| Summary: | Topological matter: magnetooptical characterization Opening a gap in a Dirac fermion system leads to the formation of a trivial or a non-trivial phase. A non-trivial phase exhibits conductive surface or edge states, and can be attributed to a non-zero topological index related to the parity of the conduction and valence bands. This index is usually inferred from the characterization of the edge or surface states through angle resolved photoemission spectroscopy or quantum spin Hall effect. Now, an international team led by Yves Guldner at Ecole Normale Supérieure reports that the effective velocity of bulk massive Dirac fermions depends on the topological index. Infrared magnetooptical Landau level spectroscopy allows to accurately determining this velocity and therefore to directly extract the topological index, as exemplified with the 3D topological insulators Pb1-x Sn x Se and Pb1-x Sn x Te.
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