Discovery of \^C$_2$ rotation anomaly in topological crystalline insulator SrPb

TL;DR

This paper reports the experimental discovery of a C_2 rotation anomaly in the topological crystalline insulator SrPb, confirmed by first-principles calculations and ARPES measurements, expanding the understanding of topological phases protected by crystalline symmetries.

Contribution

It presents the first experimental realization of the C_2 rotation anomaly in a binary compound, demonstrating two Dirac surface states protected by symmetry.

Findings

Identification of two Dirac surface states in SrPb

Confirmation of C_2 rotation anomaly via ARPES

Enrichment of topological phase classification

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.