# Coexistence of New Fermions in Topological Semimetal TaS

## TL;DR

This paper predicts that TaS is a stable topological semimetal hosting various topological fermions, including nodal lines, Weyl points, and triply degenerate points, with unique surface states and potential for studying their interplay.

## Contribution

It introduces TaS as a new topological semimetal hosting coexistence of multiple topological fermions and analyzes their electronic structures with and without spin-orbit coupling.

## Key findings

- TaS hosts ring-shaped nodal lines and triply degenerate points without SOC.
- SOC causes TDP splitting and transforms some nodal rings into Weyl points.
- Unique Fermi arcs and Lifshitz transition are observed on certain surfaces.

## Abstract

We theoretically propose that, the single crystal formed TaS is a new type of topological semimetal, hosting ring-shaped gapless nodal lines and triply degenerate points (TDPs) in the absence of spin-orbit coupling (SOC). In the presence of SOC, the each TDP splits into four TDPs along the high symmetric line in the momentum space, and one of the nodal ring remains closed due to the protection of the mirror reflection symmetry, while another nodal ring is fully gapped and transforms into six pairs ofWeyl points (WPs) carrying opposite chirality. The electronic structures of the projected surfaces are also discussed, the unique Fermi arcs are observed and the chirality remains or vanishes depending on the projection directions. On the (010) projected surface, one may observe a Lifshitz transition. The new type of topological semimetal TaS is stable and experimentally achievable, and the coexistence of topological nodal lines, WPs and TDPs states in TaS makes it a potential candidate to study the interplay between different types of topological fermions.