1,423 research outputs found
Linear and planar molecules formed by coupled P donors in silicon
Using the effective mass theory and the multi-valley envelope function
representation, we have developed a theoretical framework for computing the
single-electron electronic structure of several phosphorus donors interacting
in an arbitrary geometrical configuration in silicon taking into account the
valley-orbit coupling. The methodology is applied to three coupled phosphorus
donors, arranged in a linear chain and in a triangle, and to six donors
arranged in a regular hexagon. The results of the simulations evidence that the
valley composition of the single-electron states strongly depends on the
geometry of the dopant molecule and its orientation relative to the
crystallographic axes of silicon. The electron binding energy of the triatomic
linear molecules is larger than that of the diatomic molecule oriented along
the same crystallographic axis, but the energy gap between the ground state and
the first excited state is not significantly different for internuclear
distances from 1.5 to 6.6 nm. Three donor atoms arranged in a triangle geometry
have larger binding energies than a triatomic linear chain of dopants with the
same internuclear distances. The planar donor molecules are characterized by a
strong polarization in favor of the valleys oriented perpendicular to the plane
of the molecule. The polarization increases with number of atoms forming the
planar molecule
Multiple transitions of the spin configuration in quantum dots
Single electron tunneling is studied in a many electron quantum dot in high
magnetic fields. For such a system multiple transitions of the spin
configuration are theoretically predicted. With a combination of spin blockade
and Kondo effect we are able to detect five regions with different spin
configurations. Transitions are induced with changing electron numbers.Comment: 4 pages, 5 figure
Correlation between molecular orbitals and doping dependence of the electrical conductivity in electron-doped Metal-Phthalocyanine compounds
We have performed a comparative study of the electronic properties of six
different electron-doped metal phthalocyanine (MPc) compounds (ZnPc, CuPc,
NiPc, CoPc, FePc, and MnPc), in which the electron density is controlled by
means of potassium intercalation. In spite of the complexity of these systems,
we find that the nature of the underlying molecular orbitals produce observable
effects in the doping dependence of the electrical conductivity of the
materials. For all the MPc's in which the added electrons are expected to
occupy orbitals centered on the ligands (ZnPc, CuPc, and NiPc), the doping
dependence of the conductivity has an essentially identical shape. This shape
is different from that observed in MPc materials in which electrons are also
added to orbitals centered on the metal atom (CoPc, FePc, and MnPc). The
observed relation between the macroscopic electronic properties of the MPc
compounds and the properties of the molecular orbitals of the constituent
molecules, clearly indicates the richness of the alkali-doped
metal-phthalocyanines as a model class of compounds for the investigation of
the electronic properties of molecular systems
Enhanced tunneling across nanometer-scale metal-semiconductor interfaces
We have measured electrical transport across epitaxial, nanometer-sized
metal-semiconductor interfaces by contacting CoSi2-islands grown on Si(111)
with an STM-tip. The conductance per unit area was found to increase with
decreasing diode area. Indeed, the zero-bias conductance was found to be about
10^4 times larger than expected from downscaling a conventional diode. These
observations are explained by a model, which predicts a narrower barrier for
small diodes and therefore a greatly increased contribution of tunneling to the
electrical transport.Comment: 3 pages, 2 EPS-figures; accepted for publication in Appl. Phys. Let
Interaction-Induced Spin Polarization in Quantum Dots
The electronic states of lateral many electron quantum dots in high magnetic
fields are analyzed in terms of energy and spin. In a regime with two Landau
levels in the dot, several Coulomb blockade peaks are measured. A zig-zag
pattern is found as it is known from the Fock-Darwin spectrum. However, only
data from Landau level 0 show the typical spin-induced bimodality, whereas
features from Landau level 1 cannot be explained with the Fock-Darwin picture.
Instead, by including the interaction effects within spin-density-functional
theory a good agreement between experiment and theory is obtained. The absence
of bimodality on Landau level 1 is found to be due to strong spin polarization.Comment: 4 pages, 5 figure
Non-invasive detection of molecular bonds in quantum dots
We performed charge detection on a lateral triple quantum dot with star-like
geometry. The setup allows us to interpret the results in terms of two double
dots with one common dot. One double dot features weak tunnel coupling and can
be understood with atom-like electronic states, the other one is strongly
coupled forming molecule-like states. In nonlinear measurements we identified
patterns that can be analyzed in terms of the symmetry of tunneling rates.
Those patterns strongly depend on the strength of interdot tunnel coupling and
are completely different for atomic- or molecule-like coupled quantum dots
allowing the non-invasive detection of molecular bonds.Comment: 4 pages, 4 figure
Scaling of nano-Schottky-diodes
A generally applicable model is presented to describe the potential barrier
shape in ultra small Schottky diodes. It is shown that for diodes smaller than
a characteristic length (associated with the semiconductor doping level)
the conventional description no longer holds. For such small diodes the
Schottky barrier thickness decreases with decreasing diode size. As a
consequence, the resistance of the diode is strongly reduced, due to enhanced
tunneling. Without the necessity of assuming a reduced (non-bulk) Schottky
barrier height, this effect provides an explanation for several experimental
observations of enhanced conduction in small Schottky diodes.Comment: 4 pages, 4 figures, accepted for publication in Appl. Phys. Lett.,
some minor additions and correction
Probing a Kondo correlated quantum dot with spin spectroscopy
We investigate Kondo effect and spin blockade observed on a many-electron
quantum dot and study the magnetic field dependence. At lower fields a
pronounced Kondo effect is found which is replaced by spin blockade at higher
fields. In an intermediate regime both effects are visible. We make use of this
combined effect to gain information about the internal spin configuration of
our quantum dot. We find that the data cannot be explained assuming regular
filling of electronic orbitals. Instead spin polarized filling seems to be
probable.Comment: 4 pages, 5 figure
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