[Paper Review] Effect of Stacking Order on the Electronic State of 1T-TaS$_2$

Zongxiu Wu, Kunliang Bu+8 more|arXiv (Cornell University)|May 18, 2021

2D Materials and Applications28 references4 citations

TL;DR

This study uses high-resolution scanning tunneling microscopy and spectroscopy to investigate how stacking order affects the electronic state in 1T-TaS2. It finds that the large insulating gap is intrinsic to single-layer behavior and arises from a Mott insulator mechanism, while stacking variations only induce small-gap or metallic states near step edges due to surface effects, not bulk properties.

ABSTRACT

New theoretical proposals and experimental findings on transition metal dichalcogenide 1T-TaS$_2$ have revived interests in its possible Mott insulating state. We perform a comprehensive scanning tunneling microscopy and spectroscopy experiment on different single-step areas in pristine 1T-TaS$_2$. After accurately determining the relative displacement of Star-of-David super-lattices in two layers, we find different stacking orders can correspond to the similar large-gap spectrum on the upper terrace. When the measurement is performed away from the step edge, the large gap spectrum can always be maintained. The stacking order seems rarely disturb the large-gap spectrum in the ideal bulk material. We conclude that the large insulating gap is from the single-layer property, which is a correlation-induced Mott gap based on the single-band Hubbard model. Specific stacking orders can perturb the state and induce a small-gap or metallic spectrum for a limited area around the step edge, which we attribute to a surface and edge phenomenon. Our work provides more evidence about the surface electronic state and deepens our understanding of the Mott insulating state in 1T-TaS$_2$.

Motivation & Objective

• To determine the influence of stacking order on the electronic structure of 1T-TaS2.
• To resolve the long-standing debate on whether the large gap in 1T-TaS2 is due to a Mott insulator or a band insulator.
• To clarify whether stacking order in bulk crystals induces distinct electronic phases across the material.
• To investigate the role of surface and edge effects in modifying electronic states near step edges.
• To provide experimental evidence for the origin of the Mott gap in 1T-TaS2 using single-step areas in cleaved samples.

Proposed method

• Performed comprehensive scanning tunneling microscopy (STM) and spectroscopy (STS) on single-step areas of cleaved 1T-TaS2 crystals.
• Accurately determined interlayer alignment of Star-of-David (SD) super-lattices by analyzing STM topography and dI/dV spectra.
• Mapped dI/dV spectra across different terraces and step edges to compare electronic states under varying stacking orders.
• Used atomically resolved imaging to identify defects and confirm atomic lattice structure, including a three-petal defect linked to missing Ta atoms.
• Analyzed multi-step areas to test stacking sequence models, including AA-AC-... vs. AA-AA-AA stacking.
• Correlated spectral features (e.g., suppressed Hubbard bands) with stacking configurations to infer electronic behavior.

Experimental results

Research questions

• RQ1 Does stacking order in 1T-TaS2 significantly alter the large insulating gap observed in STM spectra?
• RQ2 Is the large gap in 1T-TaS2 a result of Mott physics in single layers or a band insulator effect from stacking?
• RQ3 To what extent do edge and surface effects modify the electronic state compared to the bulk?
• RQ4 Can different stacking orders (e.g., AA vs. AC) be experimentally distinguished via STS and STM topography?
• RQ5 What is the origin of the small-gap spectrum observed near step edges, and is it intrinsic or surface-induced?

Key findings

• Different stacking orders, including AA and AC, can all give rise to a large-gap spectrum on the upper terrace, indicating the gap is not dependent on stacking.
• The large insulating gap is preserved in the bulk-like region away from step edges, supporting its origin in single-layer Mott physics.
• Small-gap or metallic spectra only appear in limited regions near step edges, indicating a surface or edge effect rather than a bulk property.
• The large gap is consistent with a correlation-induced Mott gap based on the single-band Hubbard model, not a band gap from stacking.
• A three-petal defect feature corresponds to next-nearest-neighbor S atoms and is associated with a missing central Ta atom, suppressing the upper Hubbard band.
• Multi-step areas showing AA-AA-AA stacking contradict the unit-cell doubling model, excluding a periodic stacking-induced band gap as the origin of the large gap.