102 research outputs found
Two-electron lateral quantum-dot molecules in a magnetic field
Laterally coupled quantum dot molecules are studied using exact
diagonalization techniques. We examine the two-electron singlet-triplet energy
difference as a function of magnetic field strength and investigate the
magnetization and vortex formation of two- and four-minima lateral quantum dot
molecules. Special attention is paid to the analysis of how the distorted
symmetry affects the properties of quantum-dot molecules.Comment: 18 pages, 26 figure
Influence of shape of quantum dots on their far-infrared absorption
We investigate the effects of the shape of quantum dots on their far-infrared
absorption in an external magnetic field by a model calculation. We focus our
attention on dots with a parabolic confinement potential deviating from the
common circular symmetry, and dots having circular doughnut shape. For a
confinement where the generalized Kohn theorem does not hold we are able to
interprete the results in terms of a mixture of a center-of-mass mode and
collective modes reflecting an excitation of relative motion of the electrons.
The calculations are performed within the time-dependent Hartree approximation
and the results are compared to available experimental results.Comment: RevTeX, 16 pages with 10 postscript figures included. Submitted to
Phys. Rev.
Rectangular quantum dots in high magnetic fields
We use density-functional methods to study the effects of an external
magnetic field on two-dimensional quantum dots with a rectangular hard-wall
confining potential. The increasing magnetic field leads to spin polarization
and formation of a highly inhomogeneous maximum-density droplet at the
predicted magnetic field strength. At higher fields, we find an oscillating
behavior in the electron density and in the magnetization of the dot. We
identify a rich variety of phenomena behind the periodicity and analyze the
complicated many-electron dynamics, which is shown to be highly dependent on
the shape of the quantum dot.Comment: 6 pages, 6 figures, submitted to Phys. Rev.
Classical double-layer atoms: artificial molecules
The groundstate configuration and the eigenmodes of two parallel
two-dimensional classical atoms are obtained as function of the inter-atomic
distance (d). The classical particles are confined by identical harmonic wells
and repel each other through a Coulomb potential. As function of d we find
several structural transitions which are of first or second order. For first
(second) order transitions the first (second) derivative of the energy with
respect to d is discontinuous, the radial position of the particles changes
discontinuously (continuously) and the frequency of the eigenmodes exhibit a
jump (one mode becomes soft, i.e. its frequency becomes zero).Comment: 4 pages, RevTex, 5 ps figures, to appear in Phys.Rev.Let
Quasiperiodic Hubbard chains
Low energy properties of half-filled Fibonacci Hubbard models are studied by
weak coupling renormalization group and density matrix renormalization group
method. In the case of diagonal modulation, weak Coulomb repulsion is
irrelevant and the system behaves as a free Fibonacci chain, while for strong
Coulomb repulsion, the charge sector is a Mott insulator and the spin sector
behaves as a uniform Heisenberg antiferromagnetic chain. The off-diagonal
modulation always drives the charge sector to a Mott insulator and the spin
sector to a Fibonacci antiferromagnetic Heisenberg chain.Comment: 4 pages, 4 figures; Final version to appear in Phys. Rev. Let
Short-range interactions in a two-electron system: energy levels and magnetic properties
The problem of two electrons in a square billiard interacting via a
finite-range repulsive Yukawa potential and subjected to a constant magnetic
field is considered. We compute the energy spectrum for both singlet and
triplet states, and for all symmetry classes, as a function of the strength and
range of the interaction and of the magnetic field. We show that the
short-range nature of the potential suppresses the formation of ``Wigner
molecule'' states for the ground state, even in the strong interaction limit.
The magnetic susceptibility shows low-temperature paramagnetic peaks
due to exchange induced singlet-triplet oscillations. The position, number and
intensity of these peaks depend on the range and strength of the interaction.
The contribution of the interaction to the susceptibility displays paramagnetic
and diamagnetic phases as a function of .Comment: 12 pages,6 figures; to appear in Phys. Rev.
Energy levels and far-infrared spectroscopy for two electrons in a semiconductor nanoring
The effects of electron-electron interaction of a two-electron nanoring on
the energy levels and far-infrared (FIR) spectroscopy have been investigated
based on a model calculation which is performed within the exactly numerical
diagonalization. It is found that the interaction changes the energy spectra
dramatically, and also shows significant influence on the FIR spectroscopy. The
crossings between the lowest spin-singlet and triplet states induced by the
coulomb interaction are clearly revealed. Our results are related to the
experiment recently carried out by A. Lorke et al. [Phys. Rev. Lett. 84, 2223
(2000)].Comment: 17 pages, 6 figures, revised and accepted by Phys. Rev. B (Dec. 15
Measuring Black Hole Formations by Entanglement Entropy via Coarse-Graining
We argue that the entanglement entropy offers us a useful coarse-grained
entropy in time-dependent AdS/CFT. We show that the total von-Neumann entropy
remains vanishing even when a black hole is created in a gravity dual, being
consistent with the fact that its corresponding CFT is described by a
time-dependent pure state. We analytically calculate the time evolution of
entanglement entropy for a free Dirac fermion on a circle following a quantum
quench. This is interpreted as a toy holographic dual of black hole creations
and annihilations. It is manifestly free from the black hole information
problem.Comment: 25 pages, Latex, 8 figure
Electronic structure of rectangular quantum dots
We study the ground state properties of rectangular quantum dots by using the
spin-density-functional theory and quantum Monte Carlo methods. The dot
geometry is determined by an infinite hard-wall potential to enable comparison
to manufactured, rectangular-shaped quantum dots. We show that the electronic
structure is very sensitive to the deformation, and at realistic sizes the
non-interacting picture determines the general behavior. However, close to the
degenerate points where Hund's rule applies, we find spin-density-wave-like
solutions bracketing the partially polarized states. In the
quasi-one-dimensional limit we find permanent charge-density waves, and at a
sufficiently large deformation or low density, there are strongly localized
stable states with a broken spin-symmetry.Comment: 8 pages, 9 figures, submitted to PR
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