# Evidence for bulk nodal loops and universality of Dirac-node arc surface states in ZrGeXc (Xc = S, Se, Te)

## TL;DR

This study uses ARPES to reveal bulk nodal loops and universal Dirac-node-arc surface states in ZrGeXc compounds, highlighting the role of spin-orbit coupling and multiple nodal loops in topological line-node semimetals.

## Contribution

It demonstrates the existence of bulk nodal loops and universal Dirac-node-arc surface states across ZrGeXc compounds, linking these features to spin-orbit coupling and multiple nodal loops.

## Key findings

- Bulk nodal loops protected by glide mirror symmetry.
- Universal Dirac-node-arc surface states in ZrGeXc.
- Role of spin-orbit coupling in surface state formation.

## Abstract

We have performed angle-resolved photoemission spectroscopy (ARPES) on layered ternary compounds ZrGeXc (Xc = S, Se, and Te) with square Ge lattices. ARPES measurements with bulk-sensitive soft-x-ray photons revealed a quasi-two-dimensional bulk-band structure with the bulk nodal loops protected by glide mirror symmetry of the crystal lattice. Moreover, high-resolution ARPES measurements near the Fermi level with vacuum-ultraviolet photons combined with first-principles band-structure calculations elucidated a Dirac-node-arc surface state traversing a tiny spin-orbit gap associated with the nodal loops. We found that this surface state commonly exists in ZrGeXc despite the difference in the shape of nodal loops. The present results suggest that the spin-orbit coupling and the multiple nodal loops cooperatively play a key role in creating the exotic Dirac-node-arc surface states in this class of topological line-node semimetals.