Symmetry-protected three-dimensional nodal fermions in a pyrochlore superconductor*

The pyrochlore network, corner-sharing tetrahedra, is a three-dimensional (3D) geometrically frustrated lattice. Due to its complex atomic arrangement, pyrochlore lattice has been known to carry abundant correlated topological phases depending on its spin-orbit coupling (SOC) and electron correlation strength. However, the topological electronic band structure of pyrochlore lattice in the strong SOC limit has rarely been pioneered since most studies have mainly focused on the exotic magnetic properties originating from geometrical frustration. Here, we present the topological electronic band structure in the normal state of pyrochlore superconductor RbBi₂ measured by angle-resolved photoemission spectroscopy (ARPES). Thanks to the strong SOC of the Bi pyrochlore network, we experimentally confirm the multiple 3D nodal Fermions protected by unique crystalline symmetries of pyrochlore lattice. First of all, we found the symmetry-enforced 3D Dirac fermions protected by nonsymmorphic symmetry. In addition, quadratic band crossing protected by cubic symmetry was observed at the Γ point. Lastly, we identify additional 3D Dirac band crossing associated with band inversion protected by rotation symmetry. These observations disclose the uncharted non-trivial topological band structure of the pyrochlore lattice in the strong SOC regime and would initiate the realization of the rich correlated topological phenomena in the pyrochlore lattice such as topological superconductivity.