Magnetic breakdown phenomenon in quasi-two-dimensional organic conductors: A quantum model inspired by a realistic band structure

Abstract

We investigate the phenomenon of magnetic breakdown in quasi-two-dimensional organic conductors such as $\ensuremath{\alpha}\ensuremath{-}{(\mathrm{ET})}_{2}{\mathrm{KHg}(\mathrm{SCN})}_{4}$ and $\ensuremath{\kappa}\ensuremath{-}{(\mathrm{ET})}_{2}{\mathrm{Cu}(\mathrm{NCS})}_{2}$ by constructing a tight-binding model based on a realistic band structure which is derived from the crystallographic data. We solve the model numerically to compute the magnetic field dependence of the magnetization and show that the present model accounts naturally for the experimentally observed magnetization oscillation frequencies that are forbidden in the semiclassical picture. The computed values of the fundamental and magnetic breakdown frequencies with no adjustable parameters are close to the experimentally measured values. For completeness, we carry out the computation for both canonical (fixed number of particles) and grand canonical (fixed chemical potential) ensembles, and show that the forbidden frequencies appear in both cases. Hence, the appearance of anomalous frequencies in the de Haas--van Alphen effect has a quantum-mechanical origin and arises from the interplay of electronic states from two partially occupied bands near the Fermi energy as a function of magnetic field. We also compute the temperature dependence of the magnetization and apply ad hoc the Lifshitz-Kosevich analysis to the amplitudes of the Fourier components at moderately high temperatures. This yields effective mass values for $\ensuremath{\alpha}\ensuremath{-}{(\mathrm{ET})}_{2}{\mathrm{KHg}(\mathrm{SCN})}_{4}$ in good agreement with experimental values.