Magnetotransport and thermopower properties of the quasi-two-dimensional charge-density-wave compounds (PO2)(4) (WO3)(2m) (m=4,6)

Magnetoresistance, Hall effect, and thermopower have been measured between 4.2 and 300 K on the monophosphate tungsten bronzes (PO2)(4)(WO3)(2m), with m=4 and 6. The Peierls transitions associated with charge-density-wave gap openings appear clearly on all explored properties. In the normal state, both compounds behave as nearly compensated metals, the transport and cyclotron carriers having different signs as a consequence of the complex morphology of the Fermi surface, in agreement with previous band-structure calculations. The anisotropic thermopower found in the charge-density-wave state of the m=6 compound gives information on the carrier pockets left by the gap openings along different crystallographic directions. This result is discussed in relation to previous x-ray determination of the nesting wave vectors. A two-band model used to evaluate the sizes of these pockets shows that they are smaller in the m=6 than in the m=4 phase. This finding is attributed to a stronger low-dimensional character in the m=6 material, which leads to better nesting properties of the Fermi surface.