The surfaces of Mo bicrystals studied by low-energy ion scattering

Molybdenum bicrystals having two crystallographic orientations at the surface were studied by low-energy ion scattering (LEIS). Since the surfaces are part of the same specimen, their history and treatments are identical and thus provide an ideal possibility to compare segregation and annealing processes and to verify the quantification of LEIS. The crystallographic orientation of the seeds, the grown bicrystals and the bicrystalline samples was checked by means of X-ray Laue patterns as well as LEED patterns. The misorientation angles were about 1–2°. Because of the high surface sensitivity of LEIS, the scattered ion signals for the Mo(1 1 0)/Mo(1 0 0) bicrystal grains should reflect the atomic densities of the outermost atomic layers of their surfaces. The atomic density of the (1 0 0) surface is found to be 76% of that of the (1 1 0) surface. The difference with the theoretical value for the outer surface (71%) is ascribed to a small contribution of the second atomic layer for the open (1 0 0) surface. For the faces of a Mo(1 1 0)/Mo(1 1 0) bicrystal identical signals are found. At higher ion doses the bombardment leads to (partial) amorphization of the surface. It is found that the annealing of this layer starts already at 700 K for the (1 1 0), but at 1300 K for the (1 0 0) surface. A strong carbon segregation is already found at 1100 K for Mo(1 0 0), while there is still no segregation for the (1 1 0) surface. The results are explained in terms of differences in surface free energies, atomic mobilities and crystal structure

the interplay between cerebellum and basal ganglia in motor adaptation: A modeling study

Motor adaptation to perturbations is provided by learning mechanisms operating in the cerebellum and basal ganglia. The cerebellum normally performs motor adaptation through supervised learning using information about movement error provided by visual feedback. However, if visual feedback is critically distorted, the system may disengage cerebellar error-based learning and switch to reinforcement learning mechanisms mediated by basal ganglia. Yet, the exact conditions and mechanisms of cerebellum and basal ganglia involvement in motor adaptation remain unknown. We use mathematical modeling to simulate control of planar reaching movements that relies on both error-based and non-error-based learning mechanisms. We show that for learning to be efficient only one of these mechanisms should be active at a time. We suggest that switching between the mechanisms is provided by a special circuit that effectively suppresses the learning process in one structure and enables it in the other. To do so, this circuit modulates learning rate in the cerebellum and dopamine release in basal ganglia depending on error-based learning efficiency. We use the model to explain and interpret experimental data on error- and non-error-based motor adaptation under different conditions. © 2019 Todorov et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited

Interaction of low energy protons, deuterons,

We have performed measurements of energy-angle distributions for low energy (E ≲ 10 keV) hydrogen ions transmitted through thin amorphous carbon foils. The applicability of standard potentials for the description of the multiple scattering is tested by making comparisons of the experimental results with theoretical calculations using the model of Sigmund and Winterbon on a broad angular scale. The angular dependence of the energy loss for protons and deuterons is analyzed using the three-components model of Famá et al., that separately considers the contribution of elastic and inelastic mechanisms and the roughness effect. Additionally, the velocity dependence of the stopping power and the possible existence of isotopic and molecular effects in the energy loss is investigated by measurements with H+, D+, \hbox{${\rm H}_2^+$} and \hbox{${\rm D}_2^+$} beams of velocities between 0.15 and 0.6 a.u