The Nobel Prize in Physics was awarded for the discovery of the topological properties of solids in 2016. Our current research was focused on study to one of the first discovered topological insulators Bi2Se3. A material that acts as an insulator inside but contains free electrons on the surface.
In order to study the manifestation of these electrons it was necessary to realize the measurement of electrical properties on a very thin nano-meter layer. The structure where the conductance quantum was measured was prepared by electron beam lithography using a scanning electron microscope.
The significant manifestations of “topology” of the material is that the electron states are topologically protected, which in simplicity means that if the electron in the material encounters by non-magnetic impurity, 180 degrees backscattering is forbidden. As a result, the material exhibits greater electrical conductivity. If there is a magnetic impurity in the topological material, this property is lost. This is also associated with the loss of linearity of the dispersion spectrum and the creation of an energy gap Δ. The aim of our work was to test the existing theoretical prediction for the change of the κ parameter in the external magnetic field. This parameter provides information how rapidly the electrical conductivity decreases with decreasing temperature at low temperatures. Our experiments have shown that the existing theoretical prediction well describes the behavior of pure Bi2Se3, where the κ changes by +0.5.
However, it contradicts our observations in Bi2Se3 doped with magnetic manganese. At first glance, the opening of the energy gap brings the correct change of the parameter κ about -0.5. Unfortunately, the energy gap Δ presented in our system is not large enough to cause that big change. There is a conflict with the existing theory and another phenomenon must be responsible for κ changing by -0.5.
The result of our work is the discovery of a fundamental gap in our understanding of magnetically doped topological insulators. A possible explanation for the change in the sign κ by -0.5 is the presence of the ferromagnetic order of manganese magnetic moments. The results of our study were published in the prestigious American Physical Society journal [V. Tkáč et al., Physical Review Letters 123 (2019) 036406] with impact factor of 8.839.
DOI: https://doi.org/10.1103/PhysRevLett.123.036406
