238 research outputs found
Electronic Properties of Mn-Compounds Under Strain
We study the physical properties of MnAs under strain by using accurate
first-principles pseudopotential calculations. Our results provide new insight
on the physics of strained multilayer that are grown epitaxially on different
lattice mismatched substrates and which are presently of interest for
spintronic applications. We compute the strain dependence of the structural
parameters, electronic bands, density of states and magnetization. In the
region of strain/stress that is easily directly accessible to measurements, the
effects on these physical quantities are linear. We also address the case of
uniaxial stress inducing sizeable and strongly non linear effects on electronic
and magnetic properties.Comment: 8 pages, 6 figure
Band offsets and stability of BeTe/ZnSe (100) heterojunctions
We present ab-initio studies of band offsets, formation energy, and stability
of (100) heterojunctions between (Zn,Be)(Se,Te) zincblende compounds, and in
particular of the lattice-matched BeTe/ZnSe interface. Equal band offsets are
found at Be/Se and Zn/Te abrupt interfaces, as well as at mixed interfaces, in
agreement with the established understanding of band offsets at isovalent
heterojunctions. Thermodynamical arguments suggest that islands of non-nominal
composition may form at the interface, causing offset variations over about 0.8
eV depending on growth conditions. Our findings reconcile recent experiments on
BeTe/ZnSe with the accepted theoretical description.Comment: RevTeX 5 pages, 3 embedded figure
Interaction of carbon monoxide with Cu nanoclusters grown on alumina surface
The present work addresses the interaction of carbon monoxide with copper nanoclusters supported on an ultrathin alumina film grown on the Ni3Al(111) termination, acting as a template for a highly ordered nucleation. Through accurate
quantum-mechanical calculations combined with experimental data, it has been found that the dissociation of carbon monoxide occurs at the copper nanoclusters, at variance with extended surfaces. The detailed mechanism is explained at the atomic level, unveiling the effects of cluster finite size, reconstruction, support, and carbon monoxide coverage. The small size of the nanoclusters allows to achieve an exceptionally high local concentration of molecules at the cluster surface, considerably higher than the saturation limit for the single crystal surfaces. The high coverage facilitates the dissociation of the molecules, accompanied by carbon incorporation into the particles. We discuss the possibility of using other transition metals for an optimal seeding of the supported nanoparticles. In agreement with empirical findings, Pd is confirmed to be the best choice for a highly ordered nucleation
Cross-sectional imaging of sharp Si interlayers embedded in gallium arsenide
We investigate the electronic properties of the (110) cross-sectional surface
of Si-doped GaAs using first-principles techniques. We focus on doping
configurations with an equal concentration of Si impurities in cationic and
anionic sites, such as occurring in a self-compensating doping regime. In
particular we study a bilayer of Si atoms uniformly distributed over two
consecutive (001) atomic layers. The simulated cross-sectional scanning
tunneling microscopy images show a bright signal at negative bias, which is
strongly attenuated when the bias is reversed. This scenario is consistent with
experimental results which had been attributed to hitherto unidentified Si
complexes.Comment: 10 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
Floating bonds and gap states in a-Si and a-Si:H from first principles calculations
We study in detail by means of ab-initio pseudopotential calculations the
electronic structure of five-fold coordinated (T_5) defects in a-Si and a-Si:H,
also during their formation and their evolution upon hydrogenation. The
atom-projected densities of states (DOS) and an accurate analysis of the
valence charge distribution clearly indicate the fundamental contribution of
T_5 defects in originating gap states through their nearest neighbors. The
interaction with hydrogen can reduce the DOS in the gap annihilating T_5
defects.Comment: To appear in Europhysics Let
Coordination defects in a-Si and a-Si:H : a characterization from first principles calculations
We study by means of first-principles pseudopotential method the coordination
defects in a-Si and a-Si:H, also in their formation and their evolution upon
hydrogen interaction. An accurate analysis of the valence charge distribution
and of the ``electron localization function'' (ELF) allows to resolve possible
ambiguities in the bonding configuration, and in particular to identify clearly
three-fold (T_3) and five-fold (T_5) coordinated defects. We found that
electronic states in the gap can be associated to both kind of defects, and
that in both cases the interaction with hydrogen can reduce the density of
states in the gap.Comment: To appear in Philos. Ma
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