Scanning tunnelling microscopy of 1‐D and 2‐D charge‐density wave systems

We have used a new variable temperature scanning tunnelling microscope (STM) to study quasi‐1D and 2‐D charge‐density wave (CDW) systems. The 1‐D systems, typified by NbSe₃ and TaS₃, are of special interest since they exhibit unusual transport phenomena associated with moving CDW above a threshold electric field. In the case of NbSe₃, room temperature STM images show both major and subtle details of the lattice structure. At present, however, images taken below the Peierls transition temperature of T_P=144 K resolve major lattice details but are not sufficiently clear to resolve the CDW. On the other hand, for the fully gapped CDW system orthorhombic‐TaS₃, the CDW modulation superimposed on the lattice structure and having the correct period of four times the S‐S spacing of 3·3 Å, is observed below T_P=215 K. Above T_P, the main observable feature is the S‐S spacing along the chains. STM measurements have also been performed on the 2‐D CDW system 1T‐TaS₂ in its incommensurate, nearly commensurate, fully commensurate and trigonal phases. For the nearly commensurate phase, STM images show uniform commensurability with a relatively low concentration of small, time‐varying discommensurations in contrast to models pradicting a regular domain structure. In the trigonal phase, however, evidence is seen for the striped phase composed of long, nearly parallel discommensurations.