Origin of nonlinear photocurrents in chiral multifold semimetal CoSi studied by terahertz emission spectroscopy

Yao-Jui Chan^1*, Syed Mohammed Faizanuddin^1,2,3, Raju Kalaivanan¹, Sankar Raman¹, Hsin Lin¹, Uddipta Kar¹, Akhilesh Kr. Singh¹, Wei-Li Lee¹, Ranganayakulu K. Vankayala¹, Min-Nan Ou¹, Yu-Chieh Wen¹

¹Institute of Physics, Academia Sinica, Taipei, Taiwan
²Taiwan International Graduate Program, Academia Sinica, Taipei, Taiwan
³Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan

* Presenter:Yao-Jui Chan, email:yaojuichan@gmail.com

Investigating nonlinear photocurrents in topological semimetals with spectroscopic information provides an important pathway for uncovering unconventional light-matter interactions and developing advanced photovoltaic technologies. This is particularly significant for studying the circular photogalvanic effect (CPGE) in chiral multifold semimetals, especially in the transition metal silicides of the CoSi family which belong to the P₂₁₃ space group. For materials in this group, a quantized circular photogalvanic effect induced by the topology in band structure has been proposed. In our study, we utilized THz emission spectroscopy to probe nonlinear photocurrent in CoSi at photon energy ranging from 0.25 to 1 eV by detecting the emitted THz radiation. This contact-free approach allows us to explore the intrinsic material properties and avoid the errors typically induced by the contact resistance, thermal current, and nonlocal diffusion of carriers to the contacts in the conventional DC measurements. To extract the second-order nonlinear optical conductivity with better quantitative accuracy, we refined the TES analysis by considering the model that includes the linear, circular photogalvanic effect and photon-drag effect. We find a large linear shift current (17𝜇A/V²) and confirm the giant longitudinal injection conductivity (167𝜇A/V²) in the undoped CoSi. Besides, a bulk transverse injection current and weak photon drag effect are also identified by incorporating the symmetry analysis. Our results not only suggest that CoSi is a promising material for advanced photovoltaic applications but also consolidate TES as an effective tool for the qualitative study of nonlinear optical conductivity.

Keywords: Terahertz emission spectroscopy, chiral multifold fermion, nonlinear photocurrent