Jungiusstr. 9, Hörsaal II
SFB 668 - Kolloquium
Prof. T. Dahm (Universität Bielefeld):
Topological Insulators and Ferromagnets: appearance of flat surface bands
The new class of materials called "topological insulators" is characterized by an insulating bulk, but conducting surface states. These surface states are topologically protected by time-reversal symmetry and thus cannot be destroyed by perturbations that respect time-reversal symmetry, like nonmagnetic impurity scattering.
In this talk we study how these surface states are modified, if a time-reversal breaking ferromagnetic exchange field is applied to the topological insulator. Such an exchange field can be introduced either by doping with ferromagnetic dopants or by proximity to a ferromagnetic material. We show that ferromagnetism can "tune" the velocity of the surface electrons. A certain critical strength of the exchange field leads to a phase transition into a topological phase, in which the surface states become flat bands, i.e. the velocity of the electrons vanishes. We discuss the necessary preconditions for such flat bands to appear and the topological invariants guaranteeing the existence of these flat bands.
Using known parameters of Bi2Se3, we show that there is a good chance that this phase can be achieved experimentally in thin Bi2Se3 strips covered by a ferromagnetic thin film.