Exploring Dirac Electrons in Ultraclean Graphene by Applying On-Chip Terahertz Photocurrent Waveguide

Chan-Shan Yang^1*, Patrick Gallagher², Feng Wang²

¹Institute of Electro-Optical Engineering, National Taiwan Normal University, Taipei, Taiwan
²Department of Physics, University of California at Berkeley, Berkeley, CA, USA

* Presenter:Chan-Shan Yang, email:yangchanshan@gmail.com

Monolayer graphene is a gapless two-dimensional (2D) material with the hexagonal lattice and distinctive Dirac electronic structure. While the Fermi liquid is the canonical picture with which we understand conventional 3D metals screening long-range Coulomb interactions, the Dirac liquid can be used to theoretically describe 2D Dirac fermions near its quantum critical point (QCP). The undoped and ultraclean graphene near the QCP with marginally irrelevant Coulomb interactions is the key to understand the pure role of electron-electron interactions.
In this invited talk, I will describe how we observe the proof of Dirac liquid in the ultraclean graphene system by developing and applying the extremely sensitive on-chip terahertz (THz) time-domain integrated waveguide in the past one year. We investigate the response of frequency-dependent complex photoconductivities contributed from hot electrons upon near-infrared excitation at 77⁰K, and show the semiconducting-to-metallic crossover by sweeping the gate voltages. According to the results of fitting the change in conductivities (Δσ), the electronic temperatures and the order of electron-electron-scattering rates can be extracted and well compared to Dirac liquid predictions.

Keywords: Ultraclean graphene, Terahertz, Dirac liquid, Quantum critical point, Waveguide