Electron backscattering on single-wall carbon nanotubes observed by scanning tunneling microscopy

– Single-wall carbon nanotubes, seamless cylindrical molecules formed from a graphene sheet, are either conducting or semiconducting, depending on the particular “wrapping vector” that defines the waist of the tube. Scanning tunneling microscopy experiments have tested this idea by simultaneously measuring a tube’s lattice structure and electronic properties. Here we present a series of STM images of single-wall carbon nanotubes with a strikingly rich set of superstructures. The observed patterns can be understood as due to interference between propagating electron waves that are reflected from defects on the tube walls and ends, or as intrinsic to states propagating on semiconducting tubes. The measured broken symmetries can be used to directly probe electronic backscattering on the tube and provide a key element in the understanding of low-energy electron transport on these structures. Introduction. – Single-wall carbon nanotubes (SWNTs) [1-3] are thought to be almost ideal one-dimensional quantum systems. This property is a direct consequence of the formation of a cylinder from a perfect two-dimensional “graphene” sheet. Such sheets can also pack in a well-ordered ABAB stacking sequence to form highly ordered pyrolytic graphite (HOPG). Scanning tunneling microscopy (STM) studies of the surface of this layered crystal [4] find that the low-energy band structure determines the observed contrast, rather than the atomic configuration alone. In particular, the collapse of the Fermi surface into six “K” points at the Brillouin zone edge [5] results in STM images where only the periodicity of the two-atom unit cell is seen [6]. If the few available electronic states around the Fermi energy EF are distorted by elastic scattering at defect sites, the STM contrast can change locally to more complex patterns with larger but commensurate wavelengths [7, 8]. By wrapping a graphene sheet into a one-dimensional quantum cylinder, the general shape of the electronic bandstructure (∗) Permanent address: Institute of Applied Physics, University of Tuebingen, Germany. E-mail: [email protected]