Experimental Evidence of Stable 2$H$ Phase on the Surface of Layered 1$T'$-TaTe$_2$

TL;DR

This study reveals that the surface of layered 1$T'$-TaTe$_2$ exhibits a stable 2$H$ phase up to 180 K, with a complex electronic structure that differs from its bulk and isovalent compounds, challenging the Fermi surface nesting explanation for CDW.

Contribution

The paper provides experimental evidence of a stable 2$H$ phase on the surface of 1$T'$-TaTe$_2$, supported by ARPES and DFT, and clarifies the surface-bulk electronic structure differences.

Findings

Surface 2$H$ phase is stable up to 180 K.

Surface electronic structure resembles 2$H$-TaTe$_2$, bulk resembles 1$T$-TaTe$_2$.

CDW order is not due to Fermi surface nesting.

Abstract

We report on the low-energy electronic structure of Tantalum ditelluride (1$T'$-TaTe$_2$), one of the charge density wave (CDW) materials from the group V transition metal dichalcogenides using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). We find that the Fermi surface topology of TaTe$_2$ is quite complicated compared to its isovalent compounds such as TaS$_2$, TaSe$_2$, and isostructural compound NbTe$_2$. More importantly, we discover that the surface electronic structure of 1$T'$-TaTe$_2$ has more resemblance to the 2$H$-TaTe$_2$, while the bulk electronic structure has more resemblance to the hypothetical 1$T$-TaTe$_2$. These experimental observations are thoroughly compared with our DFT calculations performed on 1$T$-, 2$H$- and 2$H$ (monolayer)/1$T$- TaTe$_2$. We further notice that the Fermi surface topology is temperature independent up to 180 K, confirming that the 2$H$ phase on the surface is stable up to 180 K and the CDW order is not due to the Fermi surface nesting.