Quantum oscillations and magnetic field induced Fermi surface reconstruction in the charge density wave state of A0.9Mo6O17 (A=Na,K)

We present a detailed study of the magnetotransport properties of the charge density wave state in quasi-two-dimensional purple bronze ${\mathrm{Na}}_{0.9}{\mathrm{Mo}}_{6}{\mathrm{O}}_{17}$ under pulsed fields up to 62 T. Pronounced Shubnikov--de Haas (SdH) oscillations of three frequencies have been observed in the field induced charge density/spin density wave state of the present compound. The two slow oscillations are attributed to the reconstructed electron and hole pockets near the ${M}^{\ensuremath{'}}$ point of the Brillouin zone (BZ). The highest-frequency oscillations correspond to an orbit occupying 18.1% of the BZ, which is consistent with the hexagonal electron Fermi surface in the normal state. The appearance of the highest-frequency oscillations can be explained by the magnetic breakdown between the electron pockets at the ${M}^{\ensuremath{'}}$ point. This magnetic breakdown behavior has also been observed in ${\mathrm{K}}_{0.9}{\mathrm{Mo}}_{6}{\mathrm{O}}_{17}$ under very high field range up to 80 T. Based on analyses of the high field SdH oscillations, we propose a possible Fermi surface structure of ${\mathrm{Na}}_{0.9}{\mathrm{Mo}}_{6}{\mathrm{O}}_{17}$.