119 research outputs found
Broken Symmetries in the Reconstruction of v=1 Quantum Hall Edges
Spin-polarized reconstruction of the v=1 quantum Hall edge is accompanied by
a spatial modulation of the charge density along the edge. We find that this is
also the case for finite quantum Hall droplets: current spin density functional
calculations show that the so-called Chamon-Wen edge forms a ring of apparently
localized electrons around the maximum density droplet (MDD). The boundaries of
these different phases qualitatively agree with recent experiments. For very
soft confinement, Chern-Simons Ginzburg-Landau theory indicates formation of a
non-translational invariant edge with vortices (holes) trapped in the edge
region.Comment: Proceedings of the EP2DS, Ottawa (1999) (submitted to Physica E
Quantum magnetism without lattices in strongly interacting one-dimensional spinor gases
We show that strongly interacting multicomponent gases in one dimension realize an effective spin chain, offering an alternative simple scenario for the study of one-dimensional (1D) quantum magnetism in cold gases in the absence of an optical lattice. The spin-chain model allows for an intuitive understanding of recent experiments and for a simple calculation of relevant observables. We analyze the adiabatic preparation of antiferromagnetic and ferromagnetic ground states, and show that many-body spin states may be efficiently probed in tunneling experiments. The spin-chain model is valid for more than two components, opening the possibility of realizing SU(N) quantum magnetism in strongly interacting 1D alkaline-earth-metal or ytterbium Fermi gases. © 2014 American Physical Society.DFG/EXC/QUESTGerman-Israeli foundationSwiss SNFNCCR Quantum Science and TechnologySwedish Research CouncilLund Universit
Rotational and vibrational spectra of quantum rings
One can confine the two-dimensional electron gas in semiconductor
heterostructures electrostatically or by etching techniques such that a small
electron island is formed. These man-made ``artificial atoms'' provide the
experimental realization of a text-book example of many-particle physics: a
finite number of quantum particles in a trap. Much effort was spent on making
such "quantum dots" smaller and going from the mesoscopic to the quantum
regime. Far-reaching analogies to the physics of atoms, nuclei or metal
clusters were obvious from the very beginning: The concepts of shell structure
and Hund's rules were found to apply -- just as in real atoms! In this Letter,
we report the discovery that electrons confined in ring-shaped quantum dots
form rather rigid molecules with antiferromagnetic order in the ground state.
This can be seen best from an analysis of the rotational and vibrational
excitations
Symmetry Constraints and the Electronic Structures of a Quantum Dot with Thirteen Electrons
The symmetry constraints imposing on the quantum states of a dot with 13
electrons has been investigated. Based on this study, the favorable structures
(FSs) of each state has been identified. Numerical calculations have been
performed to inspect the role played by the FSs. It was found that, if a
first-state has a remarkably competitive FS, this FS would be pursued and the
state would be crystal-like and have a specific core-ring structure associated
with the FS. The magic numbers are found to be closely related to the FSs.Comment: 13 pages, 5 figure
Electron-hole bilayer quantum dots: Phase diagram and exciton localization
We studied a vertical ``quantum dot molecule'', where one of the dots is
occupied with electrons and the other with holes. We find that different phases
occur in the ground state, depending on the carrier density and the interdot
distance. When the system is dominated by shell structure, orbital degeneracies
can be removed either by Hund's rule, or by Jahn-Teller deformation. Both
mechanisms can lead to a maximum of the addition energy at mid-shell. At low
densities and large interdot distances, bound electron-hole pairs are formed.Comment: 10 pages, 3 figure
Current-spin-density functional study of persistent currents in quantum rings
We present a numerical study of persistent currents in quantum rings using
current spin density functional theory (CSDFT). This formalism allows for a
systematic study of the joint effects of both spin, interactions and impurities
for realistic systems. It is illustrated that CSDFT is suitable for describing
the physical effects related to Aharonov-Bohm phases by comparing energy
spectra of impurity-free rings to existing exact diagonalization and
experimental results. Further, we examine the effects of a symmetry-breaking
impurity potential on the density and current characteristics of the system and
propose that narrowing the confining potential at fixed impurity potential will
suppress the persistent current in a characteristic way.Comment: 7 pages REVTeX, including 8 postscript figure
Roto-vibrational spectrum and Wigner crystallization in two-electron parabolic quantum dots
We provide a quantitative determination of the crystallization onset for two
electrons in a parabolic two-dimensional confinement. This system is shown to
be well described by a roto-vibrational model, Wigner crystallization occurring
when the rotational motion gets decoupled from the vibrational one. The Wigner
molecule thus formed is characterized by its moment of inertia and by the
corresponding sequence of rotational excited states. The role of a vertical
magnetic field is also considered. Additional support to the analysis is given
by the Hartree-Fock phase diagram for the ground state and by the random-phase
approximation for the moment of inertia and vibron excitations.Comment: 10 pages, 8 figures, replaced by the published versio
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.
Broken Symmetry in Density-Functional Theory: Analysis and Cure
We present a detailed analysis of the broken-symmetry mean-field solutions
using a four-electron rectangular quantum dot as a model system. Comparisons of
the density-functional theory predictions with the exact ones show that the
symmetry breaking results from the single-configuration wave function used in
the mean-field approach. As a general cure we present a scheme that
systematically incorporates several configurations into the density-functional
theory and restores the symmetry. This cure is easily applicable to any
density-functional approach.Comment: 4 pages, 4 figures, submitted to PR
Temperature dependence of the ``0.7'' 2(e^2)/h quasi plateau in strongly confined quantum point contacts
We present new results of the ``0.7'' 2(e^2)/h structure or quasi plateau in
some of the most strongly confined point contacts so far reported. This strong
confinement is obtained by a combination of shallow etching and metal gate
deposition on modulation doped GaAs/GaAlAs heterostructures. The resulting
subband separations are up to 20 meV, and as a consequence the quantized
conductance can be followed at temperatures up to 30 K, an order of magnitude
higher than in conventional split gate devices. We observe pronounced quasi
plateaus at several of the lowest conductance steps all the way from their
formation around 1 K to 30 K, where the entire conductance quantization is
smeared out thermally. We study the deviation of the conductance from ideal
integer quantization as a function of temperature, and we find an activated
behavior, exp(-T_a/T), with a density dependent activation temperature T_a of
the order of 2 K. We analyze our results in terms of a simple theoretical model
involving scattering against plasmons in the constriction.Comment: RevTex (4 pages) including 2 postscript figures. To appear in Physica
B, 199
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