383 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
Magnetic forces and localized resonances in electron transfer through quantum rings
We study the current flow through semiconductor quantum rings. In high
magnetic field the current is usually injected to the arm of the ring preferred
by classical magnetic forces. However, for narrow magnetic field intervals that
appear periodically on the magnetic field scale the current is injected to the
other arm of the ring. We indicate that the appearance of the anomalous --
non-classical -- current circulation results from Fano interference involving
localized resonant states. The identification of the Fano interference is based
on the comparison of the solution of the scattering problem with the results of
the stabilization method. The latter employs the bound-state type calculations
and allows to extract both the energy of metastable states localized within the
ring and the width of resonances by analysis of the energy spectrum of a finite
size system in function of its length. The Fano resonances involving states of
anomalous current circulation become extremely narrow on both magnetic field
and energy scales. This is consistent with the orientation of the Lorentz force
that tends to keep the electron within the ring and thus increases the lifetime
of the electron localization within the ring. Absence of periodic Fano
resonances in electron transfer probability through a quantum ring containing
an elastic scatterer is also explained.Comment: This paper explains the origins of anomalous (non-classical) current
circulation reported in http://arxiv.org/abs/1004.219
Few-electron artificial molecules formed by laterally coupled quantum rings
We study the artificial molecular states formed in laterally coupled double
semiconductor nanorings by systems containing one, two and three electrons. An
interplay of the interring tunneling and the electron-electron interaction is
described and its consequences for the magnetization and charging properties of
the system are determined. It is shown that both the magnetic dipole moment
generated by the double ring structure and the chemical potential of the system
as function of the external magnetic field strongly depend on the number of
electrons and the interring barrier thickness. Both the magnetization and
chemical potentials exhibit cusps at the magnetic fields inducing ground-state
parity and / or spin transformations. The symmetry transformations are
discussed for various tunnel coupling strengths: from rings coupled only
electrostatically to the limit of coalesced rings. We find that in the
ground-states for rings of different radii the magnetic field transfers the
electron charge from one ring to the other. The calculations are performed with
the configuration interaction method based on an approach of Gaussian functions
centered on a rectangular array of points covering the studied structure.
Electron-electron correlation is also discussed
Few-electron eigenstates of concentric double quantum rings
Few-electron eigenstates confined in coupled concentric double quantum rings
are studied by the exact diagonalization technique. We show that the magnetic
field suppresses the tunnel coupling between the rings localizing the
single-electron states in the internal ring, and the few-electron states in the
external ring. The magnetic fields inducing the ground-state angular momentum
transitions are determined by the distribution of the electron charge between
the rings. The charge redistribution is translated into modifications of the
fractional Aharonov-Bohm period. We demonstrate that the electron distribution
can be deduced from the cusp pattern of the chemical potentials governing the
single-electron charging properties of the system. The evolution of the
electron-electron correlations to the high field limit of a classical Wigner
molecule is discussed.Comment: to appear in Physical Review
Magnetic-field-induced binding of few-electron systems in shallow quantum dots
Binding of few-electron systems in two-dimensional potential cavities in the
presence of an external magnetic field is studied with the exact
diagonalization approach. We demonstrate that for shallow cavities the
few-electron system becomes bound only under the application of a strong
magnetic field. The critical value of the depth of the cavity allowing the
formation of a bound state decreases with magnetic field in a non-smooth
fashion, due to the increasing angular momentum of the first bound state. In
the high magnetic field limit the binding energies and the critical values for
the depth of the potential cavity allowing the formation of a bound system tend
to the classical values
Gated combo nanodevice for sequential operations on single electron spin
An idea for a nanodevice in which an arbitrary sequence of three basic
quantum single qubit gates - negation, Hadamard and phase shift - can be
performed on a single electron spin. The spin state is manipulated using the
spin-orbit coupling and the electron trajectory is controlled by the electron
wave function self-focusing mechanism due to the electron interaction with the
charge induced on metal gates. We present results of simulations based on
iterative solution of the time dependent Schr\"odinger equation in which the
subsequent operations on the electron spin can be followed and controlled.
Description of the moving electron wave packet requires evaluation of the
electric field within the entire nanodevice in each time step
Magnetic-field asymmetry of electron wave packet transmission in bent channels capacitively coupled to a metal gate
We study the electron wave packet moving through a bent channel. We
demonstrate that the packet transmission probability becomes an uneven function
of the magnetic field when the electron packet is capacitively coupled to a
metal plate. The coupling occurs through a non-linear potential which
translates a different kinetics of the transport for opposite magnetic field
orientations into a different potential felt by the scattered electron
Electron spin and charge switching in a coupled quantum dot quantum ring system
Few-electron systems confined in a quantum dot laterally coupled to a
surrounding quantum ring in the presence of an external magnetic field are
studied by exact diagonalization. The distribution of electrons between the dot
and the ring is influenced by the relative strength of the dot and ring
confinement, the gate voltage and the magnetic field which induces transitions
of electrons between the two parts of the system. These transitions are
accompanied by changes in the periodicity of the Aharonov-Bohm oscillations of
the ground-state angular momentum. The singlet-triplet splitting for a two
electron system with one electron confined in the dot and the other in the ring
exhibits piecewise linear dependence on the external field due to the
Aharonov-Bohm effect for the ring-confined electron, in contrast to smooth
oscillatory dependence of the exchange energy for laterally coupled dots in the
side-by-side geometry.Comment: to appear in PRB in August 200
Stark effect on the exciton spectra of vertically coupled quantum dots: horizontal field orientation and non-aligned dots
We study the effect of an electric-field on an electron-hole pair in an
asymmetric system of vertically coupled self-assembled quantum dots taking into
account their non-perfect alignment. We show that the non-perfect alignment
does not qualitatively influence the exciton Stark effect for the electric
field applied in the growth direction, but can be detected by application of a
perpendicular electric field. We demonstrate that the direction of the shift
between the axes of non-aligned dots can be detected by rotation of a weak
electric field within the plane of confinement. Already for a nearly perfect
alignment the two-lowest energy bright exciton states possess antilocked
extrema as function of the orientation angle of the horizontal field which
appear when the field is parallel to the direction of the shift between the dot
centers
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