Double Dirac Nodal Line Semimetal with Torus Surface State

TL;DR

This paper introduces a new class of nodal line semimetals with an eight-fold degenerate double Dirac nodal line, predicts their unique surface states, and identifies LiBH as an ideal material hosting these features.

Contribution

The study identifies specific space groups hosting double Dirac nodal lines, predicts two distinct surface states, and demonstrates LiBH as an ideal candidate for experimental realization.

Findings

Double Dirac nodal line exists in 5 of 230 space groups.

LiBH hosts the DDNL at the Fermi level with a large linear energy range.

Unique surface states include a torus surface state and no surface state.

Abstract

We propose a class of nodal line semimetals that host an eight-fold degenerate double Dirac nodal line (DDNL) with negligible spin-orbit coupling. We find only 5 of the 230 space groups host the DDNL. The DDNL can be considered as a combination of two Dirac nodal lines, and has a trivial Berry phase. This leads to two possible but completely different surface states, namely, a torus surface state covering the whole surface Brillouin zone and no surface state at all. Based on first-principles calculations, we predict that the hydrogen storage material LiBH is an ideal DDNL semimetal, where the line resides at Fermi level, is relatively flat in energy, and exhibits a large linear energy range. Interestingly, both the two novel surface states of DDNL can be realized in LiBH. Further, we predict that with a magnetic field parallel to DDNL, the Landau levels of DDNL are doubly degenerate due to Kramers-like degeneracy and have a doubly degenerate zero-mode.