Two-Dimensional Valley Electrons and Excitons in Noncentrosymmetric 3R MoS$_{2}$

TL;DR

This study demonstrates that in 3R-stacked MoS₂, valley electrons and excitons are confined to two dimensions due to stacking symmetry, unlike in 2H MoS₂ where they are three-dimensional, enabling stacking-controlled valleytronics.

Contribution

The paper reveals that stacking order in MoS₂ determines the dimensionality of valley electrons and excitons, with 3R stacking confining them to two dimensions, a novel control mechanism for valleytronics.

Findings

Valley electrons in 3R MoS₂ are confined to two dimensions.

Valley excitons in 3R MoS₂ exhibit hydrogen-like spectral series.

In 2H MoS₂, valley excitons are three-dimensional.

Abstract

We find that the motion of the valley electrons -- electronic states close to the ${\rm K}$ and ${\rm K'}$ points of the Brillouin zone -- is confined into two dimension when the layers of MoS$_{2}$ follow the 3R stacking, while in the 2H polytype the bands have dispersion in all the three dimensions. According to our first-principles band structure calculations, the valley states have no interlayer hopping in 3R-MoS$_{2}$, which is proved to be the consequence of the rotational symmetry of the Bloch functions. By measuring the reflectivity spectra and analyzing an anisotropic hydrogen atomic model, we confirm that the valley excitons in 3R-MoS$_{2}$ have two-dimensional hydrogen-like spectral series, and the spreads of the wave function are smaller than the interlayer distance. In contrast, the valley excitons in 2H-MoS$_{2}$ are well described by the three-dimensional model and thus not confined in a single layer. Our results indicate that the dimensionality of the valley degree of freedom can be controlled simply by the stacking geometry, which can be utilized in future valleytronics.