Interview for Faculty Position

Berry phase: The missing ingredient in the electron theory of materials

Di Xiao

Oak Ridge National Laboratory

The theory of band structure in crystalline solids provides the fundamental basis for understanding materials and phenomena. It is generally believed that for most physical applications the energy dispersion alone carries sufficient information to give proper account of various thermodynamic and transport properties. Recently, this belief is challenged by the realization that the Berry phase of the electronic wave function can also have a profound effect on materials properties and is responsible for a spectrum of phenomena, such as the quantum/anomalous/spin Hall effects and ferroelectricity.

In my talk, I will show that the Berry phase effects can be systematically studied through its modification to the electron dynamics and the density of states. The important consequence of this modification is demonstrated by several examples, including valley Hall effect in graphene, orbital magnetization, and ferroelectricity in inhomogeneous materials such as multiferroics. Given its broad range of applications and essential role in understanding these phenomena, it is clear that the Berry phase should be included as a basic ingredient in the electron theory of materials.