New fermions on the line in topological symmorphic metals

TL;DR

This paper introduces a new class of topological metals with triply-degenerate band crossings in symmorphic lattices, revealing novel fermionic quasiparticles and unique magneto-transport phenomena beyond Weyl and Dirac semimetals.

Contribution

It identifies a topological metal with unconventional triply-degenerate band crossings, expanding the understanding of topological phases in symmorphic crystals.

Findings

Discovery of triply-degenerate band crossings in symmorphic topological metals

Distinct Landau level spectrum indicating new magneto-transport responses

Materials candidates exhibiting these novel topological features

Abstract

Topological metals and semimetals (TMs) have recently drawn significant interest. These materials give rise to condensed matter realizations of many important concepts in high-energy physics, leading to wide-ranging protected properties in transport and spectroscopic experiments. The most studied TMs, i.e., Weyl and Dirac semimetals, feature quasiparticles that are direct analogues of the textbook elementary particles. Moreover, the TMs known so far can be characterized based on the dimensionality of the band crossing. While Weyl and Dirac semimetals feature zero-dimensional points, the band crossing of nodal-line semimetals forms a one-dimensional closed loop. In this paper, we identify a TM which breaks the above paradigms. Firstly, the TM features triply-degenerate band crossing in a symmorphic lattice, hence realizing emergent fermionic quasiparticles not present in quantum field theory. Secondly, the band crossing is neither 0D nor 1D. Instead, it consists of two isolated triply-degenerate nodes interconnected by multi-segments of lines with two-fold degeneracy. We present materials candidates. We further show that triplydegenerate band crossings in symmorphic crystals give rise to a Landau level spectrum distinct from the known TMs, suggesting novel magneto-transport responses. Our results open the door for realizing new topological phenomena and fermions including transport anomalies and spectroscopic responses in metallic crystals with nontrivial topology beyond the Weyl/Dirac paradigm.