Charge-density-wave instabilities in the low-dimensional molybdenum bronzes and oxides

The molybdenum bronzes A0·30MoO3, A0·9Mo6O17 and Mo4O11 oxides all show low-dimensional electronic properties, due to anisotropic crystal structures, which lead to charge-density-wave (CDW) instabilities. In the CDW states of the quasi-one-dimensional blue bronzes, non-linear transport properties are attributed to the sliding of the CDW. A lock-in transition from an incommensurate to a commensurate CDW is found in the best crystals. The non-linear properties are associated with noise voltage at both low (around 1 Hz) and high (around 10 kHz) frequencies as well as with hysteresis and time-dependent phenomena, showing the importance of metastability. The purple bronzes A0·9Mo6O17 and the oxides η-MO4O11 and γ-Mo4O11 have quasi-two-dimensional properties and quasi-cylindrical Fermi surfaces. In the CDW state, galvano-magnetic studies indicate the presence of small electron and/or hole pockets induced by the CDW gap openings. In the bronze Li0·9Mo6O17, the low-temperature properties reflect the competition between the CDW instability and superconductivity.