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.
Highlights
Topological semimetals, which are characterized by linear band crossings near the Fermi level, have attracted intense interest in the condensed matter physics community in the last decade
We show that the band splitting observed here is not the spin splitting of energy bands due to spin-orbit coupling (SOC), which is beyond the resolution of our angle-resolved photoemission spectroscopy (ARPES)
By combining the core level measurements and theoretical calculations, we demonstrate that the Dirac band splitting is a result of surface charge polarization due to the loss of iodine atoms
Summary
Topological semimetals, which are characterized by linear band crossings near the Fermi level, have attracted intense interest in the condensed matter physics community in the last decade. Based on the physical origin of the band crossing, topological semimetals are categorized as Dirac semimetals, Weyl semimetals, nodal-line semimetals, and others [1,2]. Due to the spin degeneracy, the Dirac node in Dirac semimetals is composed of two massless Weyl nodes with opposite chirality, which overlap with each other at the same momentum. By breaking either time reversal or inversion symmetry, the Dirac band can split into a pair of Weyl bands. The Dirac semimetal state was discovered in Na3Bi [3] and Cd3As2 [4–6]. The noncentrosymmetric Weyl semimetal state was observed in TaAs [7–11], while the magnetic Weyl semimetal state was recently discovered in Co3Sn2S2 [12,13]
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