Origin of anomalous magnetic breakdown frequencies in quasi-two-dimensional organic conductors

We investigate the origin of anomalous magnetic breakdown frequencies in the de Haas--van Alphen (dHvA) effect in quasi-two-dimensional organic conductors such as α-(BEDT-TTF)₂KHg(SCN)₄ and κ-(BEDT-TTF)₂Cu(NCS)₂. A tight-binding model based on their realistic band structure is constructed and solved numerically to compute the field dependence of the magnetization. The present model provides a natural description for the phenomenon of magnetic breakdown between coexisting closed and open Fermi surfaces and accounts for the experimentally observed frequencies that are forbidden in the semiclassical picture. We find that the appearance of these anomalous frequencies in the dHvA signal is a quantum-mechanical effect which arises from differences in field dependence of the states in the two partially occupied bands near the Fermi level.