Charge density wave pinning and disorder in two dimensions

Abstract

The structures of the two-dimensional incommensurate charge density wave (CDW) in Nb-doped 1T-TaS2 (NbxTa1-xS2) have been elucidated by scanning tunneling microscopy and quantitative image analysis procedures. Analyses of the STM images demonstrate that Nb impurities introduce topological defects, dislocations, into the CDW lattice. Quantitative analysis of the density of dislocations and comparisons of this data with theoretical scaling arguments demonstrate unambiguously that the pinning of the CDW by Nb impurities is weak. In addition, the structure factor, radial distribution function, and translational and orientational correlation functions have been investigated as a function of impurity concentration. Calculations of the translational correlation function demonstrate that translational order decays exponentially over several lattice constants for x(Nb) > 0. In contrast, calculations of the orientational correlation function show that the orientational order of the system is long range for 0 < x(Nb) ≤ 0.04, while orientational order decays within a few lattice constants for x(Nb) ≥ 0.07. These data suggest that the CDW lattice evolves continuously from a crystalline solid, x(Nb) = 0, to a hexatic glass, 0 < x(Nb) ≤ 0.04, and finally to an amorphous state, x(Nb) ≥ 0.07. Comparison of these results with equilibrium melting in 2D is discussed. © 1993 American Chemical Society.