Exotic Node Line in ternary MgSrSi-type crystals

TL;DR

This study identifies and characterizes new topological nodal line semimetals in MgSrSi-type crystals, revealing complex multi-loop structures and novel topological features like nexus points and Hopf-links, expanding the understanding of topological materials.

Contribution

First principles screening of MgSrSi-type crystals uncovers over 70 NLSMs with novel multi-loop nodal line structures and topological features like nexus points and Hopf-links.

Findings

Over 70 MgSrSi-type crystals are NLSMs.

Discovery of multi-loop nodal line structures with nexus points and Hopf-links.

Prediction of complex topological surface states in these materials.

Abstract

Node line (NL) band-touchings protected by mirror symmetry (named as $m$-NL), the product of inversion and time reversal symmetry $\mathcal{S=PT}$ ($s$-NL), or nonsymmorphic symmetry are nontrivial topological objects of topological semimetals (SMs) in the Brillouin Zone (BZ). In this work, we screened the family of MgSrSi-type crystals using first principles calculations, and discovered that more than 70 members are NLSMs. A new type of multi-loop structure was found in AsRhTi that a $s$-NL touches robustly with a $m$-NL at some "nexus point", and in the meanwhile a second $m$-NL crosses with the $s$-NL to form a Hopf-link. Unlike the previously proposed Hopf-link formed by two $s$-NLs or two $m$-NLs, a Hopf-link formed by a $s$-NL and a $m$-NL requires a minimal three-band model to characterize its essential electronic structure. The associated topological surface states on different surfaces of AsRhTi crystal were also obtained. Even more complicated and exotic multi-loop structure of NLs were predicted in AsFeNa and PNiNb. Our work may shed light on search for exotic multi-loop NLSMs in real materials.