Abstract：

In quantum materials, the interplay of symmetry, topology, quantum geometry and interactions can produce new phases of matter with fundamentally new electronic and optoelectronic properties. Two dimensional van der Waals materials, with highly tunable symmetry, band structure, carrier density and interactions, allow for reaching previously inaccessible experimental parameter regimes. Such highly tunable material platforms provide exciting possibilities to discover new fundamental quantum physics, which can in turn guide the engineering of new quantum technologies for sensing, communications and computations. 

In this talk, I will present our recent research along this line. First, I will show that the optoelectronic response of graphene can be controlled among highly distinct regimes by tuning the interaction dynamics of photo-excited carriers. In particular, we observe a new type of photocurrent that appears exclusively in charge neutral graphene with unusual ultra-relativistic electron scattering kinematics. Second, I will show that the interplay between nontrivial topology and novel crystalline symmetries in atomically thin WTe₂ gives rise to new Berry curvature physics, which can be detected by nonlinear electrical transport and infrared photocurrent measurements. Remarkably, the nonlinear electrical transport in bilayer WTe₂ uncovers a new type of electrical Hall effect under the time-reversal-symmetric condition. Such a nonlinear Hall response directly measures the energy-dependent “dipole moment” of the Berry curvature that arises from layer-polarized Dirac fermions. Our work highlights the potential for using intrinsic quantum and topological properties for nonlinear applications including frequency-doubling and rectification in the GHz and infrared regimes. In the end, I will discuss new opportunities towards fundamental physics and device functionalities with the combination of highly tunable material platform and advanced characterization and manipulation methods.