Chiral edge mode for single-cone Dirac fermions

Confinement of 2D massless Dirac fermions by a mass boundary does not produce an edge mode. This applies to the surface of a 3D topological insulator at an interface to a magnetic insulator with a perpendicular magnetization. Surprisingly enough, an edge mode does appear if the magnetization is tilted away from the normal. The edge mode is an ``arc state'', ranging over a finite momentum interval, as a 2D analogue of the surface Fermi arc familiar from 3D Weyl semimetals.