Rotation Anomaly and Topological Crystalline Insulators

We show that in the presence of n-fold rotation symmetries and time-reversal symmetry, the number of fermion flavors must be a multiple of $2n$ ($n=2,3,4,6$) on two-dimensional lattices, a stronger version of the well-known fermion doubling theorem in the presence of only time-reversal symmetry. The violation of the multiplication theorems indicates anomalies, and may only occur on the surface of new classes of topological crystalline insulators. Put on a cylinder, these states have n Dirac cones on the top and on the bottom surfaces, connected by n helical edge modes on the side surface.