In our previous studies, we revealed the existence of jumps of the electric polarization when a two-dimensional insulating system changes across the Weyl semimetal phase [1,2], We showed that singularities of Berry curvature at Weyl points cause the jump of polarization and the jump is fully determined by the Weyl dipole.

 Similar to the electric polarization, second-order nonlinear photocurrent responses are known to be in a close relationship with geometric quantities. Since Weyl points are singularities of geometric phases, they are expected to give nontrivial effects in optical responses as well. In this study, we focus on the shift current responses of two-dimensional systems and study their behavior in the limit where the energy gap of the system closes and the system approaches the Weyl semimetal phase. We show the relation between the shift current and Weyl points using a generic effective model of Weyl points.

 

[1] H. Yoshida, T. Zhang, and S. Murakami, Phys. Rev. B 107, 035122 (2023)

[2] H. Yoshida, T. Zhang, and S. Murakami, Phys. Rev. B 108, 075160 (2023)