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

Abstract

SUMMARY 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 TP=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 A, is observed below TP=215 K. Above TP, 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.