Distortions dynamic scattering

Large differences between experiment and theory are often the indication of systematic errors, such, deficiency in theoretical model (as in the case kinematics approximation for dynamically scattered electrons) an measurement artifacts (such as uncorrected distortions). 

The scattering from a distorted surface region is no longer governed by the dynamical scattering in the bulk of the crystal. In the bulk, due to multiple scattering processes, the refractive index is not quite unity and the result is that... 

We have so far discussed only materials of negative dielectric anisotropy. Electrohydrodynamic distortions are observed even in weakly positive materials,but only when the initial orientation of the director is perpendicular to the applied field. Striations appear above a threshold voltage but vanish at still higher voltages and there is no dynamic scattering. The frequency dependence of the threshold voltage is shown in fig. 3.10.6. 

It is energetically unfavorable to alter the layer thickness of the smectic phase, therefore any process that relies on this feature is very unlikely to occur. However, bending of the smectic layers is possible, because this need not cause a change in the layer thickness thus the molecules can undergo a splay (A, ) distortion. In a smectic film, any point distortion of the layer, e.g., arising from a surface feature, propagates for some distance into the film this feature is important for electrically addressed dynamic scattering devices. Bend ( 33) and twist ( "22) elastic constants approach infinity at the nematic (N) to smectic transition. 

The second major contrast mechanism is extinction contrast. Here the distortion of the lattice arotmd a defect gives rise to a different scattering power from that of the surrotmding matrix. In all cases, it arises from a breakdown or change of the dynamical diffraction in the perfect ciystal. In classical structure analysis, the name extinction was used to describe the observation that the integrated intensity was less than that predicted by the kinematical theoiy. 

Fig. 11. Schematic diagram for (a) the dynamic Jahn-Teller effect and (b) the Moskalenko-Suhl-Kondo mechanism. In the former, the distorted Cg molecule undergoes the tunneling motion between three equivalent configurations. This results in the formation of the orbital-singlet state. In the latter, as shown by the black arrows, the Cooper pairs are transferred from one Fermi sphere to another, which is the pair-transfer process, a remarkable feature of multiband superconductors, and stabilizes the superconductivity. Also, as shown by the white arrows, the Cooper pairs are scattered coherently within each Fermi sphere, which is the pair-scattering process in usual superconductors.

We have presented a sample of resonance phenomena and calculations in reactive and non-reactive three-body systems. In all cases a two-mathematical dimensional dynamical space was considered> leading to a great simplification in the computational effort. For the H-K 0 system, low-energy coupled-channel calculations are planned in the future to test the reliablity of the approximations used here, i.e., the scattering path hamiltonian as well as the distorted wave Born approximation. Hopefully these approximations will prove useful in larger systems where coupled-channel calculations would be prohibitively difficult to do. Such approximations will be necessary as resonance phenomena will continue to attract the attention of experimentalists and theorists for many years. 

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