Geometric and electronic surface reconstructions determine the physical and chemical properties of surfaces and, consequently, their functionality in applications. The reconstruction of a surface minimizes its surface free energy in otherwise thermodynamically unstable situations, typically caused by dangling bonds, lattice stress, or a divergent surface potential, and it is achieved by a cooperative modification of the atomic and electronic structure. Here, we combined first-principles calculations and surface techniques (scanning tunneling microscopy, non-contact atomic force microscopy, scanning tunneling spectroscopy) to report that the repulsion between negatively charged polaronic quasiparticles, formed by the interaction between excess electrons and the lattice phonon field, plays a key role in surface reconstructions. As a paradigmatic example, we explain the (1 × 1) to (1 × 2) transition in rutile TiO 2 ð110Þ.

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Nanocomposites enable us to tune parameters that are crucial for use of such materials for neutron-optics applications. By careful choice of properties such as species (isotope) and concentration of contained nanoparticles, diffractive optical elements for long-wavelength neutrons are feasible. Nanocomposites for neutron optics have so far been tested successfully in protonated form, containing high amounts of 1 H atoms, which exhibit rather strong neutron absorption and incoherent scattering. At a future stage of development, chemicals containing 1 H could be replaced by components containing more favourable isotopes, such as 2 H or 19 F. In this note, we present results of Monte-Carlo simulations of the transmissivity of various nanocomposite materials for thermal and verycold neutron spectra. Our simulation results for deuterated and fluorinated nanocomposite materials predict the losses due to absorption and scattering to be as low as 2%, as well as the broadening of the beam cross section to be negligible.