Realization of Practical Eightfold Fermions and Fourfold van Hove Singularity in TaCo$_2$Te$_2$

TL;DR

This paper reports the experimental realization of eightfold fermions and fourfold van Hove singularities in TaCo$_2$Te$_2$, a nonsymmorphic crystal with negligible SOC, serving as a platform for topological quantum phases.

Contribution

It demonstrates the first experimental observation of practical eightfold fermions in TaCo$_2$Te$_2$, combining experimental and theoretical analyses to reveal its topological properties and tunability.

Findings

First observation of eightfold fermions in TaCo$_2$Te$_2$

Identification of fourfold van Hove singularity

Potential to tune phases via strain, magnetic field, or SOC

Abstract

Space groups describing the symmetry of lattice structure allow the emergence of fermionic quasiparticles with various degeneracy in the band structure. Theoretical efforts have predicted many materials hosting fermions with the highest degeneracy, i.e., eightfold fermions, yet lacking experimental realization. Here, we explore the band degeneracies in TaCo$_2$Te$_2$ crystals. Through systematic experimental and theoretical analyses, we establish TaCo$_2$Te$_2$ as a nonsymmorphic crystal with negligible spin-orbit coupling (SOC) and long-range magnetic order. These critical properties guarantee the first realization of practical eightfold fermions and fourfold van Hove singularity, as directly observed by photoemission spectroscopy. TaCo$_2$Te$_2$ serves as a topological quantum critical platform, which can be tuned into various magnetic, topologically trivial, and nontrivial phases by adding strain, magnetic field, or SOC. The latter is demonstrated by our first-principles calculations, which show that enhancing SOC in TaCo$_2$Te$_2$ will promote the experimental observation of bulk hourglass fermions. Our results establish TaCo$_2$Te$_2$ as a unique platform to explore states of matter intertwining magnetism, correlation, symmetry, and band topology.