N*

We study the shift-current optical response in a noncentrosymmetric polytype of the layered semiconductor BC₂N. We employ a recently developed first-principles Wannier-interpolation technique [1] implemented in the latest release of Wannier90 [2]. We find that the nonlinear photoconductivity is strongly anisotropic, due to the vanishing of particular tensor components not foretold by phenomenological symmetry arguments; this is a consequence of dipole selection rules imposed by mirror symmetry, which imply that the relative parities between valence and conduction bands are key for determining the directionality of the band-edge response. The implications of the dipole selection rules should apply to a broad class of nonlinear responses [3].

[1] J. Ibañez-Azpiroz, S. S. Tsirkin, and I. Souza, PRB 97, 245143 (2018)
[2] G. Pizzi et. al., J. Phys. Cond. Matt. 32, 165902 (2020).
[3] J. Ibañez-Azpiroz, I. Souza, and F. de Juan, PRR 2, 013263 (2020)