@article{BonačićKouteckyPittnerPouAmerigoetal.1997, author = {Bonačić-Kouteck{\´y}, Vlasta and Pittner, Jiř{\i}́ and Pou-Am{\´e}rigo, Rosendo and Hartmann, Michael}, title = {Ab-initio study of structural and optical properties of nonstoichiometric alkalimetal- oxides}, series = {Zeitschrift f{\"u}r Physik D}, volume = {40}, journal = {Zeitschrift f{\"u}r Physik D}, number = {1}, url = {http://nbn-resolving.de/urn:nbn:de:0298-opus4-5748}, pages = {445 -- 447}, year = {1997}, abstract = {Structural and optical properties of nonstoichiometric LinO and NanO (n = 3, 4) clusters containing one and two excess electrons are studied using ab-initio methods accounting for electron correlation. We show that calculated absorption patterns are excellent fingerprints of structural and bonding properties. The optical response of Li4O and Na4O clusters with the most stable tetrahedral type structures is characterized by a common feature, that is the appearance of a dominant intense transition in infrared regime although excess of electrons are not localized, as it is the case for small alkali-halide clusters, with cuboid corner vacancy (surface F-center in finite systems).}, language = {en} } @article{RoeselerHartmann1997, author = {R{\"o}seler, J. and Hartmann, Michael}, title = {An exactly solvable model for the large polaron}, series = {Zeitschrift f{\"u}r Physik B}, volume = {102}, journal = {Zeitschrift f{\"u}r Physik B}, number = {4}, url = {http://nbn-resolving.de/urn:nbn:de:0298-opus4-5782}, pages = {531 -- 540}, year = {1997}, abstract = {We present an exactly diagonalizable model Hamiltonian for the large polaron derived by analyzing the variational ansatz by Haga-Larsen (HL) for the Fr{\"o}hlich Hamiltonian. The lowest energy eigenvalue of the model Hamiltonian for fixed wave numbers reproduces the energy of the variational ansatz by Haga-Larsen and is, therefore, an upper bound with respect to the corresponding energy eigenvalue of the Fr{\"o}hlich Hamiltonian. This is valid for any momentum which is proven by extending the Haga-Larsen approach. Furthermore, since all integrations can be performed analytically, the model Hamiltonian is easily tractable. The energy eigenvalue spectrum of the model Hamiltonian is studied below and above the phonon-emission threshold. The quality of the model Hamiltonian is determined by the variational ansatz of Haga and Larsen. Incorporating an improved energy-momentum relation, a generalized model Hamiltonian is derived possessing a larger validity range with respect to the coupling strength. Furthermore, a second exactly diagonalizable model Hamiltonian based on improved Wigner-Brillouin perturbation theory due to Warmenbol, Peeters, and Devreese (WPD) is presented. It is briefly demonstrated that one is able to construct all mentioned model Hamiltonians also in the 2D polaron problem. In contrast to the 3D case, where the HL-type model Hamiltonian possesses the higher quality for any momentum, in the 2D case, it works well only for small momenta. For large momenta, only the WPD-type model Hamiltonian describes the energy-momentum relation correctly. We demonstrate the usefulness of the model Hamiltonian concept by exactly calculating the one-electron Green's function for all mentioned model Hamiltonians and comment why significant advantages of the model Hamilton concept for the treating of low-dimensional systems (planar semiconducting quantum-well structures) can be expected.}, language = {en} }