Network equilibration and first-principles liquid water.

Motivated by the very low diffusivity recently found in ab initio simulations of liquid water, we have studied its dependence with temperature, system size, and duration of the simulations. We use ab initio molecular dynamics (AIMD), following the Born-Oppenheimer forces obtained from density-functional theory (DFT). The linear-scaling capability of our method allows the consideration of larger system sizes (up to 128 molecules in this study), even if the main emphasis of this work is in the time scale. We obtain diffusivities that are substantially lower than the experimental values, in agreement with recent findings using similar methods. A fairly good agreement with D(T) experiments is obtained if the simulation temperature is scaled down by approximately 20%. It is still an open question whether the deviation is due to the limited accuracy of present density functionals or to quantum fluctuations, but neither technical approximations (basis set, localization for linear scaling) nor the system size (down to 32 molecules) deteriorate the DFT description in an appreciable way. We find that the need for long equilibration times is consequence of the slow process of rearranging the H-bond network (at least 20 ps at AIMDs room temperature). The diffusivity is observed to be very directly linked to network imperfection. This link does not appear an artifact of the simulations, but a genuine property of liquid water

Growth of Twin-Free and Low-Doped Topological Insulators on BaF₂(111)

We demonstrate the growth of twin-free Bi₂Te₃ and Sb₂Te₃ topological insulators by molecular beam epitaxy and a sizable reduction of the twin density in Bi₂Se₃ on lattice-matched BaF₂(111) substrates. Using X-ray diffraction, electron diffraction and atomic force microscopy, we systematically investigate the parameters influencing the formation of twin domains and the morphology of the films, and show that Se- and Te-based alloys differ by their growth mechanism. Optimum growth parameters are shown to result in intrinsically low-doped films, as probed by angle-resolved photoelectron spectroscopy. In contrast to previous approaches in which twin-free Bi₂Se₃ films are achieved by increasing the substrate roughness, the quality of our Bi₂Te₃ is superior on the flattest BaF₂ substrates. This finding indicates that, during nucleation, the films not only interact with the topmost atomic substrate layer but also with buried layers that provide the necessary stacking information to promote a single twin, an observation that is supported by <i>ab initio</i> calculations

Legislative Documents

Also, variously referred to as: House bills; House documents; House legislative documents; legislative documents; General Court documents