Asymptotic self-consistency in quantum transport calculations

Ab initio simulations of quantum transport commonly focus on a central region which is considered to be connected to infinite leads through which the current flows. The electronic structure of these distant leads is normally obtained from an equilibrium calculation, ignoring the self-consistent response of the leads to the current. We examine the consequences of this, and show that the electrostatic potential Delta phi is effectively being approximated by the difference between electrochemical potentials Delta mu, and that this approximation is incompatible with asymptotic charge neutrality. In a test calculation for a simple metal-vacuum-metal junction, we find significant errors in the nonequilibrium properties calculated with this approximation, in the limit of small vacuum gaps. We provide a scheme by which these errors may be corrected

Levinson's theorem and scattering phase shift contributions to the partition function of interacting gases in two dimensions

We consider scattering state contributions to the partition function of a two-dimensional (2D) plasma in addition to the bound-state sum. A partition function continuity requirement is used to provide a statistical mechanical heuristic proof of Levinson's theorem in two dimensions. We show that a proper account of scattering eliminates singularities in thermodynamic properties of the nonideal 2D gas caused by the emergence of additional bound states as the strength of an attractive potential is increased. The bound-state contribution to the partition function of the 2D gas, with a weak short-range attraction between its particles, is found to vanish logarithmically as the binding energy decreases. A consistent treatment of bound and scattering states in a screened Coulomb potential allowed us to calculate the quantum-mechanical second virial coefficient of the dilute 2D electron-hole plasma and to establish the difference between the nearly ideal electron-hole gas in GaAs and the strongly correlated exciton/free-carrier plasma in wide-gap semiconductors such as ZnSe or GaN.Comment: 10 pages, 3 figures; new version corrects some minor typo

Blue luminescence from ultrathin GaAs layers embedded in AlAs

Our investigations focus on low-temperature luminescence experiments on a set of type-II GaAs/AlAs multiple-quantum-well (MQW) samples grown by low-pressure metal-organic vapor-phase epitaxy. The layered structures consists of 50 periods of either 2 monolayers (ML), 4, 5, 6, or 7 ML GaAs embedded in 28 ML AlAs. For (001) GaAs substrates, 6 degrees misoriented towards the nearest (111) plane of group-V atoms, monolayer steps at the AlAs/GaAs interfaces with regular terrace widths (2.7 nm) can be seen by high-resolution transmission-electron microscopy. In the photoluminescence spectra of these MQW samples, type-I luminescence is found to be dominant even at room temperature. The peak wavelength of the type-I emission depends strongly on the GaAs layer thickness; it ranges from about 620-440 nm. The intense type-I emission seems to be connected with the interface peculiarities. Our astonishing observation might be explained as follows: (i) The perfect interface structure pl events the loss of photoexcited carriers from GaAs layers to the surrounding AlAs materials, i.e., the energy loss by optical-phonon scattering is reduced. (ii) For our well thicknesses two-dimensional (2D) phonons must be coupled with 3D electrons leading also to a reduction of the electron-phonon interaction. (iii) The regular interface steps should favor a coherent interaction (quantum interferences) of excitons and/or electrons confined in the GaAs wells with energetically resonant continuum states of the AlAs barriers. The experimentally observed optical transition energies of the type-I and type-II recombination are compared with model calculations applying an effective-mass approach and empirical tight-binding Green's-function scheme