417 research outputs found
Exact broken-symmetry states and Hartree-Fock solutions for quantum dots at high magnetic fields
Wigner molecules formed at high magnetic fields in circular and elliptic
quantum dots are studied by exact diagonalization (ED) and unrestricted
Hartree-Fock (UHF) methods with multicenter basis of displaced lowest Landau
level wave functions. The broken symmetry states with semi-classical charge
density constructed from superpositions of the ED solutions are compared to the
UHF results. UHF overlooks the dependence of the few-electron wave function on
the actual relative positions of electrons localized in different charge
puddles and partially compensates for this neglect by an exaggerated separation
of charge islands which are more strongly localized than in the exact
broken-symmetry states.Comment: QD2004 proceedings under press in Physica
Distance Digital Algorithm Immune to Saturation of Current Transformer
Saturation of current transformers due to slowly decaying primary current D.C. components cause errors in reproduction of the current fundamental harmonic. Most of the relays are more or less sensitive to the errors. If the saturation occures before the given relay operated. the operation may be delayed up to about 2 time constants of the D.C.component. In some, although rare cases, transient CT errors may cause unselective operation of the relay. Whether the current transformer saturates and when it occures depend mainly on the accuracy limit factor and on the residual flux in the transformer core. However bearing in mind the very high expected values of short circuit currents and the long D.C. time constants one may conclude, that the design of CT-s which never saturate would end in bulky and expensive units. Therefore most of the protective CT-s which are in service saturate during severe transients. It is a duty of protection engineers to design the relays in such a way, that the errors caused by the saturation neither cause maloperation, nor bring about excessive delay
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
Simulative Analysis of a New Type Wide Range Estimator of Protection Criterion Values
In the paper a new adaptive estimator of protection criterion values insensitive to wide range power system frequency changes is proposed and analysed. Its operation is based on a coarse calculation of signal frequency and appropriate correction of used orthogonal filters frequency responses as well as adaptation of chosen parameters of the measurement algorithm. The main features of the new method has been compared with standard criterion values estimators both theoretically and by simulations. The proposed estimators assure good accuracy of measurement within the frequency range from 5 to 95 Hz
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
Spin rotations induced by electron running on closed trajectories in gated semiconductor nanodevices
A design for a quantum gate performing transformations of a single electron
spin is presented. The spin rotations are performed by the electron going
around the closed loops in a gated semiconductor device. We demonstrate the
operation of NOT, phase-flip and Hadamard quantum gates, i.e. the single-qubit
gates which are most commonly used in the algorithms. The proposed devices
employ the self-focusing effect for the electron wave packet interacting with
the electron gas on the electrodes and the Rashba spin-orbit coupling. Due to
the self-focusing effect the electron moves in a compact wave packet. The
spin-orbit coupling translates the spatial motion of the electron into the
rotations of the spin. The device does not require microwave radiation and
operates using low constant voltages. It is therefore suitable for selective
single-spin rotations in larger registers.Comment: submitte
Exciton and negative trion dissociation by an external electric field in vertically coupled quantum dots
We study the Stark effect for an exciton confined in a pair of vertically
coupled quantum dots. A single-band approximation for the hole and a parabolic
lateral confinement potential are adopted which allows for the separation of
the lateral center-of-mass motion and consequently for an exact numerical
solution of the Schr\"odinger equation. We show that for intermediate tunnel
coupling the external electric field leads to the dissociation of the exciton
via an avoided crossing of bright and dark exciton energy levels which results
in an atypical form of the Stark shift. The electric-field-induced dissociation
of the negative trion is studied using the approximation of frozen lateral
degrees of freedom. It is shown that in a symmetric system of coupled dots the
trion is more stable against dissociation than the exciton. For an asymmetric
system of coupled dots the trion dissociation is accompanied by a positive
curvature of the recombination energy line as a function of the electric field.Comment: PRB - in prin
Quantum dot defined in two-dimensional electron gas at n-AlGaAs/GaAs heterojunction: simulation of electrostatic potential and charging properties
We present a self-consistent Schroedinger-Poisson scheme for simulation of
electrostatic quantum dots defined in gated two-dimensional electron gas formed
at n-AlGaAs/GaAs heterojunction. The computational method is applied to a
quantitative description of transport properties studied experimentally by
Elzermann et al. [Appl. Phys. Lett. {\bf 84}, 4617 (2004)]. The
three-dimensional model describes the electrostatics of the entire device with
a quantum dot that changes shape and floats inside a gated region when the
applied voltages are varied. Our approach accounts for the metal electrodes of
arbitrary geometry and configuration, includes magnetic field applied
perpendicular to the growth direction, electron-electron correlation in the
confined electron system and its interaction with the electron reservoir
surrounding the quantum dot. We calculate the electric field, the space charge
distribution as well as energies and wave functions of confined electrons to
describe opening of two transport channels between the reservoir and the
confined charge puddle. We determine the voltages for charging the dot with up
to 4 electrons.
The results are in a qualitative and quantitative agreement with the
experimental data
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