Hourglass Dirac Chain metal in Rhenium Dioxide

Nonsymmorphic symmetries, which involve fractional lattice translations in crystalline materials, can generate exotic types of fermionic excitations that are robust against spin-orbit coupling. Here we report on a new topological phase arising from nonsymmorphic symmetries—the hourglass Dirac chain metal, and demonstrate its realization in three-dimensional rhenium
dioxide crystals. We show that ReO₂ features hourglass-type dispersion in the bulk electronic structure dictated by its nonsymmorphic space group. Due to time reversal and inversion symmetries, each band has an additional two-fold degeneracy, making the neck crossing-point of the hourglass four-fold degenerate. Remarkably, close to the Fermi level, the neck crossing-point traces out a Dirac chain—a chain of connected four-fold-degenerate Dirac loops—in the momentum space. The symmetry protection, the transformation under symmetry-breaking, and the associated topological surface states of the hourglass Dirac chain are revealed. Our results open the door to a new class of topological state of matter, and provide a promising platform to explore the intriguing physics of the novel emergent fermions.