The surface of SrTiO3 (001) is considered to be weakly polar, and in this work, we study the validity of this notion. It exhibits a surface structural distortion, quantified here using low energy electron diffraction at room temperature. Structural analysis shows the presence of strong surface rumpling in the TiO2 terminated surface with the oxygen atoms moving outward and Ti atoms moving inward. Density functional calculations confirm the measured rumpling, and experimental data show the distortion is localized at the surface. Angle-dependent core-level X-ray photoemission spectroscopy (XPS) shows that the surface rumpling strongly impacts the electronic structure of the surface. This observation is reinforced by density functional theory, which demonstrates that the valence state of Ti at the surface is reduced while O is enhanced, where we found the Ti–O bonds are more covalent near the surface. Our results show that surface rumpling is accompanied by a change in the bond hybridization of Ti–O at the surface. Changes in the XPS satellite structures at the surface are consistent with this picture of the change in bonding, indicating that the (001) surface of SrTiO3 is not polar and charge rearrangement is a consequence of surface rumpling.

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Introducing a fluid microjet into the boundary layer to increase fluid momentum and hence delay separation is a method for actively controlling a flow separation region. The present work numerically analyzed the control of a separation bubble behind a ramp. For this purpose, we first verified the steady-state numerical results for a flow (without a jet) over the ramp against reliable experimental studies from the literature. Next, the effects of introducing a microjet to the flow were also verified. A jet was then placed at three different distances above the ramp to study its impact on various parameters, including velocities, Reynolds stresses, pressure, vorticity, streamlines, and the separation bubble size. As the jet was moved further back, the jet-induced upwash region grew considerably. Finally, the effects of using three identical jets were studied and compared against those of a single jet. The results indicated that using a three-jet array shrank the separation bubble. Using an array with d/ D = 15 (distance between microjets over microjet diameter) can limit laterally the separation bubble about 2.75 times smaller than a single jet in the z-direction. Also, the employment of the jet managed to decrease the length of the separation zone in the x-direction up to 78%, in the case of Lx/ L1 = 0.0143 (longitudinal distance of microjet from above the ramp over ramp length) and d/ D = 10.

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Abstract:It has been reported theoretically that the intercalation of nitrogen in the voids of the rather open cubic structure of bulk Cu 3 N build up a magnetic structure. In an extended effort to study this system, we have investigated spin polarization in bulk and thin films of nitrogen intercalated Cu 3 N (Cu 3 N 2 ) structure by means of first-principles calculations based on Kohn-Sham density functional theory and ultrasoft pseudopotentials technique. Contrary to the previous study, the results show that after an accurate structural relaxation of the system, magnetism in the bulk structure vanishes. This effect is due to the migration of the intercalated nitrogen atom from the body center of the cell to the nearness of one of the cell faces. Similar study for the thin films of 5, 7, 9 and 11 2 monolayers thickness was performed and it was found that initial relaxation of structures with 7 and 11 monolayers show a net magnetic moment of 2.6 µ B . By a more extended survey of the energy surfaces, the film with 7 monolayers loses its magnetic moment similar to the bulk structure but the film with 11 monolayers maintains its magnetic moment. It is possibly a new quantum size effect that keeps the intercalated nitrogen atom of the middlemost cell at the body center site. Electron density map of this film clearly confirms the spin polarization upon the intercalated atom.