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

Abstract 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 YbB 12 without invoking neutral Fermi surfaces or other exotic quasiparticles.