Topological Aspect and Quantum Magnetoresistance of $\beta$-Ag$_2$Te
  To explain the unusual non-saturating linear magnetoresistance observed in silver chalcogenides, the quantum scenario has been proposed based on the assumption of gapless linear energy spectrum. Here we show, by first principles calculations, that $\beta$-Ag$_2$Te with distorted anti-fluorite structure is in fact a topological insulator with gapless Dirac-type surface states. The characteristic feature of this new binary topological insulator is the highly anisotropic Dirac cone, in contrast to known examples, such as Bi$_2$Te$_3$ and Bi$_2$Se$_3$. The Fermi velocity varies an order of magnitude by rotating the crystal axis. 
Wei Zhang, Rui Yu, Wanxiang Feng, Yugui Yao, Hongming Weng, Xi Dai,   Zhong Fang
cond-mat.mtrl-sci