Emergence of a weak topological insulator from the Bi x Se y family

We report the emergence of a weak topological insulator (WTI), BiSe, of the Bi-chalcogenide family with an indirect bandgap of 42 meV. Its structural unit consists of a bismuth bilayer (Bi2), a known quantum spin hall insulator sandwiched between two units of Bi2Se3 which are three dimensional strong topological insulators. Our density functional theory calculations confirm the WTI phase and angle resolved photo-emission spectroscopy measurements carried out on cleaved single crystal flakes show Rashba states that closely agree with our theoretical predictions. Finally, we present a comparison between electronic and magneto-transport properties measured on single crystal flakes and thin films of BiSe.