Science http://doi.org/bvbj (2016)
Bismuth has been a central element in designing materials with topologically protected electronic states, largely due to its strong spin–orbit coupling. Despite this, the electronic topology of pure bismuth itself has remained somewhat controversial. This is partly due to its very small band-gap, which, along with some other features, makes it quite difficult to conclusively determine the topological nature of the electronic states.
By probing thin films of different thicknesses, exploiting quantum confinement effects to distinguish the surface and bulk electronic structures, Suguru Ito and co-workers believe that they have unambiguously proved that pure bismuth is topologically nontrivial. But just as one mystery of bismuth was being solved, a second emerged.
Due to a combination of factors, such as the low carrier density, pure crystals of bismuth were not expected to exhibit a superconducting state under ambient pressure. But experiments by Om Prakash and colleagues provide evidence that they can indeed become superconducting when cooled to low enough temperatures. As the adiabatic approximation of conventional Bardeen–Cooper–Schrieffer theory does not hold true for bismuth, the physics of this material seems to be far from trivial.
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Fleet, L. Not a trivial matter. Nature Phys 13, 8 (2017). https://doi.org/10.1038/nphys4014