Nodal Loop and Nodal Surface States in Ti3Al Family Materials

TL;DR

This paper predicts that Ti3X compounds are topological semimetals featuring coexisting nodal loops and surfaces, with potential for exploring new topological phases.

Contribution

It introduces a new family of titanium-based compounds as topological semimetals with unique nodal features, supported by first-principles calculations and symmetry analysis.

Findings

Ti3Al has an almost ideal, nearly flat nodal loop near the Fermi level.

The nodal loop is protected by spacetime inversion and mirror symmetries.

The nodal surface is protected by nonsymmorphic screw symmetry and time reversal.

Abstract

Topological metals and semimetals are new states of matter which attract great interest in current research. Here, based on first-principles calculations and symmetry analysis, we propose that the family of titanium-based compounds Ti3X (X=Al, Ga, Sn, Pb) are unexplored topological semimetals. These materials feature the coexistence of a nodal loop and a nodal surface in their low-energy band structure. Taking Ti3Al as an example, we show that the material has an almost ideal nodal loop in the sense that the loop is close to the Fermi level and it is nearly flat in energy with energy variation <0.25 meV. The loop is protected by either one of the two independent symmetries: the combined spacetime inversion symmetry and the mirror reflection symmetry. The nodal surface at the k_z=\pi plane is guaranteed by the nonsymmorphic screw rotational symmetry and the time reversal symmetry. We discuss the effect of spin-orbit coupling and construct an effective model for describing the nodal loop. Our findings indicate that the Ti3Al family compounds can serve as an excellent material platform for studying new topological phases and particularly the interplay between nodal-loop and nodal-surface fermions.