Cell-by-cell mapping of carrier concentrations in high-temperature superconductors

The correlation between atomic and electronic structure in the vicinity of grain boundaries in YBa₂Cu₃O_7- is investigated on the scale of the coherence length, using a combination of Z-contrast imaging and electron energy loss spectroscopy in the scanning transmission electron microscope. The detector arrangement of the microscope enables both techniques to be performed simultaneously, allowing the exact crystallographic location of the probe to be determined from the image. As the pre-edge feature of the oxygen K-absorption edge is very sensitive to the partial density of unoccupied states, it can be calibrated to give an accurate measure of local hole concentrations (±5%). Spatial resolution is limited only by the 2.2 Å probe size, thus allowing the electronic properties of the material to be inferred on a scale less than the coherence length. Results are discussed from the study of YBa₂Cu₃O_7- films prepared by laser ablation on yttria-stabilized-zirconia (YSZ) substrates, where a symmetric (near 5) grain boundary shows no significant hole depletion in the boundary region, while an asymmetric (near 17) boundary shows depletion extending over a region far greater than the atomic disorder shown in the image. Application of this technique to the study of interfaces in other superconducting systems is discussed.