A Coherent Physics Picture of Topological Insulators at Single-Particle   Level
  The study of topological property of band insulators is an interesting branch of condensed matter physics. Two types of topologically nontrivial insulators have been extensively studied. The first type is characterized by a nonzero TKNN invariant or Chern number[1] which is directly related to the quantization of Hall conductance in the integer quantum Hall effect. Haledane propose a model with this type of band structure even in the absence of a macroscopic magnetic field[2]. We refer to such materials "Chern insulator". The second type called "Z2 topological insulators" is proposed recently[3, 4]. Quantum spin Hall effect has been predicted and observed in such systems.[5, 6]. Despite the recent intensively study there are still some fundamental problems that aren't quite clear about Z2 insulators even at the single-particle level. For example, it's claimed that Z2 insulators will return to its origin state after two cycles, thus coupling to the reservoirs is important for the Z2 insulators to continuously pump spin[7]. Theoretical and experimental results show quantum spin Hall effect is an edge state transport property of the materials and coupling to the reservoir seems not play an important role. So the Z2 picture is not satisfactory in explaining these phenomena. We study the relationship of the ground states of Z2 insulators and that of Chern insulator. Combined with the results of recent researches on polarization of Chern insulators[8] and topology of edge states[9] we propose a coherent physics picture of topological insulators. 
Yi-Dong Wu
cond-mat.mtrl-sci