Direct Observation of Mono-, Bi-, and Tri-layer Charge Density Waves in 1T-TaS_2 by Transmission Electron Microscopy without a Substrate

TL;DR

This study directly visualizes charge density waves in mono-, bi-, and tri-layer 1T-TaS_2 using substrate-free STEM, revealing new order states and defect types, and highlights the role of electron-phonon interactions in ultra-thin CDW systems.

Contribution

It provides the first real-space observation of CDWs in ultra-thin 1T-TaS_2 without substrate influence, identifying new order states and defect structures related to electron-phonon interactions.

Findings

Mono-layer exhibits a new triclinic stripe CDW order.

Defects are line-type due to strong electron-phonon interaction.

Absence of triple q condition in mono-layer CDW.

Abstract

Charge-density-waves (CDW) which occur mainly in low-dimensional systems have a macroscopic wave function similar to superfluids and superconductors. Kosterlitz-Thouless (KT) transition is observed in superfluids and superconductors, but the presence of KT transition in ultra-thin CDW systems has been an open problem. We report the direct real-space observation of CDWs with new order states in mono-, bi-, and tri-layer 1T-TaS_2 crystal by using a low voltage scanning-transmission-electron-microscope (STEM) without a substrate. This method is ideal to observe local atomic structures and possible defects. We clearly observed that the mono-layer crystal has a new triclinic stripe CDW order without the triple q condition q_1 + q_2 + q_3 = 0. A strong electron-phonon interaction gives rise to new crevasse (line) type defects instead of disclination (point) type defects due to the KT transition. These results reaffirm the importance of the electron-phonon interaction in mono-layer nanophysics.