ORIGIN OF ANOMALOUS MAGNETIC BREAKDOWN FREQUENCIES IN QUASI-2-DIMENSIONAL ORGANIC CONDUCTORS

We investigate the origin of anomalous magnetic breakdown frequencies in the de Haas-van Alphen (dHvA effect in quasi-two-dimensional organi c conductors such as alpha-(BEDT-TTF)(2)KHg(SCN)(4) and kappa-(BEDT-TT F)(2)Cu(NCS)(2). A tight-binding model based on their realistic band s tructure is constructed and solved numerically to compute the field de pendence of the magnetization. The present model provides a natural de scription for the phenomenon of magnetic breakdown between coexisting closed and open Fermi surfaces and accounts for the experimentally obs erved frequencies that are forbidden in the semiclassical picture. We find that the appearance of these anomalous frequencies in the dHvA si gnal is a quantum-mechanical effect which arises from differences in f ield dependence of the states in the two partially occupied bands near the Fermi level. [S0163-1829(97)06341-8].