Topological materials – materials whose surface properties are very different to those in their bulk – have come to the fore in recent years and are currently revolutionizing modern condensed matter physics thanks to their unique properties.
Topological phases of matter are so-called because they are described by global invariants that are not at all affected by imperfections, such as defects or other variations, in a material. Mathematically, these invariants are constructed as integrals of a local property over a closed parameter space. And although they show much promise for use in a host of applications, including error-resistant spintronics and quantum computation, they have been considered as global, and not local, quantities thus far.
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