCIs. The
findings enrich the classification of topological phases, which pave the way
for exploring exotic behaviour of the new classes of TCIs.
