  The proposed high-spin superconductivity in the half-Heusler compounds
changes the landscape of superconductivity research. While superconducting
instability is possible only in systems with quantum mechanically coherent
quasiparticles, it has not been verified for any proposed high-spin Fermi
surfaces. Here we report an observation of anomalous Shubnikov-de Haas effect
in half-Heusler YPtBi, which is compatible with a coherent $j=3/2$ Fermi
surface. The quantum oscillation (QO) signal in cubic YPtBi manifests extreme
anisotropy upon rotation of the magnetic field from [100] to [110]
crystallographic direction where the QO signal drastically vanishes near [110].
This radical anisotropy for a cubic system cannot be explained by trivial
scenarios for QO involving effective mass or impurity scattering, but it is
naturally explained by the warping feature of the $j=3/2$ Fermi surface YPtBi.
Our results prove the high-spin nature of the quasiparticle in the half-Heusler
compounds, which makes the realization of the unprecedented high-spin
superconductivity more plausible.
