Probing complex stacking in a layered material via electron-nuclear quadrupolar coupling

TL;DR

This study introduces a novel NMR-based method to characterize interlayer stacking in layered materials like 1T-TaS$_2$, revealing distinct stacking patterns and their electronic implications through combined experimental and computational analysis.

Contribution

The paper demonstrates a new approach using $^{33}$S NMR and first-principles calculations to probe and distinguish stacking patterns in layered materials.

Findings

Distinct NMR signatures for different stacking orders

Quantitative match between simulation and experimental spectra

Identification of coexisting interfacial environments

Abstract

For layered materials, the interlayer stacking is a critical degree of freedom tuning electronic properties, while its microscopic characterization faces great challenges. The transition-metal dichalcogenide 1T-TaS$_2$ represents a novel example, in which the stacking pattern is not only enriched by the spontaneous occurrence of the intralayer charge density wave, but also recognized as a key to understand the nature of the low-temperature insulating phase. We exploit the $^{33}\rm{S}$ nuclei in a 1T-TaS$_2$ single crystal as sensitive probes of the local stacking pattern via quadrupolar coupling to the electron density distribution nearby, by combining nuclear magnetic resonance (NMR) measurements with the state-of-the-art first-principles electric-field gradient calculations. The applicability of our proposal is analyzed through temperature, magnetic-field, and angle dependent NMR spectra. Systematic simulations of a single 1T-TaS$_2$ layer, bilayers with different stacking patterns, and typical stacking orders in three-dimensional (3D) structures unravel distinct NMR characteristics. Particularly, one 3D structure achieves a quantitative agreement with the experimental spectrum, which clearly rationalizes the coexistence of two types of interfacial environments. Our method may find general applications in the studies of layered materials.