375 research outputs found
An Anderson-Fano Resonance and Shake-Up Processes in the Magneto-Photoluminescence of a Two-Dimensional Electron System
We report an anomalous doublet structure and low-energy satellite in the
magneto-photoluminescence spectra of a two-dimensional electron system. The
doublet structure moves to higher energy with increasing magnetic field and is
most prominent at odd filling factors 5 and 3. The lower-energy satellite peak
tunes to lower energy for increasing magnetic field between filling factor 6
and 2. These features occur at energies below the fundamental band of
recombination originating from the lowest Landau level and display striking
magnetic field and temperature dependence that indicates a many-body origin.
Drawing on a recent theoretical description of Hawrylak and Potemski, we show
that distinct mechanisms are responsible for each feature.Comment: 14 pages including 5 figures. To appear in the April 15th edition of
Phy. Rev. B. rapid com
Tunable Negative Differential Resistance controlled by Spin Blockade in Single Electron Transistors
We demonstrate a tunable negative differential resistance controlled by spin
blockade in single electron transistors. The single electron transistors
containing a few electrons and spin polarized source and drain contacts were
formed in GaAs/GaAlAs heterojunctions using metallic gates. Coulomb blockade
measurements performed as a function of applied source-drain bias, electron
number and magnetic field reveal well defined regimes where a decrease in the
current is observed with increasing bias. We establish that the origin of the
negative differential regime is the spin-polarized detection of electrons
combined with a long spin relaxation time in the dot. These results indicate
new functionalities that may be utilized in nano-spintronic devices in which
the spin state is electro-statically controlled via the electron occupation
number.Comment: 8 pages, 4 figure
Optical properties of charged quantum dots doped with a single magnetic impurity
We present a microscopic theory of the optical properties of self-assembled
quantum dots doped with a single magnetic manganese (Mn) impurity and
containing a controlled number of electrons. The single-particle electron and
heavy-hole electronic shells are described by two-dimensional harmonic
oscillators. The electron-electron, electron-hole Coulomb as well as the
short-range electron spin-Mn spin and hole spin-Mn spin contact exchange
interactions are included. The electronic states of the photo-excited
electron-hole-Mn complex and of the final electron-Mn complex are expanded in a
finite number of configurations and the full interacting Hamiltonian is
diagonalized numerically. The emission spectrum is predicted as a function of
photon energy for a given number of electrons and different number of confined
electronic quantum dot shells. We show how emission spectra allow to identify
the number of electronic shells, the number of electrons populating these
shells and, most importantly, their spin. We show that electrons not
interacting directly with the spin of Mn ion do so via electron-electron
interactions. This indirect interaction is a strong effect even when Mn
impurity is away from the quantum dot center.Comment: 12 pages, 10 figure
Theory of electronic properties and quantum spin blockade in a gated linear triple quantum dot with one electron spin each
We present a theory of electronic properties and the spin blockade phenomena
in a gated linear triple quantum dot. Quadruple points where four different
charge configurations are on resonance, particularly involving (1,1,1)
configuration, are considered. In the symmetric case, the central dot is biased
to higher energy and a single electron tunnels through the device when (1,1,1)
configuration is resonant with (1,0,1),(2,0,1),(1,0,2) configurations. The
electronic properties of a triple quantum dot are described by a Hubbard model
containing two orbitals in the two unbiased dots and a single orbital in the
biased dot. The transport through the triple quantum dot molecule involves both
singly and doubly occupied configurations and necessitates the description of
the (1,1,1) configuration beyond the Heisenberg model. Exact eigenstates of the
triple quantum dot molecule with up to three electrons are used to compute
current assuming weak coupling to the leads and non-equilibrium occupation of
quantum molecule states obtained from the rate equation. The intra-molecular
relaxation processes due to acoustic phonons and cotunneling with the leads are
included, and are shown to play a crucial role in the spin blockade effect. We
find a quantum interference-based spin blockade phenomenon at low source-drain
bias and a distinct spin blockade due to a trap state at higher bias. We also
show that, for an asymmetric quadruple point with
(0,1,1),(1,1,1,),(0,2,1),(0,1,2) configurations on resonance, the spin blockade
is analogous to the spin blockade in a double quantum dot
Spin relaxation in a two-electron quantum dot
We discuss the rate of relaxation of the total spin in the two-electron
droplet in the vicinity of the magnetic field driven singlet-triplet
transition. The total spin relaxation is attributed to spin-orbit and
electron-phonon interactions. The relaxation process is found to depend on the
spin of ground and excited states. This asymmetry is used to explain puzzles in
recent high source-drain transport experiments.Comment: 9 pages in the PDF format, 1 figur
Magnetism and correlations in fractionally filled degenerate shells of graphene quantum dots
When an electron is confined to a triangular atomic thick layer of graphene
[1-5] with zig-zag edges, its energy spectrum collapses to a shell of
degenerate states at the Fermi level (Dirac point) [6-9]. The degeneracy is
proportional to the edge size and can be made macroscopic. This opens up the
possibility to design a strongly correlated electronic system as a function of
fractional filling of the zero-energy shell, in analogy to the fractional
quantum Hall effect in a quasi-two-dimensional electron gas[10], but without
the need for a high magnetic field. In this work we show that electronic
correlations, beyond the Hubbard model[6,7] and mean-field density functional
theory (DFT) [7,8] play a crucial role in determining the nature of the ground
state and the excitation spectrum of triangular graphene quantum dots as a
function of dot size and filling fraction of the shell of zero-energy states.
The interactions are treated by a combination of DFT, tight-binding,
Hartree-Fock and configuration interaction methods (TB-HF-CI) and include all
scattering and exchange terms within second nearest neighbors as well as
interaction with metallic gate. We show that a half filled charge neutral shell
leads to full spin polarization of the island but this magnetic moment is
completely destroyed by the addition of a single electron, in analogy to the
effect of skyrmions on the quantum Hall ferromagnet [11-14] and spin
depolarization in electrostatically defined semiconductor quantum dots[15-18].
The depolarization of the ground state is predicted to result in blocking of
current through a graphene quantum dot due to spin blockade (SB) [18].Comment: v2: minor corrections, new forma
Herzberg Circuit and Berry's Phase in Chirality-based Coded Qubit in a Triangular Triple Quantum Dot
We present a theoretical proposal for the Herzberg circuit and controlled
accumulation of Berry's phase in a chirality-based coded qubit in a triangular
triple quantum dot molecule with one electron spin each. The qubit is encoded
in the two degenerate states of a three spin complex with total spin .
Using a Hubbard and Heisenberg model the Herzberg circuit encircling the
degeneracy point is realized by adiabatically tuning the successive on-site
energies of quantum dots and tunnel couplings across a pair of neighbouring
dots. It is explicitly shown that encircling the degeneracy point leads to the
accumulation of the geometrical Berrys phase. We show that only triangular but
not linear quantum dot molecule allows for the generation of Berry's phase and
we discuss a protocol to detect this geometrical phase
The Collapse of the Spin-Singlet Phase in Quantum Dots
We present experimental and theoretical results on a new regime in quantum
dots in which the filling factor 2 singlet state is replaced by new spin
polarized phases. We make use of spin blockade spectroscopy to identify the
transition to this new regime as a function of the number of electrons. The key
experimental observation is a reversal of the phase in the systematic
oscillation of the amplitude of Coulomb blockade peaks as the number of
electrons is increased above a critical number. It is found theoretically that
correlations are crucial to the existence of the new phases.Comment: REVTeX4, 4 pages, 4 figures, to appear in PR
Valence holes as Luttinger spinor based qubits in quantum dots
We present a theory of valence holes as Luttinger spinor based qubits in
p-doped self-assembled quantum dots within the 4-band formalism. The
two qubit levels are identified with the two chiralities of the doubly
degenerate ground state. We show that single qubit operations can be
implemented with static magnetic field applied along the and
directions, acting analogously to the and
operators in the qubit subspace respectively. The coupling of two dots and
hence the double qubit operations are shown to be sensitive to the orientation
of the two quantum dots. For vertical qubit arrays, there exists an optimal
qubit separation suitable for the voltage control of qubit-qubit interactions
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