NonlinearTopo · Nonlinear Optical and Electrical Phenomena in Topological Semimetals

In the past decade, the band-structure topology and related topological materials have been intensively studied mostly by revealing their unique surface states. The current proposal sets a new paradigm by focusing on nonlinear optical phenomena in topological semimetals (TSMs). I aim to investigate the photocurrent and second-harmonic generation, as well as to discover novel nonlinear effects. The strength of TSMs lies in the fact that the giant Berry curvature in their band-crossing regions (e.g., Weyl points) can strongly boost these nonlinear effects, such as inducing a colossal photocurrent. Current understanding of the photocurrent is based on a model that considers the two-band transition within a Weyl cone. In the field of nonlinear optics, however, it is known that the photocurrent largely comes from three-band virtual transitions. Unfortunately, the nonlinear optics theory cannot be simply applied to TSMs due to the unphysical divergence of the photocurrent at band-crossing points. Therefore, I propose to bring the concept of three-band transitions to TSMs by reformulating the photocurrent theory framework. The new methodology represents the challenging and ground-breaking nature of the current proposal. Beyond the optical excitation, I further propose to explore exotic nonlinear electric and thermoelectric phenomena at the zero-frequency limit. I aim to build up a diagnostic tool that explores the nonlinear phenomena in a vast number of real TSM materials and directly probe the bulk topology by investigating their nonlinear properties. For example, my recent results have exposed a new group of Weyl points in a well-known Weyl semimetal by analysing the photocurrent distribution in its band structure. External perturbations can sensitively modify the TSM band structure, hence tune the induced photocurrent. This controllable photocurrent opens the door for novel device concepts, such as an optoelectronic transistor controlled by an external magnetic field.