Imaging and microanalysis of boundary structures

The specimens used in this study are laser ablated thin films of YBa2Cu30v (YBCO) [11.7] grown on SrTi03 bicrystal substrates. This method of boundary preparation has been chosen as it is known to produce clean grain boundaries, 

The high-Z atomic column positions at a grain boundaries can therefore be determined directly from the image during the experiment. However, on completion of the microscopy, a more accurate statistical method to obtain the column positions is to recover the Z-contrast object function through maximum entropy image analysis [11.12]. Maximum entropy image analysis is based on 

Bayesian probability theory, and is an iterative technique designed to produce an object function which, when convoluted with the microscope probe, gives the best fit to the experimental image. The iterative process starts with an object function of uniform intensity, which makes no prior assumptions about the structure of the specimen, and moves towards the best fit to the experimental image by calculating the maximum entropy while minimizing the fit with the experimental image (Fig. 11.3). This process allows the coordinates for the atom column positions to be determined to an accuracy of 0.1 A 

A detailed analysis of the electronic states in the pre-edge using dedicated EEL spectrometers has revealed the symmetry of these holes [11.14, 11.15]. These results can be used as a guide in interpreting the spectra obtained from the electron microscope, where the use of circular apertures to form the small probe averages the momentum transfers in all directions and precludes such an analysis. However, it must be stated that although it is not possible to address the symmetry of the hole states in the same detailed manner as by dedicated spectrometers, optimum collection conditions can be deflned which reduce orientation effects on the accuracy of the hole concentration measurement 

Crucial in the use of EELS to study carrier concentrations at grain boundaries in high materials is the ability to position the probe with atomic precision a precision only afforded by the Z-contrast image. In order to achieve with EELS the same atomic resolution as the image, the range over 

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