666 research outputs found
Frequency-dependent transport through a quantum dot in the Kondo regime
We study the AC conductance and equilibrium current fluctuations of a Coulomb
blockaded quantum dot. A relation between the equilibrium spectral function and
the linear AC conductance is derived which is valid for frequencies well below
the charging energy of the quantum dot. Frequency-dependent transport
measurements can thus give experimental access to the Kondo peak in the
equilibrium spectral function of a quantum dot. We illustrate this in detail
for typical experimental parameters using the numerical renormalization group
method in combination with the Kubo formalism.Comment: 4 pages, 4 figure
Quantum Phase Transition in a Multi-Level Dot
We discuss electronic transport through a lateral quantum dot close to the
singlet-triplet degeneracy in the case of a single conduction channel per lead.
By applying the Numerical Renormalization Group, we obtain rigorous results for
the linear conductance and the density of states. A new quantum phase
transition of the Kosterlitz-Thouless type is found, with an exponentially
small energy scale close to the degeneracy point. Below , the
conductance is strongly suppressed, corresponding to a universal dip in the
density of states. This explains recent transport measurements.Comment: 4 pages, 5 eps figures, published versio
Two path transport measurements on a triple quantum dot
We present an advanced lateral triple quantum dot made by local anodic
oxidation. Three dots are coupled in a starlike geometry with one lead attached
to each dot thus allowing for multiple path transport measurements with two
dots per path. In addition charge detection is implemented using a quantum
point contact. Both in charge measurements as well as in transport we observe
clear signatures of states from each dot. Resonances of two dots can be
established allowing for serial transport via the corresponding path. Quadruple
points with all three dots in resonance are prepared for different electron
numbers and analyzed concerning the interplay of the simultaneously measured
transport along both paths.Comment: 4 pages, 4 figure
Rotational levels in quantum dots
Low energy spectra of isotropic quantum dots are calculated in the regime of
low electron densities where Coulomb interaction causes strong correlations.
The earlier developed pocket state method is generalized to allow for
continuous rotations. Detailed predictions are made for dots of shallow
confinements and small particle numbers, including the occurance of spin
blockades in transport.Comment: RevTeX, 10 pages, 2 figure
Magnetically induced chessboard pattern in the conductance of a Kondo quantum dot
We quantitatively describe the main features of the magnetically induced
conductance modulation of a Kondo quantum dot -- or chessboard pattern -- in
terms of a constant-interaction double quantum dot model. We show that the
analogy with a double dot holds down to remarkably low magnetic fields. The
analysis is extended by full 3D spin density functional calculations.
Introducing an effective Kondo coupling parameter, the chessboard pattern is
self-consistently computed as a function of magnetic field and electron number,
which enables us to quantitatively explain our experimental data.Comment: 4 pages, 3 color figure
Measurements of higher order noise correlations in a quantum dot with a finite bandwidth detector
We present measurements of the fourth and fifth cumulants of the distribution
of transmitted charge in a tunable quantum dot. We investigate how the measured
statistics is influenced by the finite bandwidth of the detector and by the
finite measurement time. By including the detector when modeling the system, we
use the theory of full counting statistics to calculate the noise levels for
the combined system. The predictions of the finite-bandwidth model are in good
agreement with measured data
Electron Spins in Artificial Atoms and Molecules for Quantum Computing
Achieving control over the electron spin in quantum dots (artificial atoms)
or real atoms promises access to new technologies in conventional and in
quantum information processing. Here we review our proposal for quantum
computing with spins of electrons confined to quantum dots. We discuss the
basic requirements for implementing spin-qubits, and describe a complete set of
quantum gates for single- and two-qubit operations. We show how a quantum dot
attached to leads can be used for spin filtering and spin read-out, and as a
spin-memory device. Finally, we focus on the experimental characterization of
the quantum dot systems, and discuss transport properties of a double-dot and
show how Kondo correlations can be used to measure the Heisenberg exchange
interaction between the spins of two dots.Comment: 13 pages, 8 figures, Invited Review (Semiconductor Spintronics,
Special Issue of SST
Wigner Molecules in Nanostructures
The one-- and two-- particle densities of up to four interacting electrons
with spin, confined within a quasi one--dimensional ``quantum dot'' are
calculated by numerical diagonalization. The transition from a dense
homogeneous charge distribution to a dilute localized Wigner--type electron
arrangement is investigated. The influence of the long range part of the
Coulomb interaction is studied. When the interaction is exponentially cut off
the ``crystallized'' Wigner molecule is destroyed in favor of an inhomogeneous
charge distribution similar to a charge density wave .Comment: 10 pages (excl. Figures), Figures available on request LaTe
The Kondo Effect in the Unitary Limit
We observe a strong Kondo effect in a semiconductor quantum dot when a small
magnetic field is applied. The Coulomb blockade for electron tunneling is
overcome completely by the Kondo effect and the conductance reaches the
unitary-limit value. We compare the experimental Kondo temperature with the
theoretical predictions for the spin-1/2 Anderson impurity model. Excellent
agreement is found throughout the Kondo regime. Phase coherence is preserved
when a Kondo quantum dot is included in one of the arms of an Aharonov-Bohm
ring structure and the phase behavior differs from previous results on a
non-Kondo dot.Comment: 10 page
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