  The discovery of strong topological insulators led to enormous activity in
condensed matter physics and the discovery of new types of topological
materials. Bisumth based chalcogenides are exemplary strong three dimensional
topological insulators that host an odd number of massless Dirac fermionic
states on all surfaces. A departure from this notion is the idea of a weak
topological insulator, wherein only certain surface terminations host surface
states characterized by an even number of Dirac cones leading to exciting new
physics. Experimentally however, weak topological insulators have proven to be
elusive. Here, we report a discovery of a weak topological insulator (WTI),
BiSe, of the Bi-chalcogenide family with an indirect band gap of 42 meV. Its
structural unit consists of bismuth bilayer (Bi$_2$), a known quantum spin hall
insulator sandwiched between two units of Bi$_2$Se$_3$ which are three
dimensional strong topological insulators. Angle resolved photo-emission
spectroscopy (ARPES) measurements on cleaved single crystal flakes along with
density fucntional theory (DFT) calculations confirm the existence of weak
topological insulating state of BiSe. Additionally, we have carried out
magneto-transport measurements on single crystal flakes as well as thin films
of BiSe, which exhibit clear signatures of weak anti-localization at low
temperatures, consistent with the properties of topological insulators.
