876 research outputs found
Kondo effect in coupled quantum dots with RKKY interaction: Finite temperature and magnetic field effects
We study transport through two quantum dots coupled by an RKKY interaction as
a function of temperature and magnetic field. By applying the Numerical
Renormalization Group (NRG) method we obtain the transmission and the linear
conductance. At zero temperature and magnetic field, we observe a quantum phase
transition between the Kondo screened state and a local spin singlet as the
RKKY interaction is tuned. Above the critical RKKY coupling the Kondo peak is
split. However, we find that both finite temperature and magnetic field restore
the Kondo resonance. Our results agree well with recent transport experiments
on gold grain quantum dots in the presence of magnetic impurities.Comment: 4 pages, 5 figure
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-stage Kondo effect in side-coupled quantum dots: Renormalized perturbative scaling theory and Numerical Renormalization Group analysis
We study numerically and analytically the dynamical (AC) conductance through
a two-dot system, where only one of the dots is coupled to the leads but it is
also side-coupled to the other dot through an antiferromagnetic exchange (RKKY)
interaction. In this case the RKKY interaction gives rise to a ``two-stage
Kondo effect'' where the two spins are screened by two consecutive Kondo
effects. We formulate a renormalized scaling theory that captures remarkably
well the cross-over from the strongly conductive correlated regime to the low
temperature low conductance state. Our analytical formulas agree well with our
numerical renormalization group results. The frequency dependent current noise
spectrum is also discussed.Comment: 6 pages, 7 figure
Impurity effects in few-electron quantum dots: Incipient Wigner molecule regime
Numerically exact path-integral Monte Carlo data are presented for
strongly interacting electrons confined in a 2D parabolic quantum dot,
including a defect to break rotational symmetry. Low densities are studied,
where an incipient Wigner molecule forms. A single impurity is found to cause
drastic effects: (1) The standard shell-filling sequence with magic numbers
, corresponding to peaks in the addition energy , is
destroyed, with a new peak at N=8, (2) spin gaps decrease,
(3) for N=8, sub-Hund's rule spin S=0 is induced, and (4) spatial ordering of
the electrons becomes rather sensitive to spin. We also comment on the recently
observed bunching phenomenon.Comment: 7 pages, 1 table, 4 figures, accepted for publication in Europhysics
Letter
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
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