Topological insulators (TIs) are bulk insulating materials that nonetheless exhibit metallic conductivity on their surfaces. This conductivity is guaranteed by the bulk band structure's topology—the surface features these states as long as the symmetry defining the topological index remains the same.
In so-called strong TIs, these states are protected and so featured on all surfaces. In weak TIs however, these properties are only protected at surfaces with a certain orientation. Stacking two-dimensional TIs, that is QSHIs, to form a three-dimensional crystal, for example, generally produces a weak TI with no protected states on the top or bottom surfaces of the crystal: there are metallic surface states inherited from the edge states of the 2-D TI, but also an insulating surface plane that lies normal to the stacking direction.
Recent theoretical work, also carried out by MARVEL researchers, suggested however that this might not be the case for stacked, or bulk, jacutingaite. The research suggested a more complicated scenario—the material may be a topological crystalline insulator (TCI) as well as a weak TI. In TCIs, the topology is defined by symmetry with respect to a mirror plane and metallic surface states can be found on surfaces perpendicular to it. This state might be expected in the material because of it threefold mirror symmetry. Jacutingaite also maintains translational symmetry in the stacking of the layers though, meaning that it might also feature the properties of a weak TI. Until now, however, there have been no experimental results on the bulk band structure.
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