333 research outputs found
Structural Examination of Au/Ge(001) by Surface X-Ray Diffraction and Scanning Tunneling Microscopy
The one-dimensional reconstruction of Au/Ge(001) was investigated by means of
autocorrelation functions from surface x-ray diffraction (SXRD) and scanning
tunneling microscopy (STM). Interatomic distances found in the SXRD-Patterson
map are substantiated by results from STM. The Au coverage, recently determined
to be 3/4 of a monolayer of gold, together with SXRD leads to three
non-equivalent positions for Au within the c(8x2) unit cell. Combined with
structural information from STM topography and line profiling, two building
blocks are identified: Au-Ge hetero-dimers within the top wire architecture and
Au homo-dimers within the trenches. The incorporation of both components is
discussed using density functional theory and model based Patterson maps by
substituting Germanium atoms of the reconstructed Ge(001) surface.Comment: 5 pages, 3 figure
ZnSe/GaAs(001) heterostructures with defected interfaces: structural, thermodynamic and electronic properties
We have performed accurate \emph{ab--initio} pseudopotential calculations for
the structural and electronic properties of ZnSe/GaAs(001) heterostructures
with interface configurations accounting for charge neutrality prescriptions.
Beside the simplest configurations with atomic interdiffusion we consider also
some configurations characterized by As depletion and cation vacancies,
motivated by the recent successfull growth of ZnSe/GaAs pseudomorphic
structures with minimum stacking fault density characterized by the presence of
a defected (Zn,Ga)Se alloy in the interface region. We find that--under
particular thermodynamic conditions--some defected configurations are favoured
with respect to undefected ones with simple anion or cation mixing, and that
the calculated band offsets for some defected structures are compatible with
those measured. Although it is not possible to extract indications about the
precise interface composition and vacancy concentration, our results support
the experimental indication of (Zn,Ga)Se defected compounds in high-quality
ZnSe/GaAs(001) heterojunctions with low native stacking fault density. The
range of measured band offset suggests that different atoms at interfaces
rearrange, with possible presence of vacancies, in such a way that not only
local charges but also ionic dipoles are vanishing.Comment: 26 pages. 5 figures, revised version, in press (Physical Review B
Electronic excitations stabilized by a degenerate electron gas in semiconductors
Excitons in semiconductors and insulators consist of fermionic subsystems, electrons and holes, whose attractive interaction facilitates bound quasiparticles with quasi-bosonic character. In the presence of a degenerate electron gas, such excitons dissociate due to free carrier screening. Despite their absence, we found pronounced emission traces in the below-band-edge region of bulk, germanium-doped GaN up to a temperature of 100 K, mimicking sharp spectral features at high free electron concentrations (3.4E19–8.9E19 cm−3). Our interpretation of the data suggests that a degenerate, three-dimensional electron gas stabilizes a novel class of quasiparticles, which we name collexons. These many-particle complexes are formed by exchange of electrons with the Fermi gas. The potential observation of collexons and their stabilization with rising doping concentration is enabled by high crystal quality due to the almost ideal substitution of host atoms with dopants.DFG, 43659573, SFB 787: Semiconductor Nanophotonics: Materials, Models, Device
Field-induced delocalization and Zener breakdown in semiconductor superlattices
We investigate the energy spectrum and the electron dynamics of a band in a semiconductor superlattice as a function of the electric field. Linear optical spectroscopy shows that, for high fields, the well-known localization of the Bloch states is followed by a field-induced delocalization, associated with Zener breakdown. Using time-resolved measurements, we observe Bloch oscillations in a regime where they are damped by Zener breakdown
Frenkel and charge transfer excitons in C60
We have studied the low energy electronic excitations of C60 using momentum
dependent electron energy-loss spectroscopy in transmission. The momentum
dependent intensity of the gap excitation allows the first direct experimental
determination of the energy of the 1Hg excitation and thus also of the total
width of the multiplet resulting from the gap transition. In addition, we could
elucidate the nature of the following excitations - as either Frenkel or charge
transfer excitons.Comment: RevTEX, 3 Figures, to appear in Phys. Rev.
Self-regulated charge transfer and band tilt in nm-scale polar GaN films
To date, the generic polarization of Bernardini, Fiorentini and Vanderbilt
(PBFV) has been widely used to address the issue of polarity in III-V nitride
semiconductors, but improvements in nitride materials and the performance of
optoelectronic devices have been limited. The current first-principles
calculation for the electronic structures of nm-scale [0001] GaN films show
that the internal electric fields and the band tilt of these films are in
opposite direction to those predicted by PBFV. Additionally, it is determined
that an intrinsic self-regulated charge transfer across the film limits the
electrostatic potential difference across the film, which renders the local
conduction band energy minimum (at the Ga-terminated surface) approximately
equal to the local valence band energy maximum (at the N-terminated surface).
This effect is found to occur in films thicker than ~4nm
Photoemission Beyond the Sudden Approximation
The many-body theory of photoemission in solids is reviewed with emphasis on
methods based on response theory. The classification of diagrams into loss and
no-loss diagrams is discussed and related to Keldysh path-ordering
book-keeping. Some new results on energy losses in valence-electron
photoemission from free-electron-like metal surfaces are presented. A way to
group diagrams is presented in which spectral intensities acquire a
Golden-Rule-like form which guarantees positiveness. This way of regrouping
should be useful also in other problems involving spectral intensities, such as
the problem of improving the one-electron spectral function away from the
quasiparticle peak.Comment: 18 pages, 11 figure
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