Topological Semimetals with Triply Degenerate Nodal Points in \theta-phase Tantalum Nitride

TL;DR

This paper predicts that heta-phase Tantalum Nitride is a topological semimetal with unique triply degenerate nodal points, which are protected by symmetry and lead to novel electronic phenomena.

Contribution

It introduces a new type of topological node in heta-TaN, characterized by triply degenerate points protected by crystalline symmetry, and explores their topological and quantum properties.

Findings

Identification of triply degenerate nodal points in heta-TaN

Prediction of singular Fermi surface touchings and 3D spin textures

Discovery of a new 'helical anomaly' in quantum Landau level analysis

Abstract

Using first-principles calculation and symmetry analysis, we propose that \theta-TaN is a topological semimetal having a new type of point nodes, i.e., triply degenerate nodal points. Each node is a band crossing between degenerate and non-degenerate bands along the high-symmetry line in the Brillouin zone, and is protected by crystalline symmetries. Such new type of nodes will always generate singular touching points between different Fermi surfaces and 3D spin texture around them. Breaking the crystalline symmetry by external magnetic field or strain leads to various of topological phases. By studying the Landau levels under a small field along $c$-axis, we demonstrate that the system has a new quantum anomaly that we call "helical anomaly".