Field-dependent effective mass explains amplitude and phase of quantum oscillations in YbB 12

Recent high-field measurements of the Kondo insulator YbB 12 have revealed quantum oscillations and thermodynamic signatures that are not captured by Landau quantization with a field-independent effective mass. We show that these features follow from a field-dependent cyclotron mass m*(B) entering through a Hermitian von Roos-type kinetic-energy operator. The resulting gradient terms generate a (B)-dependent Onsager phase shift δϕ(B) ∝ ∂ B ln m*(B). This yields oscillations that persist deep in the hybridization gap and preserve the standard Lifshitz-Kosevich temperature damping. Using a single experimentally constrained m*(B), the model reproduces both the oscillation amplitude and the 39-41 T doublet in YbB12 without invoking neutral Fermi surfaces or other exotic quasiparticles.