The experimental evidence is presented of the topological insulator state in PbBi 2Te 4. A single surface Dirac cone is observed by angle-resolved photoemission spectroscopy with synchrotron radiation. Topological invariants Z 2 are calculated from the ab initio band structure to be 1;(111). The observed two-dimensional isoenergy 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. © 2012 American Physical Society.

This work was financially supported by KAKENHI (Grants No. 20340092 and No. 23340105), Grant-in-Aid for Scientific Research (B) of JSPS. We also acknowledge partial support by the Department of Education of the Basque Country Government, the University of the Basque Country (project GV-UPV/EHU, Grant No. IT-366-07), Ministerio de Ciencia e Inovación (Grant No. FIS2010-19609-C02-00).

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