An equivalent expression of Z2 Topological Invariant for band insulators   using Non-Abelian Berry's connection
  We introduce a new expression for the Z2 topological invariant of band insulators using non- Abelian Berry's connection. Our expression can identify the topological nature of a general band insulator without any of the gauge fixing problems that plague the concrete implementation of previous invariants. The new expression can be derived from the "partner switching" of the Wannier function center during time reversal pumping and is thus equivalent to the Z2 topological invariant proposed by Kane and Mele. 
Rui Yu, Xiao Liang Qi, Andrei Bernevig, Zhong Fang and Xi Dai
cond-mat.mtrl-sci