Coulomb Diamond Measurements Research Articles

Quantum transport in carbon nanotubes covalently functionalized with magnetic molecules

We report on quantum transport measurements of a carbon nanotube (CNT) quantum dot that is functionalized with magnetic {Mn4} complexes. The coupling between the spin‐5/2 MnII ions within each {Mn4} complex is predominantly antiferromagnetic. Coulomb diamond measurements at T = 4 K reveal that the covalent attachment of the complexes to the CNT framework has only little influence on the carbon nanotube quantum dot. Surprisingly, a strong increase of noise is observed upon cooling the sample to T = 30 mK. Time traces of the current taken at a diamond edge reveal a random telegraph signal. We attribute this to fluctuations of molecular spin states of the attached {Mn4} complexes.

Spin blockade with spin singlet electrons

We observe a singlet spin blockade (SSB) in two-electron vertical double quantum dots where the single-electron transport is blocked for spin singlet electrons. In contrast to the conventional Pauli spin blockade with spin triplet electrons, this singlet spin blockade is observed under high magnetic field, where the doubly occupied states in one of the dots go beyond the singlet-triplet ground-state transition. The SSB region in Coulomb diamond measurements is in agreement with the two-electron excitation spectrum. A leakage current of 10 pA order is observed in SSB, consistent with the spin singlet lifetime due to random nuclear spin fluctuations.

Observation of excited states in a graphene quantum dot

We demonstrate that excited states in single-layer graphene quantum dots can be detected via direct transport experiments. Coulomb diamond measurements show distinct features of sequential tunneling through an excited state. Moreover, the onset of inelastic cotunneling in the diamond region could be detected. For low magnetic fields, the positions of the single-particle energy levels fluctuate on the scale of a flux quantum penetrating the dot area. For higher magnetic fields, the transition to the formation of Landau levels is observed. Estimates based on the linear energy-momentum relation of graphene give carrier numbers of the order of 10 for our device.

Coulomb oscillations in three-layer graphene nanostructures

We present transport measurements on a tunable three-layer graphene single electron transistor (SET). The device consists of an etched three-layer graphene flake with two narrow constrictions separating the island from source and drain contacts. Three lateral graphene gates are used to electrostatically tune the device. An individual three-layer graphene constriction has been investigated separately showing a transport gap near the charge neutrality point. The graphene tunneling barriers show a strongly nonmonotonic coupling as a function of gate voltage indicating the presence of localized states in the constrictions. We show Coulomb oscillations and Coulomb diamond measurements proving the functionality of the graphene SET. A charging energy of ≈0.6 meV is extracted.

Charge detection in graphene quantum dots

We report measurements on a graphene quantum dot with an integrated graphene charge detector. The quantum dot device consists of a graphene island (diameter of ∼200 nm) connected to source and drain contacts via two narrow graphene constrictions. From Coulomb diamond measurements a charging energy of 4.3 meV is extracted. The charge detector is based on a 45 nm wide graphene nanoribbon placed approximately 60 nm from the island. We show that resonances in the nanoribbon can be used to detect individual charging events on the quantum dot. The charging induced potential change on the quantum dot causes a steplike change in the current in the charge detector. The relative change in the current ranges from 10% up to 60% for detecting individual charging events.

Tunable Graphene Single Electron Transistor

We report electronic transport experiments on a graphene single electron transistor. The device consists of a graphene island connected to source and drain electrodes via two narrow graphene constrictions. It is electrostatically tunable by three lateral graphene gates and an additional back gate. The tunneling coupling is a strongly nonmonotonic function of gate voltage indicating the presence of localized states in the barriers. We investigate energy scales for the tunneling gap, the resonances in the constrictions, and for the Coulomb blockade resonances. From Coulomb diamond measurements in different device configurations (i.e., barrier configurations) we extract a charging energy of approximately 3.4 meV and estimate a characteristic energy scale for the constriction resonances of approximately 10 meV.

Tunable Coulomb blockade in nanostructured graphene

We report on Coulomb blockade and Coulomb diamond measurements on an etched, tunable single-layer graphene quantum dot. The device consisting of a graphene island connected via two narrow graphene constrictions is fully tunable by three lateral graphene gates. Coulomb blockade resonances are observed and from Coulomb diamond measurements, a charging energy of ≈3.5meV is extracted. For increasing temperatures, we detect a peak broadening and a transmission increase of the nanostructured graphene barriers.

Transport and TEM on dysprosium metallofullerene peapods

AbstractSingle‐walled carbon nanotubes (SWNTs) produced by the arc‐discharge method using non‐magnetic catalysts were filled with Dy@C82 endohedral metallofullerenes. Transport measurements were performed on (Dy@C82)@SWNT metallofullerene peapod samples in field‐effect transistor configuration at 4.2 K. After the transport investigations the substrates were partially etched away enabling us to take TEM images of the very same metallofullerene peapod samples on which transport had been carried out before. This procedure gives essential information on whether the sample measured was a single tube or a bundle and on the degree of tube filling. In the case of the presented measurements, TEM investigations revealed one sample to be a metallofullerene peapod bundle and another one to be an individual tube being not or not completely filled despite of a high overall sample filling. Both samples behave as semiconductors, where the latter sample exhibits Coulomb blockade oscillations for low bias voltages. Thus we present the first combined Coulomb blockade diamond and TEM measurements on a single nanotube and moreover the first transport data including the transfer characteristics I sd(V g) of a sample which is confirmed to be a metallofullerene peapod bundle. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Single-hole transistor in p-type GaAs∕AlGaAs heterostructures

A single-hole transistor is patterned in a p-type, C-doped GaAs∕AlGaAs heterostructure by scanning probe oxidation lithography. Clear Coulomb blockade resonances have been observed at Thole=300mK. A charging energy of ∼1.5meV is extracted from Coulomb diamond measurements, in agreement with the lithographic dimensions of the dot. The absence of excited states in Coulomb diamond measurements, as well as the temperature dependence of Coulomb peak heights indicate that the dot is in the multilevel transport regime. Fluctuations in peak spacings larger than the estimated mean single-particle level spacing are observed.