A topological insulator is an insulator that does not conduct electricity inside a substance, but exhibits metallic behavior on the surface.A completely new phenomenon has been predicted and discovered by adding the function of a magnet to this topological insulator.One is called the "anomaly Hall effect", which produces a frictionless current along the edge of the sample without an external magnetic field.The other is the "topological electromagnetic effect," which is a phenomenon in which magnetization is generated by an electric field and electric polarization is generated by a magnetic field.And although an unconventional effect was predicted for "antiferromagnetic topological insulators", such a substance was undiscovered.

　The research team predicts that MnBi2Te4 (manganese, bismuth, tellurium) can be an antiferromagnetic topological insulator by advanced theoretical calculations.Next, when the single crystal sample was prepared and its characteristics were investigated, it was found to be a layered antiferromagnet (a substance with zero magnetization due to the reverse arrangement of the magnetic moments of adjacent atoms).The sample was tested by angle-resolved photoelectron spectroscopy at Hiroshima University, and the state of Dirac electrons (electrons with zero mass) with a large energy gap on the surface was directly observed.This succeeded in realizing the world's first antiferromagnetic topological insulator.

　The discovery of antiferromagnetic topological insulators is of great significance.The observation and research of topological electromagnetic effects involving the undiscovered elementary particle "axion" will be accelerated.Using the discovered antiferromagnetic topological insulator, it is expected that various new phenomena will be discovered by Axion.

Paper information:[Nature] Prediction and observation of an antiferromagnetic topological in