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3D insulators can be topologically characterized by the value of their bulk axion field.Axion fields introduce additional terms in Maxwell’s equations for condensed-matter systems.The microscopic expression for the axion field in a crystal is given by the non-Abelian Chern–Simons integral, which depends on the Berry connection matrix of the band structure.In strong 3D topological insulators, a half-quantized surface Hall effect appears when the surface states are gapped, together with linear magnetoelectric coupling in their bulk.The axion insulator state can be realized in antiferromagnetic insulators without external fields.Materials with a non-trivial axion field can be used in dark-matter detectors and non-reciprocal thermal emitters.3D insulators can be topologically characterized by the value of their bulk axion field.Axion fields introduce additional terms in Maxwell’s equations for condensed-matter systems.The microscopic expression for the axion field in a crystal is given by the non-Abelian Chern–Simons integral, which depends on the Berry connection matrix of the band structure.In strong 3D topological insulators, a half-quantized surface Hall effect appears when the surface states are gapped, together with linear magnetoelectric coupling in their bulk.The axion insulator state can be realized in antiferromagnetic insulators without external fields.Materials with a non-trivial axion field can be used in dark-matter detectors and non-reciprocal thermal emitters. |
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