VARIOUS REGIMES OF CHARGE-DENSITY WAVES IN LAYERED COMPOUNDS

In this paper we have subjected different layered transition-metal dichalcogenides 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 a localized CDW centered around defects or impurities in the case of 1T-TiS2, via an intermediate regime governed by overlapping envelope functions in 2H-NbSe2, to a fully developed CDW in 1T-TaSe2 (as well as in a large number of other compounds). We have obtained strong indication from both direct-space observations and comparison to calculated images based upon the Ginzburg-Landau theory that localized charge-density waves do exist in conjunction with associated lattice distortions. The fact that these observations have been made in solids ranging from (dirty) semi-conductor (1T-TiS2) to semimetal (1T-TaSe2) to metallic (2H-NbSe2) points to 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.