Structural Characterization via Transmission Electron Microscopy (TEM)
Epitaxy and microstructure are important properties of high temperature superconductors that directly impact their current carrying capabilities. For example, microstructural defects can improve supercurrent carrying capacity by providing normal state regions by which magnetic vortices are �pinned�. Because defects or non-superconducting regions in a type-II superconductor form energetically favorable penetration sites for magnetic vortices, an attractive force between the magnetic field line and the defect results, and the supercurrent impeding motion of vortices is dampened. Therefore, to improve current carrying properties of materials such as YBa2Cu3Ox (YBCO), analysis and understanding of defect properties and mechanisms is crucial.
Since it is desirable that such defects are on the order of a coherence length (?ab ~ 2 nm in YBCO), transmission electron microscopy (TEM) is a valuable technique for characterization of the microstructure. High resolution TEM can characterize crystals to very small dimensions through both imaging and diffraction. TEM characterization of defects that are either intentionally inserted (such as nanoparticles) or those that result from other structural modification (such as substrate-level surface modulations) gives direct evidence of their size and extent through the YBCO film, and gives some understanding of the mechanisms responsible for interesting effects such as the porous films observed by our group.
Another advantage of TEM characterization is that cross-sectional images can be obtained, thus showing the physical properties through the thickness of the film. For example, the image below shows of a cross section of a vicinal YBCO film on a miscut SrTiO3 substrate. Pores can be seen to extend through a significant portion of the film thickness. This illustrates an example of microstructurally modifying YBCO for the enhancement of its critical current.