Magnetoquantum oscillations, magnetic breakdown, and Fermi-surface modifications inNbSe3

The magnetoquantum oscillations in ${\mathrm{NbSe}}_3$ show frequency shifts and beat structures due to interaction between the pinned charge-density-wave (CDW) configuration and the normal Fermi surface (FS). Spatial variations in the phase deformation of the CDW pinned at impurities can result in changes in the local Fermi level and CDW gap, giving rise to a distribution of extremal FS areas. Magnetic breakdown (MB) between these small FS sections and the open nested sheets of FS can have striking effects on the amplitude and frequency of the quantum oscillations due to modulation of the transmission and reflection coefficients at the MB junctions. A model using a one-dimensional open-orbit network and a rectangular distribution of FS areas has been used to fit the data in the range 0–225 kG. Good fits to the beat periods and frequency shifts are obtained and provide an accurate measure of the width of the frequency distribution for different CDW configurations. The existence of these MB mechanisms also provides an explanation for hysteresis and asymmetry observed in the magnetoresistance as the CDW configuration is manipulated in electric and magnetic fields.