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4 years ago

Quick reference

Quick reference

Restart the program and

Restart the program and press PhGr Load. The phasegrating data appear on the screen. Now press Restore parameters from LOAD. The data will disappear. Pressing File you will see that a new parameter-file has been created (which contains parameters exactly identical with those found in the step3-file), and all parameters are set identical to those in the step3-file. To return to the old parameter-file, press Load and choose “STEM_parameters.m”. SAVE/EXIT. 5. Intensity on the detector for probing a point In this mode the electron probe can be set at a given position with respect to the crystal unit cell. The crystal unit cell is marked with a yellow frame in the output of the phasegrating step 3. To choose the bright Ti-position in the center of the crystal unit cell, press Point Par and enter coordinates 0.5 0.5 as shown below: The “Save results” check-button is selected, which means that a “step 7”-file will be created and the results can be reloaded if required. Done Select the Point step only and press Doit The multislice calculation is performed and the result appears on the graphics panel: The upper graphs show the probe intensity and the intensity of the object exit wave function, the upper right graph gives the intensity plotted vs. the specimen thickness. The lower left graph compares coherent intensity in the wave-function (“coherent”), HA-TDS (thermally diffuse scattered intensity on the HAADF-detector), HA-coh (coherent intensity on the HAADF-detector) and coh+HA-TDS (total intensity on the detector).

The image in figure 2 depicts a color-coded map of the intensity distribution on the detectorplane (Fourier-transform of object-exit wave function), only showing the coherent (Braggscattered) intensity, but not the the TDS-intensity. The distribution of the TDS-intensity in k-space cannot be obtained using the absorptive potential approximation, as only the integral TDS-intensity on the detector ca be obtained. To see the distribution of TDS-intensity, the frozen phonon-approximation must be used. However, to obtain a rough idea on the distribution of intensity on the detector, the detector can be subdivided into N subrings. For that, one may press Detector and choose e.g. 20 “number of rings”. Then, select the steps “LUT”, “PhGr” and “Point” and press “Doit”. You will experience, that the calculations take a longer time, because 20 TDS-absorptive potentials are computed, each is connected with a certain subring of the detector. Finally, the graphic panel contains 2 further graphs in the lower row giving information on the angular distribution of the intensity on the detector: