1,400 research outputs found
Scanning Gate Spectroscopy on Nanoclusters
A gated probe for scanning tunnelling microscopy (STM) has been developed.
The probe extends normal STM operations by means of an additional electrode
fabricated next to the tunnelling tip. The extra electrode does not make
contact with the sample and can be used as a gate. We report on the recipe used
for fabricating the tunnelling tip and the gate electrode on a silicon nitride
cantilever. We demonstrate the functioning of the scanning gate probes by
performing single-electron tunnelling spectroscopy on 20-nm gold clusters for
different gate voltages.Comment: 3 pages, 4 figure
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
Electron Pair Resonance in the Coulomb Blockade
We study many-body corrections to the cotunneling current via a localized
state with energy at large bias voltages . We show that the
transfer of {\em electron pairs}, enabled by the Coulomb repulsion in the
localized level, results in ionization resonance peaks in the third derivative
of the current with respect to , centered at . Our
results predict the existence of previously unnoticed structure within
Coulomb-blockade diamonds.Comment: 5 pages, 4 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
Charge noise analysis of an AlGaAs/GaAs quantum dot using transmission-type radio-frequency single-electron transistor technique
Radio-frequency (rf)- operated single-electron transistors (SETs) are
high-sensitivity, fast-response electrometers, which are valuable for
developing new insights into single-charge dynamics. We investigate
high-frequency (up to 1 MHz) charge noise in an AlGaAs/GaAs quantum dot using a
transmission-type rf-SET technique. The electron capture and emission kinetics
on a trap in the vicinity of the quantum dot are dominated by a Poisson
process. The maximum bandwidth for measuring single trapping events is about 1
MHz, which is the same as that required for observing single-electron tunneling
oscillations in a measurable current (~0.1pA).Comment: 4 pages, 4 figures, to be published in Appl. Phys. Let
Evolution of SU(4) Transport Regimes in Carbon Nanotube Quantum Dots
We study the evolution of conductance regimes in carbon nanotubes with doubly
degenerate orbitals (``shells'') by controlling the contact transparency within
the same sample. For sufficiently open contacts, Kondo behavior is observed for
1, 2, and 3 electrons in the topmost shell. As the contacts are opened more,
the sample enters the ``mixed valence'' regime, where different charge states
are strongly hybridized by electron tunneling. Here, the conductance as a
function of gate voltage shows pronounced modulations with a period of four
electrons, and all single-electron features are washed away at low temperature.
We successfully describe this behavior by a simple formula with no fitting
parameters. Finally, we find a surprisingly small energy scale that controls
the temperature evolution of conductance and the tunneling density of states in
the mixed valence regime.Comment: 4 pages + supplementary info. The second part of the original
submission is now split off as a separate paper (0709.1288
Detection of Quantum Noise from an Electrically-Driven Two-Level System
Quantum mechanics can strongly influence the noise properties of mesoscopic
devices. To probe this effect we have measured the current fluctuations at
high-frequency (5-90 GHz) using a superconductor-insulator-superconductor
tunnel junction as an on-chip spectrum analyser. By coupling this
frequency-resolved noise detector to a quantum device we can measure the
high-frequency, non-symmetrized noise as demonstrated for a Josephson junction.
The same scheme is used to detect the current fluctuations arising from
coherent charge oscillations in a two-level system, a superconducting charge
qubit. A narrow band peak is observed in the spectral noise density at the
frequency of the coherent charge oscillations.Comment: 16 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
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