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

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

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

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

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

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 $T^*$ close to the degeneracy point. Below $T^*$, 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

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

SU(4) and SU(2) Kondo Effects in Carbon Nanotube Quantum Dots

We study the SU(4) Kondo effect in carbon nanotube quantum dots, where doubly degenerate orbitals form 4-electron ``shells''. The SU(4) Kondo behavior is investigated for one, two and three electrons in the topmost shell. While the Kondo state of two electrons is quenched by magnetic field, in case of an odd number of electrons two types of SU(2) Kondo effect may survive. Namely, the spin SU(2) state is realized in the magnetic field parallel to the nanotube (inducing primarily orbital splitting). Application of the perpendicular field (inducing Zeeman splitting) results in the orbital SU(2) Kondo effect.Comment: 5 pages. Some material was previously posted in cond-mat/0608573, v

Non Equilibrium Noise as a Probe of the Kondo Effect in Mesoscopic Wires

We study the non-equilibrium noise in mesoscopic diffusive wires hosting magnetic impurities. We find that the shot-noise to current ratio develops a peak at intermediate source-drain biases of the order of the Kondo temperature. The enhanced impurity contribution at intermediate biases is also manifested in the effective distribution. The predicted peak represents increased inelastic scattering rate at the non-equilibrium Kondo crossover.Comment: 4+ pages, 4 figures, published versio