###### Nano Optics Seminar

**Zhiqiang Li**

*Sichuan University*

**"Magneto-optical Study of Graphene Heterostructures and TCI"**

In this talk, I will present our recent work on graphene/BN heterostructures and topological crystalline insulators (TCI). We performed photocurrent study of graphene/BN in magnetic field. A record-high zero-bias photoresponsivity of 0.3 A/W is observed corresponding to the collection of at least five carriers per absorbed photon. We show that this effect arises from the enhanced Nernst coefficient through Lifshtiz transition at low-energy Van Hove singularities. Remarkably, the photocurrent diagram in high field exhibits a self-similar recursive pattern, which captures the fractal structure of the Hofstadter spectrum. These behaviors are inherently linked to the fractal quantum Hall states.

In many topological materials, it remains a challenge to probe optical and transport properties of the topological surface states. We will report our infrared study of a TCI, Pb0.7Sn0.3Se. We demonstrate that the far-infrared conductivity is dominated by the surface states as a result of their unique band structure. Moreover, our experiments yield a surface mobility of 40,000 cm2 V-1 s-1, which suggests the viability of surface-dominated conduction in thin TCI crystals. This study is the first step towards exploring the exotic physics predicted for TCIs.