Quantum oscillations reveal sixfold fermions in cubic $\beta$-PtBi$_2$

We report a study of de Haas-van Alphen oscillations in high-quality single crystals of cubic $\beta$-PtBi$_2$. In combination with density functional theory calculations, we identify quantum oscillations associated with all Fermi surface sheets predicted by theory. Our results uncover three small electron pockets centered on a sixfold band-touching point located approximately 25 meV below the Fermi level at the $R$ point of the Brillouin zone. These findings firmly establish the presence of sixfold fermions in close proximity to the Fermi energy of $\beta$-PtBi$_2$.