Contrast reversal of the charge density wave STM image in purple potassium molybdenum bronze K0.9Mo6O17

We report on a scanning tunneling microscopy (STM) investigation of the charge density wave (CDW) state of a layered oxide compound, the purple bronze K0.9Mo6O17 The experiments have been carried out in an ultra-high-vacuum variable-temperature STM. At low temperature, we have observed a (2X2) superstructure due to the CDW state, superimposed on the atomic lattice. Spectroscopic measurements reveal a strong decrease of the density of states near the Fermi energy, consistent with the suppression of a sizable fraction of the Fermi surface below the CDW transition temperature. Finally, by simultaneous imaging at negative and positive biases, we show a complete spatial separation between occupied and empty states involved in the CDW modulation, resulting in a contrast reversal of the CDW images. [S0163-1829(99)03127-6].