The first experimental evidence is presented of the topological insulator
state in PbBi2Te4. A single surface Dirac cone is observed by
angle-resolved photoemission spectroscopy (ARPES) with synchrotron radiation.
Topological invariants Z2 are calculated from the {\it ab initio}
band structure to be 1; (111). The observed two-dimensional iso-energy contours
in the bulk energy gap are found to be the largest among the known
three-dimensional topological insulators. This opens a pathway to achieving a
sufficiently large spin current density in future spintronic devices.Comment: 5 pages, 5 figures, accepted for publication in Phys. Rev. Let