UNC-CH PHYSICS AND ASTRONOMY COLLOQUIUM
Babak Seradjeh, University of Indiana
“Dynamical Generation and Detection of Topological Phases of Matter”
A gapped topological phase of matter is characterized by a nontrivial twist in its electronic energy bands that cannot be untied without closing an energy gap. When a material with twisted energy bands faces a material with untwisted energy bands (such as the regular vacuum) something interesting must happen at the interface: there, electrons move with no apparent energy gap, just as in a conductor. A hallmark of a topological phase is to host such topological bound states. As understood recently, topological phases may be generated in an otherwise normal material by a time-periodic driving force. These phases are necessarily dynamical: they can only occur when the system is driven out of equilibrium.
In this talk, I present our recent work to answer two main questions: (1) What are the promising experimental schemes of realizing and detecting driven topological phases? (2) What are the fundamentally new device applications of these topological phases? For example, a two-dimensional system of Dirac fermions, such as graphene, irradiated by a circularly polarized laser realizes a phase known as Floquet topological insulator. I present an effective theory of the Floquet topological insulator in irradiated graphene and use it to study its transport signature. Remarkably, we find that disorder can enhance transport in some cases by several orders of magnitude. I also show how such a system can be used to create a new type of “valley” transistor. A Floquet topological superconductor, on the other hand, is characterized by dynamical generation of Majorana fermion excitations. I present a proposal to realize these excitations in tunable, coupled, and driven quantum dots. Finally, I discuss how these dynamical Majorana fermions can be detected by measuring a quantized conductance sum rule and show that this effect is robust against disorder.