476 research outputs found
Accuracy of the Hartree-Fock method for Wigner molecules at high magnetic fields
Few-electron systems confined in two-dimensional parabolic quantum dots at
high magnetic fields are studied by the Hartree-Fock (HF) and exact
diagonalization methods. A generalized multicenter Gaussian basis is proposed
in the HF method. A comparison of the HF and exact results allows us to discuss
the relevance of the symmetry of the charge density distribution for the
accuracy of the HF method. It is shown that the energy estimates obtained with
the broken-symmetry HF wave functions become exact in the infinite
magnetic-field limit. In this limit the charge density of the broken-symmetry
solution can be identified with the classical charge distribution.Comment: to appear in EPJ
Effect of confinement potential shape on exchange interaction in coupled quantum dots
Exchange interaction has been studied for electrons in coupled quantum dots
(QD's) by a configuration interaction method using confinement potentials with
different profiles. The confinement potential has been parametrized by a
two-centre power-exponential function, which allows us to investigate various
types of QD's described by either soft or hard potentials of different range.
For the soft (Gaussian) confinement potential the exchange energy decreases
with increasing interdot distance due to the decreasing interdot tunnelling.
For the hard (rectangular-like) confinement potential we have found a
non-monotonic behaviour of the exchange interaction as a function of distance
between the confinement potential centres. In this case, the exchange
interaction energy exhibits a pronounced maximum for the confinement potential
profile which corresponds to the nanostructure composed of the small inner QD
with a deep potential well embedded in the large outer QD with a shallow
potential well. This effect results from the strong localization of electrons
in the inner QD, which leads to the large singlet-triplet splitting.
Implications of this finding for quantum logic operations have been discussed.Comment: 16 pages, including 11 figure
Controlled exchange interaction for quantum logic operations with spin qubits in coupled quantum dots
A two-electron system confined in two coupled semiconductor quantum dots is
investigated as a candidate for performing quantum logic operations on spin
qubits. We study different processes of swapping the electron spins by
controlled switching on/off the exchange interaction. The resulting spin swap
corresponds to an elementary operation in quantum information processing. We
perform a direct time evolution simulations of the time-dependent Schroedinger
equation. Our results show that -- in order to obtain the full interchange of
spins -- the exchange interaction should change smoothly in time. The presence
of jumps and spikes in the corresponding time characteristics leads to a
considerable increase of the spin swap time. We propose several mechanisms to
modify the exchange interaction by changing the confinement potential profile
and discuss their advantages and disadvantages
Stability of negative and positive trions in quantum wires
Binding energies of negative () and positive trions () in quantum
wires are studied for strong quantum confinement of carriers which results in a
numerical exactly solvable model. The relative electron and hole localization
has a strong effect on the stability of trions. For equal hole and electron
confinement, is more stable but a small imbalance of the particle
localization towards a stronger hole localization e.g. due to its larger
effective mass, leads to the interchange of and recombination lines
in the photoluminescent spectrum as was recently observed experimentally. In
case of larger stability, a magnetic field oriented parallel to the wire
axis leads to a stronger increase of the binding energy resulting in a
crossing of the and lines
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