# Colloquium: Probing Topology with Magnetism: A View from the Surface

Date:

10/13/2017 - 3:30pm to 4:30pm

Location:

CP 155

Speaker(s) / Presenter(s):

Professor Herb Fertig (Indiana University)

Type of Event (for grouping events):

Topological crystalline insulators (TCI's) are a class of materials which support non-trivial topology in their electronic structure, "protected" by an underlying crystal symmetry. We will discuss how certain aspects of this topology can be uncovered at surfaces of such crystals when they are magnetically doped, and show that the system hosts a surprisingly rich variety of ferromagnetic

states and phase transitions. After discussing how band topology can be characterized, and the unavoidable presence of conducting surface states even as the bulk is insulating when the topology is non-trivial, we examine the impact of magnetic dopants on the surface states. By breaking the symmetry that protects the topology, the system can become ferromagnetic, but the number of degenerate groundstates is dependent on both the symmetry of the surface as well as the density of electrons there. Moreover, the same surface may support a very "stiff" ferromagnet or a rather "floppy" one. The nature of the ferromagnet realized is in principle externally controllable, and for different cases it disorders at finite temperature through phase transitions of different universality classes. The type of system realized for a specific set of circumstances can be probed via the unique properties of domain walls which appear when the system is thermally excited.

states and phase transitions. After discussing how band topology can be characterized, and the unavoidable presence of conducting surface states even as the bulk is insulating when the topology is non-trivial, we examine the impact of magnetic dopants on the surface states. By breaking the symmetry that protects the topology, the system can become ferromagnetic, but the number of degenerate groundstates is dependent on both the symmetry of the surface as well as the density of electrons there. Moreover, the same surface may support a very "stiff" ferromagnet or a rather "floppy" one. The nature of the ferromagnet realized is in principle externally controllable, and for different cases it disorders at finite temperature through phase transitions of different universality classes. The type of system realized for a specific set of circumstances can be probed via the unique properties of domain walls which appear when the system is thermally excited.