Weyl-Transition-Driven Giant Reversible Orbital Hall Conductivity

The orbital Hall conductivity (OHC) is central to orbitronics, yet its microscopic control remains largely unexplored. Here we identify a general mechanism in which tilted Weyl crossings formed by orbitally distinct bands generate a strongly asymmetric orbital Berry curvature (OBC) distribution, whose imbalance survives Brillouin-zone integration and yields a sizable OHC already at zeroth order. Using first-principles calculations, we show that monolayer PtBi2 realizes this mechanism and hosts a giant OHC dominated by a type-II Weyl point. A small biaxial tensile strain drives a type-II → type-I → type-II Weyl-cone tilting transition, reversibly switching the sign of the OHC through the chiral orbital texture of the crossing bands, assisted by a strain-induced structural phase transition. Our results establish Weyl engineering of orbital quantum geometry as a route to generating and reversibly controlling OHC in polar multi-orbital materials.