Introduction

It is clear from the wealth of transport measurements involving both thin films [11.1–11.3] and bulk materials [11.4] that grain boundaries have a strong effect on the transport properties of high Tc materials. Perhaps the most likely source of this effect is the small superconducting coherence length (5–15 Å), which makes the high Tc superconductors extremely sensitive to defects that distort the perfect crystal structure. Electron microscopy is the only experimental technique capable of analyzing these defects on the scale of the coherence length. In particular, the scanning transmission electron microscope (STEM) allows both Z-contrast imaging and electron energy loss spectroscopy (EELS) to be performed with atomic resolution (∼2.2 Å). By using these techniques it is possible to relate the changes in hole concentration, measured from the energy loss spectrum, with defined atomic locations at grain boundaries observed in the image [11.5,11.6]. Such a combination of techniques therefore provides insight into the structure–property relationships of grain boundaries at the fundamental atomic level. In this chapter, the application of these techniques to investigate the dramatic changes in carrier concentrations associated with grain boundaries in YBa2Cu3O7–δ will be discussed.

Imaging and microanalysis of boundary structures

The specimens used in this study are laser ablated thin films of YBa2Cu3O7–δ (YBCO) [11.7] grown on SrTiO3 bicrystal substrates. This method of boundary preparation has been chosen as it is known to produce clean grain boundaries, i.e. free from impurity segregation or copper enrichment, in which the oxygen stoichiometry can be well controlled through annealing. Such specimens therefore contain the inherent limits to the superconducting properties of YBCO grain boundaries, which can only be degraded further by copper enrichment and oxygen deficiency.