Surface charge induced Dirac band splitting in a charge density wave material (TaSe_{4})_{2}I

Abstract

(TaSe_{4})_{2}I, a quasi-one-dimensional (1D) crystal, shows a characteristic temperature-driven metal-insulator phase transition. Above the charge density wave (CDW) temperature T_{c}, (TaSe_{4})_{2}I has been predicted to harbor a Weyl semimetal phase. Below T_{c}, it becomes an axion insulator. Here, we performed angle-resolved photoemission spectroscopy measurements on the (110) surface of (TaSe_{4})_{2}I and observed two sets of Dirac-like energy bands in the first Brillouin zone, which agree well with our first-principles calculations. Moreover, we found that each Dirac band exhibits an energy splitting of hundreds of millielectron volts 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 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.