Effects of defects on charge density waves in layered dichalcogenides

We have measured the Charge Density Wave (CDW) onset temperature, T₀, and the normal state resistivity increase, dϱ/dc, in the layered compounds 2H-NbSe₂, 2H-TaS₂ and 1T-VSe₂ containing a well controlled concentration, c, of irradiation-induced defects. When T₀ increases from 35 K (2H-NbSe₂) to 110 K (1T-VSe₂), — dT₀/dc decreases from 110 to 20 K/%, in spite of the fourfold increase of dθ/dc. This correlation cannot be explained by supposing that a common defect scattering rate controls both T₀ and ϱ. Electron diffraction results give evidence of CDW distortions even in the absence of macroscopic onset transitions indicating further that the main effect of defects is to destroy the phase coherence of the CDW instead of its amplitude.