  Motivated by recent experiments on Kondo insulators, we theoretically study
quantum oscillations from disorder-induced in-gap states in small-gap
insulators. By solving a non-Hermitian Landau level problem that incorporates
the imaginary part of electron's self-energy, we show that the oscillation
period is determined by the Fermi surface area in the absence of the
hybridization gap, and derive an analytical formula for the oscillation
amplitude as a function of the indirect band gap, scattering rates, and
temperature. Over a wide parameter range, we find that the effective mass is
controlled by scattering rates, while the Dingle factor is controlled by the
indirect band gap. We also show the important effect of scattering rates in
reshaping the quasiparticle dispersion in connection with angle-resolved
photoemission measurements on heavy fermion materials.
