Melting of Charge Density Waves in Low Dimensions

Charge density waves (CDWs) are collective electronic states that can reshape and melt, even while confined within a rigid atomic crystal. In two dimensions, melting is predicted to be distinct, proceeding through partially ordered nematic and hexatic states that are neither liquid nor crystal. Here we measure and explain how continuous, hexatic melting of incommensurate CDWs occurs in low-dimensional materials. As a CDW is thermally excited, disorder emerges progressively---initially through smooth elastic deformations that modulate the local wavelength, and subsequently via the nucleation of topological defects. Experimentally, we track three hallmark signatures of CDW melting---azimuthal superlattice peak broadening, wavevector contraction, and integrated intensity decay.