Large change of interlayer vibrational coupling with stacking in Mo$_{1-x}$W$_{x}$Te$_{2}$

TL;DR

This study investigates how stacking variations in Mo$_{1-x}$W$_{x}$Te$_{2}$ affect interlayer vibrational coupling, revealing significant differences in interlayer force constants across phases and highlighting the role of stacking in material properties.

Contribution

It provides the first inelastic neutron scattering measurements of the interlayer shear mode in Mo$_{1-x}$W$_{x}$Te$_{2}$, showing how stacking influences interlayer interactions and vibrational properties.

Findings

Interlayer force constants are significantly weaker in $T^*_d$ and $1T^{\'e}$ phases compared to $T_d$.

The interlayer shear mode energies align with the linear chain model, with some intensity discrepancies.

Longer-range effects, such as stacking-induced band structure changes, likely influence interlayer vibrational coupling.

Abstract

Stacking variations in quasi-2D materials can have an important influence on material properties, such as changing the topology of the band structure. Unfortunately, the weakness of van der Waals interactions makes it difficult to compute the stacking dependence of properties, and even in a material as simple as graphite the stacking energetics remain unclear. Mo$_{1-x}$W$_{x}$Te$_{2}$ is a material in which three differently-stacked phases are conveniently accessible by temperature changes: $1T^{\prime}$, $T^*_d$, and the reported Weyl semimetal phase $T_d$. The transitions proceed via layer sliding, and the corresponding interlayer shear mode (ISM) is relevant not just for the stacking energetics, but for understanding the relationship between the Weyl physics and structural changes. However, the interlayer interactions of Mo$_{1-x}$W$_{x}$Te$_{2}$ are not well understood, with wide variation in computed properties. We report inelastic neutron scattering of the ISM in a Mo$_{0.91}$W$_{0.09}$Te$_{2}$ crystal. The ISM energies are generally consistent with the linear chain model (LCM), as expected given the weak interlayer interaction, though there are some discrepancies from predicted intensities. However, the interlayer force constants $K_x$ in the $T^*_d$ and $1T^{\prime}$ phases are substantially weaker than that of $T_d$, at 76(3)% and 83(3)%, respectively. Considering that the relative positioning of atoms in neighboring layers is approximately the same regardless of overall stacking, our results suggest that longer-range influences, such as stacking-induced band structure changes, may be responsible for the substantial change in the interlayer vibrational coupling. These findings should elucidate the stacking energetics of Mo$_{1-x}$W$_{x}$Te$_{2}$ and other van der Waals layered materials.