We investigate analytically the anisotropic dielectric properties of single crystal {\alpha}-SnS near the fundamental absorption edge by considering atomic orbitals. Most striking is the excitonic feature in the armchair- (b-) axis direction, which is particularly prominent at low temperatures. To determine the origin of this anisotropy, we perform first-principles calculations using the GW0 Bethe-Salpeter equation (BSE) including the electron-hole interaction. The results show that the anisotropic dielectric characteristics are a direct result of the natural anisotropy of p orbitals. In particular, this dominant excitonic feature originates from the py orbital at the saddle point in the {\Gamma}-Y region.
@article{arxiv.2007.03449,
title = {Anisotropic Behavior of Excitons in Single Crystal {\alpha}-SnS},
author = {Van Long Le and Do Duc Cuong and Hoang Tung Nguyen and Xuan Au Nguyen and Bogyu Kim and Kyujin Kim and Wonjun Lee and Soon Cheol Hong and Tae Jung Kim and Young Dong Kim},
journal= {arXiv preprint arXiv:2007.03449},
year = {2020}
}