Spin-Momentum Locked Topological Surface States, non-trivial Berry's   phase and magnetoelectric quantization in topological insulators
  The surface of a 3D topological insulator is a new type of two dimensional electron gas (2DEG). The exactly quantized magneto-optical or axionic response of a 3D topological insulator is a direct result of the Berry's phase protected spin-momentum locked states on its surface. (PRL Viewpoint http://physics.aps.org/node/3810). 
M. Zahid Hasan
cond-mat.mes-hall cond-mat.quant-gas cond-mat.str-el