Topological Aspect and Quantum Magnetoresistance of β-Ag₂Te

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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 β-Ag₂Te 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 Bi₂Te₃ and Bi₂Se₃. The Fermi velocity varies an order of magnitude by rotating the crystal axis.