Sotirios Fragkos1,2,Evgenia Symeonidou1,3,Polychronis Tsipas1,Panagiotis Pappas1,Evangelia Xenogiannopoulou1,Athanasios Dimoulas1
NCSR Demokritos1,University of West Attica2,Aristotle University of Thessaloniki3
Sotirios Fragkos1,2,Evgenia Symeonidou1,3,Polychronis Tsipas1,Panagiotis Pappas1,Evangelia Xenogiannopoulou1,Athanasios Dimoulas1
NCSR Demokritos1,University of West Attica2,Aristotle University of Thessaloniki3
Topological semimetals (TSMs) are candidate hosts of interesting types of low-energy quasiparticles such as type-I and type-II Dirac and Weyl fermions and have attracted considerable interest in recent years due to their topological electronic properties and possible technological applications. In particular, Dirac semimetals possess large spin Berry curvatures and thus give rise to a large spin Hall and spin Nernst effects which may be used to generate pure spin current for spintronic devices [1,2].<br/>Also, a new type-III TSM has recently emerged as an interesting theoretical possibility [3,4], which is exactly at the border between type-I and type-II, and is characterized by a unique line-like Fermi surface and a flat energy dispersion at the Fermi level. This system can be easily driven into electronic (Lifshitz) phase transition to type-I or type-II by external stimuli associated with a big change in the Fermi surface from a point-like to a metal-like configuration, likely accompanied by resistivity and thermopower changes which could be accessible to (thermo)electric transport experiments.<br/>In this work, we investigate the molecular beam epitaxial (MBE) growth of few and single-layer 1T HfTe<sub>2</sub> and ZrTe<sub>2</sub> 2D transition metal ditellurides on various substrates [5-7]. Using electron diffraction, scanning tunneling microscopy, scanning transmission electron microscopy, and synchrotron X-ray diffraction, we show compelling evidence for vdW epitaxy with excellent rotational alignment.<br/>The electronic band structure imaged by angle resolved photoemission spectroscopy (ARPES) complemented by first-principles density functional theory (DFT) calculations shows that valence and conduction bands cross at the Fermi level exhibiting abrupt linear dispersions at the zone center. The latter indicates massless Dirac fermions similar to what is expected for a topological Dirac semimetal.<br/>In addition, our first-principles calculations reveal that 1T HfTe<sub>2</sub> and ZrTe<sub>2</sub> transition metal ditellurides are type-I and type-II Dirac semimetals, respectively [8]. By alloying the two materials, a new Hf<sub>x</sub>Zr<sub>1-x</sub>Te<sub>2</sub> alloy with type-III Dirac cone emerges at x=0.2 along the Γ-Α direction of the Brillouin zone. We experimentally realize by MBE, high epitaxial quality single and few layers of Hf<sub>0.2</sub>Zr<sub>0.8</sub>Te<sub>2</sub> on technologically important InAs(111) substrate and verify by in-situ ARPES, that the Dirac point is located the Fermi level. Our synchrotron ARPES results show that the Dirac cone remains unaltered as the photon energy is varied, indicating that there is no energy dispersion along the <i>k<sub>z</sub></i> axis, as expected for type-III Dirac semimetal.<br/>We acknowledge the financial support from the European Union H2020, Contract No. 824123 - SKYTOP.<br/>1. K. Taguchi et al., Phys. Rev. B 101, 235201 (2020).<br/>2. Y. Yen and G.-Y. Guo, Phys. Rev. B 101, 064430 (2020).<br/>3. G. E. Volovik and K. Zhang, J. Low Temp. Phys. 189, 276 (2017).<br/>4. H. Liu et al., Phys. Rev. Lett. 120, 237403 (2018).<br/>5. S. A. Giamini et al., 2D Mater. 4, 015001 (2017).<br/>6. P. Tsipas et al., ACS Nano 12, 1696 (2018).<br/>7. P. Tsipas et al., APL Mater. 9, 101103 (2021).<br/>8. S. Fragkos et al., J. Appl. Phys. 129, 075104 (2021).