Discovery of Ĉ$_2$ rotation anomaly in topological crystalline insulator SrPb

Abstract: 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.