Probing complex low-dimensional solids with scanning probe microscopes: From charge density waves to high-temperature superconductivity

Scanning probe microscopy (SPM) studies of low-dimensional solids have yielded significant advances in the understanding of collective phenomena in these complex materials. In this article, SPM studies from the authors' laboratory of charge density waves (CDWs) in metal dichalcogenide materials and of high-temperature copper-oxide superconductors (HTSCs) are reviewed. Temperature-dependent SPM studies of the structural properties of CDW phases in metal-doped tantalum disulfide (TaS₂) and tantalum diselenide (TaSe₂) have directly illuminated the problem of weak and strong CDW pinning. These studies have also led to the unexpected discovery of a hexatic CDW phase in niobium-doped TaS₂. SPMs have also been used to probe the structural and electronic properties of HTSCs. SPM studies of metal-doped Bi₂Sr₂CaCu₂O₈ (BSCCO) superconductors have been used to elucidate the complex crystal chemistry of this system. Comparisons of these data with electron diffraction measurements also highlight the importance of local crystallography in these complex materials. In addition, SPM studies have been used to elucidate the interaction of magnetic flux lines with surface defects, thus enabling a quantitative assessment of pinning by surface roughness. Finally, etching has been studied by in situ SPM and shown to be a promising means for controlling the properties of HTSC surfaces.