Solid-hexatic-liquid phases in two-dimensional charge-density waves

The structural order of the two-dimensional (2D) charge-density-wave (CDW) phase in ${\mathrm{Nb}}_{\mathit{x}}$${\mathrm{Ta}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{S}}_{2}$ materials has been determined as a function of impurity concentration from the quantitative analysis of scanning tunneling microscopy images. We show that the CDW phase evolves through crystalline, hexatic glass, and liquidlike states as the impurity concentration in the lattice increases to x=0.10. These results address systematically the structural manifestations of quenched disorder in 2D systems.