Epitaxial HfTe₂, ZrTe₂ and type-III Dirac fermions in Hf_xZr_x-1Te₂ topological semimetal candidate

Topological semimetals are hosts of interesting types of low-energy quasiparticles such as type-I and type-II Dirac and Weyl fermions. Yet a type-III^1,2,3 emerges as a theoretical possibility exactly at the border between type-I and II, characterized by a line-like Fermi surface and a flat energy dispersion along one direction in the Brillouin Zone. A type-III fermion could undergo an electronic (Lifshitz) phase transition¹. We theoretically predict that 1T-HfTe₂ and 1T-ZrTe₂ transition metal dichalcogenides are type-I and type-II Dirac semimetals, respectively. By alloying the two materials, a new Hf_xZr_x-1Te₂ alloy with type-III Dirac cone emerges at x=0.2. We also provide experimental evidence that by using MBE, single and few layers of HfTe₂⁴, ZrTe₂⁵ and Hf_0.2Zr_0.8Te₂ can be grown on InAs(111) substrates, and by using in-situ ARPES, that the Dirac point lies at -or very close to- the E_F.

1. G. E. Volovik and K. Zhang, J. Low Temp. Phys. 189, 276 (2017).
2. H. Liu et al., Phys. Rev. Lett. 120, 237403 (2018).
3. L. Jin et al., Phys. Rev. B 101, 045130 (2020).
4. S. A. Giamini et al., 2D Mater. 4, 015001 (2017).
5. P. Tsipas et al., ACS Nano 12, 1696 (2018).