Nanotube Quantum Dots Research Articles

Friedel Oscillations in Carbon Nanotube Quantum Dots and Superlattices

Proceedings of the XXXVII International School of Semiconducting Compounds, Jaszowiec 2008.

Artificial atom and quantum terahertz response in carbon nanotube quantumdots

Artificial atom behaviours have been observed in single-wall carbon nanotube (SWCNT)quantum dots (QDs). Two-electron shell structures and the Zeeman splitting ofsingle-particle states were revealed in single-electron transport measurements in lowtemperatures. To demonstrate that the charging energy of the dot lies in a terahertz (THz)range, the THz photon-assisted tunnelling was tested, and was really observed as a satelliteCoulomb peak. Some satellite peaks moved as a frequency was changed, but other peaksdid not move. We give possible models to explain the existence of two different satellitepeaks.

Electron transport in carbon nanotube quantum dots in an axial magnetic field

The quantum conductance of the quantum dots (QDs) made of two kinds of primary carbon nanotubes (CNTs), i.e., armchair and zigzag CNTs, threaded by an axial magnetic field, has been studied by using the tight binding approximation and constant interaction model. It is found that under increasing axial magnetic field, each conductance shell of the zigzag CNT-QDs could split into two groups with each group of two peaks moving up or down, respectively. And the up- and down-moving two peaks would re-group with other two peaks, down- and up-moving, in the neighboring shell, forming a new four-peak shell, and then re-splitting, re-grouping again due to the Aharonov–Bohm effect, which is in agreement with those of experiments. But, in contrast, the conductance shells of the armchair CNT-QDs do not split by the magnetic field. Our subsequent theoretical studies show further that the above phenomena, i.e., the conductance shell-splitting, re-grouping, and re-splitting again with increasing the magnetic field exist in all the CNT-QDs except for the armchair one.

Friedel-like oscillations in carbon nanotube quantum dots

The interface states of all-metallic carbon nanotube quantum dots are studied based on a tight-binding approach and a Green’s function matching technique. We have found that depending on the type of metallic tube, the energy of interface states may show an oscillatory behavior. We identify these as steamming from Friedel oscillations. We comment on the possible implications of this finding on other physical properties, such as stability during the growth of nanotube junctions and magnetic interaction through carbon nanotubes.

Photoluminescence properties of highly dispersed ZnO quantum dots in polyvinylpyrrolidone nanotubes prepared by a single capillary electrospinning

Highly dispersed ZnO quantum dots (QDs) in polyvinylpyrrolidone (PVP) nanotubes have been prepared by a single capillary electrospinning. The structure and optical properties characterizations were performed by x-ray diffraction, scanning and transmission electron microscopy, absorption, photoluminescence, and resonant Raman spectra. In the composites, PVP molecules passivate the surface defects of ZnO QDs and prevent the aggregations of ZnO QDs. As a result, the composites exhibit narrower band edge emissions and less laser thermal effects. Blueshifted band gap, enlarged exciton energy, and less exciton-longitudinal optical (LO) phonon interaction due to the quantum confinement effect have also been observed.

Adiabatic Charge Pumping in Carbon Nanotube Quantum Dots

We investigate charge pumping in carbon nanotube quantum dots driven by the electric field of a surface acoustic wave. We find that, at small driving amplitudes, the pumped current reverses polarity as the conductance is tuned through a Coulomb blockade peak using a gate electrode. We study the behavior as a function of wave amplitude, frequency, and direction and develop a model in which our results can be understood as resulting from adiabatic charge redistribution between the leads and quantum dots on the nanotube.

Highly sensitive and broadband carbon nanotube radio-frequency single-electron transistor

We have investigated radio-frequency single-electron transistor operation of single-walled carbon nanotube quantum dots in the strong tunneling regime. At a temperature of 4.2 K and with a carrier frequency of 754.2 MHz, we reach a charge sensitivity of 2.3×10−6e/Hz over a bandwidth of 85 MHz. Our results indicate a gain-bandwidth product of 3.7×1013 Hz(3/2)/e, which is by one order of magnitude better than those for typical radio-frequency single-electron transistors.

Pauli spin blockade in carbon nanotube double quantum dots

We report Pauli spin blockade in a carbon nanotube double quantum dot defined by tunnel barriers at the contacts and a structural defect in the nanotube. We observe a pronounced current suppression for negative source-drain bias voltages, which is investigated for both symmetric and asymmetric coupling of the quantum dots to the leads. The measured differential conductance agrees well with a theoretical model of a double quantum dot system in the spin-blockade regime, which allows us to estimate the occupation probabilities of the relevant singlet and triplet states. This work shows that effective spin-to-charge conversion in nanotube quantum dots is feasible and opens the possibility of single-spin readout in a material that is not limited by hyperfine interaction with nuclear spins.

Spin-orbit interaction and anomalous spin relaxation in carbon nanotube quantum dots

We study spin relaxation and decoherence in nanotube quantum dots caused by electron-lattice and spin-orbit interaction and predict striking effects induced by magnetic fields B. For particular values of B, destructive interference occurs resulting in ultralong spin relaxation times T-1 exceeding tens of seconds. For small phonon frequencies omega, we find a 1/root omega spin-phonon noise spectrum-a dissipation channel for spins in quantum dots-which can reduce T-1 by many orders of magnitude. We show that nanotubes exhibit zero-field level splitting caused by spin-orbit interaction. This enables an all-electrical and phase-coherent control of spin.

Quantum phase transition in a single-molecule quantum dot

Quantum criticality is the intriguing possibility offered by the laws of quantum mechanics when the wave function of a many-particle physical system is forced to evolve continuously between two distinct, competing ground states. This phenomenon, often related to a zero-temperature magnetic phase transition, is believed to govern many of the fascinating properties of strongly correlated systems such as heavy-fermion compounds or high-temperature superconductors. In contrast to bulk materials with very complex electronic structures, artificial nanoscale devices could offer a new and simpler means of understanding quantum phase transitions. Here we demonstrate this possibility in a single-molecule quantum dot, where a gate voltage induces a crossing of two different types of electron spin state (singlet and triplet) at zero magnetic field. The quantum dot is operated in the Kondo regime, where the electron spin on the quantum dot is partially screened by metallic electrodes. This strong electronic coupling between the quantum dot and the metallic contacts provides the strong electron correlations necessary to observe quantum critical behaviour. The quantum magnetic phase transition between two different Kondo regimes is achieved by tuning gate voltages and is fundamentally different from previously observed Kondo transitions in semiconductor and nanotube quantum dots. Our work may offer new directions in terms of control and tunability for molecular spintronics.

Photon-Assisted Electron Transport for a Λ-shaped Carbon Nanotube Junction

We theoretically study the electron transport properties for two coupled single-walled carbon nanotube quantum dots connected to metallic electrodes under the irradiation of an external electromagnetic field at low temperatures. Using the standard nonequilibrium Green's function techniques, we examine the time-averaged transmission coefficient and linear conductance. It is shown that by some numerical examples, the photon-assisted inter-dot coupling causes Fano resonance and the conductance of the system is sensitive to the external field parameters. The transport dependence on the external field parameters may be used to detect the high-frequency microwave irradiation.

Gate-dependent tunneling-induced level shifts observed in carbon nanotube quantum dots

We have studied electron transport in clean single-walled carbon nanotube quantum dots. Because of the large number of the Coulomb blockade diamonds simultaneously showing both shell structure and the Kondo effect, we are able to perform a detailed analysis of tunneling renormalization effects. Thus, determining the environment induced level shifts of this artificial atom. In shells where only one of the two orbitals is strongly coupled, we observe a marked asymmetric gate dependence of the inelastic cotunneling lines together with a systematic gate dependence of the size (and shape) of the Coulomb diamonds. These effects are all given a simple explanation in terms of second-order perturbation theory in the tunnel coupling.

Electric-field-controlled spin reversal in a quantum dot with ferromagnetic contacts

Electric-field induced control of the magnetic ground state of a carbon nanotube quantum dot enables the orientation of injected spins to be reversed without using an external magnetic field.

Transport properties of double-walled carbon nanotube quantum dots

The transport properties of quantum dot (QD) systems based on double-walled carbon nanotubes (DWCNTs) are investigated. The interplay between microscopic structure and strong Coulomb interaction is treated within a bosonization framework. The linear and nonlinear current-voltage characteristics of the QD system are calculated by starting from the Liouville equation for the reduced density matrix. Depending on the intershell couplings, an eight-electron periodicity of the Coulomb blockade peak spacing in the case of commensurate DWCNT QDs and a four-electron periodicity in the incommensurate case are predicted. The contribution of excited states of DWCNTs to the nonlinear transport is investigated as well.

Terahertz photon-assisted tunneling in carbon nanotube quantum dots

The authors have studied the transport properties of carbon nanotube quantum dots under terahertz (THz) wave irradiation. The experimental data have shown that the satellite currents are generated with the THz irradiation, and that the peak position of the satellite currents varies linearly with the THz photon energy. These results provide experimental evidence for photon-assisted tunneling in the THz region. The present observation provides the interesting possibility of developing a highly sensitive and frequency-tunable THz detector capable of high-temperature operation.

Thermoelectric Effects in Carbon Nanotube Quantum Dot in the Kondo Regime

Thermoelectric Effects in Carbon Nanotube Quantum Dot in the Kondo Regime

Tunnel Magnetoresistance in Carbon Nanotube Quantum Dot

The out-of-equilibrium transport properties of carbon nanotube quantum dot in the Kondo regime are studied by means of the non-equilibrium Green function. The equation of motion method is used. The in∞uence of the polarization of electrodes and orbital level splitting, as well as leftright asymmetry, on the spin polarizations of difierential conductance are discussed. For zero bias voltage and orbitally degenerate states the SU(4) symmetry of Kondo state is preserved for antiparallel conflguration of polarizations of electrodes, whereas it is broken for parallel. In the former case a suppression of linear conductance with increasing polarization is observed. In the latter the behaviour is nonmonotonic due to splitting of the Kondo peak and bringing closer one of the peaks to the Fermi level with increasing polarization. This gives rise to giant tunnel linear magnetoresistance for large polarization.

Su(4) mixed valence regime 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.

Half-filling SU(4) Kondo state in carbon nanotubes: Numerical results

Charge transport measurements in carbon nanotube quantum dots by P. Jarillo-Herrero et al. [Nature 434 (2005) 484] have detected a strongly enhanced Kondo temperature T K ≈ 8.0 K . This Kondo state was associated with the SU(4) symmetry of the Hamiltonian at quarter-filling for an orbitally double-degenerate single-occupied electronic shell, indicating the simultaneous Kondo screening of charge and spin. Pure SU(4) symmetry can only be achieved when the orbital quantum number is preserved upon tunneling. So-called channel mixing can destroy the SU(4) state, transforming it into an SU(2)-type Kondo state (the so-called two-level SU(2) state). In this work, numerical calculations of charge transport show in detail the transition between these two Kondo states. Our results indicate that the SU(4) state seems to be quite robust, surviving to considerable amount of channel mixing. Moreover, a curious behavior of the conductance is revealed close to the two level SU(2) state.

Standing‐wave observations in single‐wall carbon nanotube quantum dots and Y‐junction rings

AbstractStanding electronic wave formation inside single walled carbon nanotube Y‐junction rings and quantum dots with a quantum‐dot‐like quadratic dispersion is reported. Remarkably, the detailed structural defect configurations play only a minor role and the standing wave oscillation dispersions are mainly triggered by the electronic properties of the pristine constituent tubes. This raises the possibility of using both structures as nanoscale tuneable electronic switching devices. Near the Fermi level, quantum dots show smooth dispersing oscillations reminiscent of the finite pristine tube eigenvalues. However, localized density of states peaks are found in YJRs, which are identified as interference effects of the double‐slit interferometer. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)