Realization of practical eightfold fermions and fourfold van Hove singularity in TaCo2Te2.

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 TaCo₂Te₂ crystals. Through systematic experimental and theoretical analyses, we establish TaCo₂Te₂ as a nonsymmorphic crystal with negligible spin–orbit coupling (SOC) and long-range magnetic order. These critical properties guarantee the realization of practical eightfold fermions and fourfold van Hove singularity, as directly observed by photoemission spectroscopy. TaCo₂Te₂ 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 TaCo₂Te₂ will promote the experimental observation of bulk hourglass fermions. Our results establish TaCo₂Te₂ as a platform to explore the interplay between symmetry and band topology. [ABSTRACT FROM AUTHOR]