Surface Charge Induced Dirac Band Splitting in a Charge Density Wave Material (TaSe4)2I

TL;DR

This study reveals how surface charge polarization causes Dirac band splitting in (TaSe4)2I, a quasi-1D material with charge density wave order, highlighting the interplay between topology and CDW phases.

Contribution

It demonstrates experimentally and theoretically that surface charge polarization induces Dirac band splitting in (TaSe4)2I, linking surface chemistry to topological electronic structure.

Findings

Dirac-like bands observed via ARPES match first-principles calculations.

Surface iodine loss leads to charge polarization and band splitting.

The Dirac band splitting is hundreds of meV under certain conditions.

Abstract

(TaSe4)2I, a quasi-one-dimensional (1D) crystal, shows a characteristic temperature-driven metal-insulator phase transition. Above the charge density wave (CDW) temperature Tc, (TaSe4)2I has been predicted to harbor a Weyl semimetal phase. Below Tc, it becomes an axion insulator. Here, we performed angle-resolved photoemission spectroscopy (ARPES) measurements on the (110) surface of (TaSe4)2I and observed two sets of Dirac-like energy bands in the first Brillion zone, which agree well with our first-principles calculations. Moreover, we found that each Dirac band exhibits an energy splitting of hundreds of meV under certain circumstances. In combination with core level measurements, our theoretical analysis showed that this Dirac band splitting is a result of surface charge polarization due to the loss of surface iodine atoms. Our findings here shed new light on the interplay between band topology and CDW order in Peierls compounds and will motivate more studies on topological properties of strongly correlated quasi-1D materials.