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Nonlinear Optical and Electrical Phenomena in Topological Semimetals

Periodic Reporting for period 2 - NonlinearTopo (Nonlinear Optical and Electrical Phenomena in Topological Semimetals)

Reporting period: 2020-07-01 to 2021-12-31

In the past decade, the band-structure topology and related topological materials have been extensively studied mostly by revealing their unique surface states. The current proposal sets a new paradigm and asks, what novel phenomena and properties topological materials will bring us? We focus on nonlinear optical and electrical phenomena in topological semimetals. Our studies can pave the paths for green-energy application (e.g. novel solar energy-harvesting strategy), photodetection, and optoelectric devices. We plan to explore the fundamental physics of what role the band topology plays in the nonlinear light-matter interaction. We aim to build up a diagnostic tool that explores the nonlinear phenomena in a vast number of real materials and directly probes the bulk topology by investigating their nonlinear properties.
We revealed a novel understanding of the nonlinear optical response theory. We re-formulated the nonlinear response theory to explore physical insights behind it. The related theory was previously established on ordinary time-reversal symmetric materials and is usually formulated with the Berry curvature. One step further, we revealed a long-ignored topological quantity, the quantum metric, to the nonlinear theory. The metric was introduced by Einstein to understand the spacetime curvature and represents the “quantum distance” in condensed matter physics. However, there is little known about how to measure the quantum metric in real materials. We discovered a practical way to detect the quantum metric by exotic nonlinear photocurrents. We further proposed novel phenomena in emerging materials, for example, the magnetic photogalvanic effect and the subgap photocurrent.

We discovered the nonlinear effect due to the interplay between topology and chirality in DNA-like molecules. It is unexpected progress that we discovered a nonlinear transport effect in DNA-like chiral molecules from the perspective of topology. In the last ten years, a mysterious correlation between chiral geometry and electronic spin was extensively studied. When they transmit through DNA-like chiral molecules, electrons get spin-polarized, and the polarization depends on the chirality. This effect is called chiral-induced spin selectivity (CISS). The high spin polarization is induced and manipulated in ways not previously imagined. However, the underlying mechanism between chiral structure and electronic spin remains debated. We approached the intriguing CISS effect from a topological perspective. We found an orbital texture in the band structure, a topological characteristic induced by chirality. We find that the orbital texture enables the chiral molecule to polarize the quantum orbital, which forms the core of CISS. The orbital effect leads to nonlinear transport phenomena in the magnetoresistance, rationalizing CISS experiments. We further predicted that the orbital-induced nonlinear transport is a general phenomenon in inversion-breaking materials.
A ground-breaking theory should have significant experimental consequences. Our nonlinear theory opened an unexplored territory with the quantum metric by predicting novel phenomena and exciting nonlinear materials. We found the counter-intuitive subgap photocurrent (photon energy smaller than the gap) and the magnetic photogalvanic effect in magnetic topological and ordinary materials. We revealed the magnetic control on devices based on photocurrent and second harmonic generation. These works pave a pathway to detect the quantum metric by exotic nonlinear photocurrents. In addition, our earlier material predictions on the nonlinear anomalous Hall effect were observed in recent experiments.

Our CISS theory brings a new understanding of the CISS effect. Furthermore, because chirality is a common feature of many chemical and most biochemical systems, the extent of the orbital polarization effect may be larger than one can imagine from the CISS. We were also provoked to investigate nonlinear effects in an emerging chiral material, the magic-angle twisted bilayer graphene, and found giant nonlinear magnetoresistance induced by the flat bands and chirality. This CISS work also inspired us to design spintronic devices using the orbital Hall effect, leading to a patent in the application.
Nonlinear light-matter interaction in a topological material
Topological electrons in DNA