Dirac Line-nodes and Effect of Spin-orbit Coupling in Non-symmorphic Critical Semimetal MSiS (M=Hf, Zr)

TL;DR

This study experimentally confirms Dirac line-node semimetals in MSiS compounds, revealing their electronic structure and the impact of spin-orbit coupling, thus expanding the understanding of topological quantum materials.

Contribution

The paper provides the first experimental observation of Dirac line-nodes in non-symmorphic MSiS compounds using ARPES, highlighting the role of symmetry and spin-orbit coupling.

Findings

Identification of Dirac line-nodes in MSiS (M=Hf, Zr)

Spin-orbit coupling lifts degeneracy away from line-nodes

Confirmation of non-symmorphic symmetry protecting the line-nodes

Abstract

Topological Dirac semimetals (TDSs) represent a new state of quantum matter recently discovered that offers a platform for realizing many exotic physical phenomena. A TDS is characterized by the linear touching of bulk (conduction and valance) bands at discrete points in the momentum space (i.e. 3D Dirac points), such as in Na3Bi and Cd3As2. More recently, new types of Dirac semimetals with robust Dirac line-nodes (with non-trivial topology or near the critical point between topological phase transitions) have been proposed that extends the bulk linear touching from discrete points to 1D lines. In this work, using angle-resolved photoemission spectroscopy (ARPES), we explored the electronic structure of the non-symmorphic crystals MSiS (M=Hf, Zr). Remarkably, by mapping out the band structure in the full 3D Brillouin Zone (BZ), we observed two sets of Dirac line-nodes in parallel with the kz-axis and their dispersions. Interestingly, along directions other than the line-nodes in the 3D BZ, the bulk degeneracy is lifted by spin-orbit coupling (SOC) in both compounds with larger magnitude in HfSiS. Our work not only experimentally confirms a new Dirac line-node semimetal family protected by non-symmorphic symmetry, but also helps understanding and further exploring the exotic properties as well as practical applications of the MSiS family of compounds.