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.
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