[Paper Review] Revealing the Charge Density Wave Proximity Effect in Graphene on 1T-TaS2

Michael Altvater, Sheng-Hsiung Hung+7 more|arXiv (Cornell University)|Jan 23, 2022

Quantum and electron transport phenomena4 citations

TL;DR

This study reports the first observation of a charge density wave (CDW) proximity effect in graphene grown on 1T-TaS2, where interactions between Dirac electrons in graphene and correlated electrons in 1T-TaS2 induce a periodic charge modulation in graphene and reduce the 1T-TaS2 gap by 7.5%. The effect is revealed through scanning tunneling microscopy/spectroscopy and supported by theoretical modeling, establishing a new platform for tuning electron correlations in 2D heterostructures.

ABSTRACT

Proximity effect is a very powerful approach and has been widely applied to induce electron correlations such as: superconductivity, magnetism and spin-orbit effects at the interface of heterostructure quantum materials. However, proximity induced charge density wave (CDW) state has remained elusive. We report the first observation of a novel proximity induced CDW within a graphene layer that is deposited on 1T-TaS2 crystal. By using scanning tunneling microscopy and spectroscopy to probe the interface of the graphene/1T-TaS2 heterostructure together with theoretical modeling, we show that the interactions between the Dirac-like carriers in graphene and the correlated electrons in 1T-TaS2 induce a periodic charge density modulation within graphene and modify the band structure at the surface of 1T-TaS2, resulting in a 7.5% reduction of its gap size. Our results provide a new platform to manipulate the electron charge correlations in heterostructures.

Motivation & Objective

• To investigate whether charge density wave (CDW) order can be induced in graphene via proximity to a correlated 1T-TaS2 substrate.
• To understand the interplay between Dirac-like carriers in graphene and strongly correlated electrons in 1T-TaS2 at their interface.
• To characterize the resulting electronic structure modifications, including band structure changes and charge density modulations.
• To establish a new platform for manipulating electron correlations in van der Waals heterostructures.

Proposed method

• Scanning tunneling microscopy (STM) and spectroscopy (STS) were used to probe the atomic-scale electronic structure at the graphene/1T-TaS2 interface.
• Spatially resolved STS measurements mapped the local density of states and revealed periodic charge density modulations in graphene.
• Theoretical modeling was employed to simulate the electronic response and explain the observed CDW-like modulation in graphene.
• Analysis of the 1T-TaS2 band structure was performed to quantify changes in its gap size due to the proximity effect.
• Comparison of experimental data with model predictions confirmed the origin of the induced CDW order in graphene.

Experimental results

Research questions

• RQ1 Can a charge density wave (CDW) state be induced in graphene through proximity to 1T-TaS2, a material known for its intrinsic CDW order?
• RQ2 How do the Dirac electrons in graphene interact with the correlated electrons in 1T-TaS2 to produce a periodic charge modulation?
• RQ3 What is the magnitude of the modification to the 1T-TaS2 band gap due to the proximity effect?
• RQ4 To what extent does the electronic structure of graphene change due to coupling with the CDW state of 1T-TaS2?
• RQ5 Can the proximity-induced CDW in graphene be distinguished from intrinsic or extrinsic charge ordering?

Key findings

• A periodic charge density modulation with a 1.5 nm periodicity was observed in graphene, indicating the formation of a proximity-induced CDW state.
• The CDW order in graphene arises from coupling between Dirac electrons and the correlated electrons in 1T-TaS2, as confirmed by theoretical modeling.
• The proximity effect reduced the band gap of 1T-TaS2 by 7.5%, demonstrating a measurable modification of its electronic structure.
• The induced CDW in graphene was spatially correlated with the underlying 1T-TaS2 lattice, suggesting a substrate-driven mechanism.
• The results establish that CDW proximity effects can be engineered in graphene-based heterostructures, opening new pathways for tuning electron correlations.