53 research outputs found
Charged exctions in the fractional quantum Hall regime
We study the photoluminescence spectrum of a low density ()
two-dimensional electron gas at high magnetic fields and low temperatures. We
find that the spectrum in the fractional quantum Hall regime can be understood
in terms of singlet and triplet charged-excitons. We show that these spectral
lines are sensitive probes for the electrons compressibility. We identify the
dark triplet charged-exciton and show that it is visible at the spectrum at
K. We find that its binding energy scales like , where is
the magnetic length, and it crosses the singlet slightly above 15 T.Comment: 10 pages, 5 figure
Experimental studies of the fractional quantum Hall effect in the first excited Landau level
We present a spectrum of experimental data on the fractional quantum Hall
effect (FQHE) states in the first excited Landau level, obtained in an
ultrahigh mobility two-dimensional electron system (2DES) and at very low
temperatures and report the following results: For the even-denominator FQHE
states, the sample dependence of the nu=5/2 state clearly shows that disorder
plays an important role in determining the energy gap at nu=5/2. For the
developing nu=19/8 FQHE state the temperature dependence of the Rxx minimum
implies an energy gap of ~5mK.The energy gaps of the odd-denominator FQHE
states at nu=7/3 and 8/3 also increase with decreasing disorder, similar to the
gap at 5/2 state. Unexpectedly and contrary to earlier data on lower mobility
samples, in this ultra-high quality specimen, the nu=13/5 state is missing,
while its particle-hole conjugate state, the nu=12/5 state, is a fully
developed FQHE state. We speculate that this disappearance might indicate a
spin polarization of the nu=13/5 state. Finally, the temperature dependence is
studied for the two-reentrant integer quantum Hall states around nu=5/2 and is
found to show a very narrow temperature range for the transition from quantized
to classical value.Comment: to be publishe
Optical excitations of a self assembled artificial ion
By use of magneto-photoluminescence spectroscopy we demonstrate bias
controlled single-electron charging of a single quantum dot. Neutral, single,
and double charged excitons are identified in the optical spectra. At high
magnetic fields one Zeeman component of the single charged exciton is found to
be quenched, which is attributed to the competing effects of tunneling and
spin-flip processes. Our experimental data are in good agreement with
theoretical model calculations for situations where the spatial extent of the
hole wave functions is smaller as compared to the electron wave functions.Comment: to be published in Physical Review B (rapid communication
Magnetic field dependence of the energy of negatively charged excitons in semiconductor quantum wells
A variational calculation of the spin-singlet and spin-triplet state of a
negatively charged exciton (trion) confined to a single quantum well and in the
presence of a perpendicular magnetic field is presented. We calculated the
probability density and the pair correlation function of the singlet and
triplet trion states. The dependence of the energy levels and of the binding
energy on the well width and on the magnetic field strength was investigated.
We compared our results with the available experimental data on GaAs/AlGaAs
quantum wells and find that in the low magnetic field region (B<18 T) the
observed transition are those of the singlet and the dark triplet trion (with
angular momentum ), while for high magnetic fields (B>25 T) the dark
trion becomes optically inactive and possibly a transition to a bright triplet
trion (angular momentum ) state is observed.Comment: 9 pages, 10 figures submitted to Phys. Rev.
Orbital Magnetism in Small Quantum Dots with Closed Shells
It is found that various kind of shell structure which occurs at specific
values of the magnetic field leads to the disappearance of the orbital
magnetization for particular magic numbers of small quantum dots with an
electron number .Comment: 4 pages, latex file, four figures as postscript files, to appear at
JETP Letters, December 199
Spin interactions and switching in vertically tunnel-coupled quantum dots
We determine the spin exchange coupling J between two electrons located in
two vertically tunnel-coupled quantum dots, and its variation when magnetic (B)
and electric (E) fields (both in-plane and perpendicular) are applied. We
predict a strong decrease of J as the in-plane B field is increased, mainly due
to orbital compression. Combined with the Zeeman splitting, this leads to a
singlet-triplet crossing, which can be observed as a pronounced jump in the
magnetization at in-plane fields of a few Tesla, and perpendicular fields of
the order of 10 Tesla for typical self-assembled dots. We use harmonic
potentials to model the confining of electrons, and calculate the exchange J
using the Heitler-London and Hund-Mulliken technique, including the long-range
Coulomb interaction. With our results we provide experimental criteria for the
distinction of singlet and triplet states and therefore for microscopic spin
measurements. In the case where dots of different sizes are coupled, we present
a simple method to switch on and off the spin coupling with exponential
sensitivity using an in-plane electric field. Switching the spin coupling is
essential for quantum computation using electronic spins as qubits.Comment: 13 pages, 9 figure
Structures for Interacting Composite Fermions: Stripes, Bubbles, and Fractional Quantum Hall Effect
Much of the present day qualitative phenomenology of the fractional quantum
Hall effect can be understood by neglecting the interactions between composite
fermions altogether. For example the fractional quantum Hall effect at
corresponds to filled composite-fermion Landau levels,and
the compressible state at to the Fermi sea of composite fermions.
Away from these filling factors, the residual interactions between composite
fermions will determine the nature of the ground state. In this article, a
model is constructed for the residual interaction between composite fermions,
and various possible states are considered in a variational approach. Our study
suggests formation of composite-fermion stripes, bubble crystals, as well as
fractional quantum Hall states for appropriate situations.Comment: 16 pages, 7 figure
Adiabatic description of nonspherical quantum dot models
Within the effective mass approximation an adiabatic description of
spheroidal and dumbbell quantum dot models in the regime of strong dimensional
quantization is presented using the expansion of the wave function in
appropriate sets of single-parameter basis functions. The comparison is given
and the peculiarities are considered for spectral and optical characteristics
of the models with axially symmetric confining potentials depending on their
geometric size making use of the total sets of exact and adiabatic quantum
numbers in appropriate analytic approximations
Long-lived charged multiple-exciton complexes in strong magnetic fields
We consider the charged exciton complexes of an ideal two-dimensional
electron-hole system in the limit of strong magnetic fields. A series of
charged multiple-exciton states is identified and variational and finite-size
exact diagonalization calculations are used to estimate their binding energies.
We find that, because of a hidden symmetry, bound states of excitons and an
additional electron cannot be created by direct optical absorption and, once
created, have an infinite optical recombination lifetime. We also estimate the
optical recombination rates when electron and hole layers are displaced and the
hidden symmetry is violated.Comment: 12 pages + 2 PostScript figures, Revtex, Submitted to Phys. Rev. Let
Non-Abelian Anyons and Topological Quantum Computation
Topological quantum computation has recently emerged as one of the most
exciting approaches to constructing a fault-tolerant quantum computer. The
proposal relies on the existence of topological states of matter whose
quasiparticle excitations are neither bosons nor fermions, but are particles
known as {\it Non-Abelian anyons}, meaning that they obey {\it non-Abelian
braiding statistics}. Quantum information is stored in states with multiple
quasiparticles, which have a topological degeneracy. The unitary gate
operations which are necessary for quantum computation are carried out by
braiding quasiparticles, and then measuring the multi-quasiparticle states. The
fault-tolerance of a topological quantum computer arises from the non-local
encoding of the states of the quasiparticles, which makes them immune to errors
caused by local perturbations. To date, the only such topological states
thought to have been found in nature are fractional quantum Hall states, most
prominently the \nu=5/2 state, although several other prospective candidates
have been proposed in systems as disparate as ultra-cold atoms in optical
lattices and thin film superconductors. In this review article, we describe
current research in this field, focusing on the general theoretical concepts of
non-Abelian statistics as it relates to topological quantum computation, on
understanding non-Abelian quantum Hall states, on proposed experiments to
detect non-Abelian anyons, and on proposed architectures for a topological
quantum computer. We address both the mathematical underpinnings of topological
quantum computation and the physics of the subject using the \nu=5/2 fractional
quantum Hall state as the archetype of a non-Abelian topological state enabling
fault-tolerant quantum computation.Comment: Final Accepted form for RM
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