QUANTUM TRANSPORT IN THE CHARGE-DENSITY-WAVE STATE OF THE QUASI-2-DIMENSIONAL BRONZES (PO2)(4)(WO3)(2M)(M=4,6)

Magnetotransport has been studied on the quasi two-dimensional monopho sphate tungsten bronzes (PO2)(4)(WO3)(2m) for m = 4 and 6, between 0.3 and 300 K in fields up to 18 T. These compounds show several charge d ensity wave transitions. Large magnetoresistance is found in the charg e-density-wave state for magnetic fields applied perpendicular to the layers. At low temperatures, Shubnikov-de Haas oscillations are attrib uted to the existence of small carrier pockets left by the charge dens ity wave gap opening. The size of these pockets is of the order of a f ew % of the two-dimensional high temperature Brillouin zone and smalle r in the case m = 6 than in m = 4. This is due to a more pronounced lo w-dimensional character and therefore to a better Fermi surface nestin g in the compound m = 6 than in m = 4.