# Topological nodal lines and hybrid Weyl nodes in YCoC$_2$

## TL;DR

This paper predicts that the noncentrosymmetric superconductor YCoC$_2$ is a topological semimetal with nodal lines and Weyl nodes, which could host topological superconductivity and Majorana modes.

## Contribution

It reveals the topological semimetal nature of YCoC$_2$, including nodal lines and Weyl nodes, based on first-principles calculations and effective models.

## Key findings

- YCoC$_2$ hosts intersecting nodal rings without SOC.
- SOC induces Weyl nodes in YCoC$_2$, including type-I and type-II.
- Fermi arcs are observed on the surface of YCoC$_2$.

## Abstract

Based on first-principles calculations and effective model analysis, we propose that the noncentrosymmetric superconductor YCoC$_2$ in normal state is a topological semimetal. In the absence of spin-orbit coupling (SOC), it can host two intersecting nodal rings protected by two mirror planes, respectively. One ring is composed of type-I nodes, where the two crossing bands have opposite slop sign in their dispersions. The other ring consists of both type-I and type-II nodes (the slop signs of the two bands are the same in certain direction). In the presence of SOC, the former nodal ring is gapped totally while the later one evolves into ten pairs of Weyl nodes, with two of them being type-I and eight being type-II. The type-II Weyl nodes are further classified into two kinds with different velocity matrix when described in Weyl equation near the nodes. Fermi arcs from topological surface states are observed in the surface projected energy dispersions. It is notably that YCoC$_2$ has been reported as a superconductor with critical temperature $T_c$ of 4.2 K. This makes it very attractive since including superconducting into topological semimetal state might result in topological superconductivity and be used to synthesize Majorana zero modes.