Experimental verification of PbBi${}_2$Te${}_4$ as a 3D topological insulator

The first experimental evidence is presented of the topological insulator state in PbBi${}_2$Te${}_4$. A single surface Dirac cone is observed by angle-resolved photoemission spectroscopy (ARPES) with synchrotron radiation. Topological invariants ${\mathbb{Z}}_2$ 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.