Scanning Tunneling Microscopy on Charge Density Waves in Layered Compounds

Different layered transition metal dichalcogenides were subjected to scanning tunneling microscopy to reveal the electronic charge distribution associated with the charge density wave (CDW) part of the superstructure, in addition to the atomic corrugation. The observations presented display three regimes ranging from localized CDW centred around defects/impurities in the case of lT-TiS2, via an intermediate regime governed by overlapping envelope functions in 2H-NbSe2, to a fully developed CDW system in 1T-TaSe2 (as well in a large number of other compounds). The fact that these observations have been made in solids ranging from (dirty) semiconductor (1T-TiS2) to semimetal (1T-TaSe2) to metallic (2H-NbSe2) points at the general applicability of the phenomenological Ginzburg-Landau theory, employed to describe the various regimes in which the formation of charge density waves and the accompanying periodic lattice distortions appear to act.