Analysis of unconventional chiral fermions in a noncentrosymmetric chiral crystal PtAl

Symmetry protected nontrivial states in chiral topological materials hold immense potential for fundamental science and technological advances. Here, we report electrical transport, quantum oscillations, and electronic structure results of a single crystal of chiral quantum material PtAl. Based on the de Haas-van Alphen (dHvA) oscillations, we show that the smallest Fermi pocket $(\ensuremath{\alpha})$ possesses a nontrivial Berry phase $1.16\ensuremath{\pi}$. The band associated with this Fermi pocket carries a linear energy dispersion over a substantial energy window of $\ensuremath{\sim}700\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ that is further consistent with the calculated optical conductivity. First principles calculations unfold that PtAl is a higher fold chiral fermion semimetal where structural chirality drives the chiral fermions to lie at the high symmetry $\mathrm{\ensuremath{\Gamma}}$ and $R$ points of the cubic Brillouin zone. In the absence of spin orbit coupling, the band crossings at $\mathrm{\ensuremath{\Gamma}}$ and $\mathrm{R}$ points are three and fourfold degenerate with a chiral charge of $\ensuremath{-}2$ and $+2$, respectively. The inclusion of spin orbit coupling transforms these crossing points into four and sixfold degenerate points with a chiral charge of $\ensuremath{-}4$ and $+4$. Nontrivial surface states on the (001) surface connect the bulk projected chiral points through the long helical Fermi arcs that spread over the entire surface Brillouin zone.