Eckehard Schöll

Chaotic front dynamics in semiconductor superlattices

Institut fu¨r Theoretische Physik, Technische Universit¨at Berlin, Hardenbergstrasse 36, 10623, Berlin, Germany(Dated: 11.12.2001)We analyze the dynamical evolution of the current and the charge density in a superlattice forfixed external dc voltage in the regime of self-sustained current oscillations, using a microscopicsequential tunneling model. Fronts of accumulation and depletion layers which are generated atthe emitter may collide and annihilate, thereby leading to a variety of different scenarios. We findcomplex chaotic regimes at high voltages and low contact conductivities.

Density-functional theory and quantum chemistry studies on "dry" and "wet" NaCl(001)

Common salt, NaCl, is a everyday material that we are all familiar with. It has been of fascination and played a central role in society since at least the middle ages, when it was known as “white gold” because of its essential role as a food preservative. However, many fundamental physical and chemical properties of NaCl and in particular NaCl surfaces remain unanswered. Given the relevance of NaCl surfaces to physical processes in the upper atmosphere or the importance of NaCl/water in biophysics, there is an increasing need to better understand salt surfaces. This thesis tackles the issue of the properties of salt surfaces with the application of a variety of electronic structure techniques. To begin, the properties of bulk NaCl, NaCl(001), and defects on NaCl(001) surfaces have been examined with density-functional theory within the plane-wave pseudopotential approach. Aiming to remedy the lack of quantitative energetic and structural knowledge of such surfaces, several DFT exchange-correlation functionals are employed to determine the surface energy and surface structure. A range of 9–15 meV/Å is obtained for the surface energy of NaCl(001), and the surface is predicted to undergo only small relaxations of the top layer atoms. The isolated step formation energy of monoatomic (100)-like steps on NaCl(001) is estimated to be about 40–60 meV/Å and the interaction energy between adjacent steps is weak. The ab initio atomistic thermodynamics has been employed to determine the relative stabilities of stoichiometric (100)-like and non-stoichiometric (111)-like steps on NaCl(001), revealing that (100)-like steps are significantly more stable than (111)-like steps at all accessible values of the chlorine chemical potential. In addition to these DFT studies, we have used Møller-Plesset perturbation theory to examine the adsorption of halogen atoms on several alkali halides surfaces. To some surprise, these studies indicate that the halogen atoms bind preferentially to halide substrate atoms on a series of alkali halide surfaces, rather than to the alkali atoms as might be anticipated. An analysis of the electronic structures in each system reveals that this novel adsorption mode is stabilized by the formation of a textbook two-center three-electron covalent bond. Finally, water adsorption on NaCl(001) is examined. It is shown that the adsorption energy is very sensitive to the exchange-correlation functionals employed yielding values that range from 280 meV to 640 meV. Considerable effort is then employed with MP2 and CCSD(T) to arrive at a reliable estimate of the adsorption energy of 517 meV. This represents one of the most reliable theoretical estimates of an adsorption energy on a solid surface obtained so far. In the end, a careful analysis of the electronic structure of the water/NaCl(001) adsorption system reveals the nature of the adsorption bond.