The Kondo effect and superconductivity are both prime examples of many-body phenomena. Here we report transport measurements on a carbon nanotube quantum dot coupled to superconducting leads that show a delicate interplay between both effects. We demonstrate that the superconductivity of the leads does not destroy the Kondo correlations on the quantum dot when the Kondo temperature, which varies for different single-electron states, exceeds the superconducting gap energy.
We explore the electric-field effect of carbon nanotubes (NTs) in electrolytes. Due to the large gate capacitance, Fermi energy (EF) shifts of order ±1 V can be induced, enabling to tune NTs from p to n-type. Consequently, large resistance changes are measured. At zero gate voltage, the NTs are hole-doped in air with |EF|≈0.3–0.5 eV, corresponding to a doping level of ≈1013 cm−2. Hole-doping increases in the electrolyte.
We have measured the differential conductance of individual multiwall carbon nanotubes. Coulomb blockade and energy level quantization are observed. The electron levels are nearly fourfold degenerate (including spin) and their evolution in magnetic field (Zeeman splitting) agrees with a g factor of 2. In zero magnetic field the sequential filling of states evolves with spin S according to S = 0-->1/2-->0.... A Kondo enhancement of the conductance is observed when the number of electrons on the tube is odd.
We report resonant multiple Andreev reflections in a multiwall carbon nanotube quantum dot coupled to superconducting leads. The position and magnitude of the subharmonic gap structure is found to depend strongly on the level positions of the single-electron states which are adjusted with a gate electrode. We discuss a theoretical model of the device and compare the calculated differential conductance with the experimental data.
We have studied tunneling of electrons into multiwall carbon nanotubes (NTs) in NT-gold and NT-NT junctions, the latter created by atomic force microscope manipulation. The tunneling conductance goes to zero as the energy (temperature and bias) is reduced, and the functional form is consistent with a power law. The exponents depend upon sample geometry. The relationship between these results and theories for tunneling into ballistic and disordered metals is discussed.
Scanning tunneling microscope spectroscopy is used to study in detail the electronic band structure of carbon nanotubes as well as to locally investigate electronic features of interesting topological sites such as nanotube ends and bends. From a large number of measurements of the tunneling density-of-states ͑DOS͒ nanotubes can be classified, according to predictions, as either semiconducting ͑two-third of the total number of tubes͒ or metallic ͑one-third͒. The energy subband separations in the tunneling DOS compare reasonably well to theoretical calculations. At nanotube ends, spatially resolved spectra show additional sharp conductance peaks that shift in energy as a function of position. Spectroscopy measurements on a nanotube kink suggest that the kink is a heterojunction between a semiconducting and a metallic nanotube.
We demonstrate charge pumping in semiconducting carbon nanotubes by a traveling potential wave. From the observation of pumping in the nanotube insulating state we deduce that transport occurs by packets of charge being carried along by the wave. By tuning the potential of a side gate, transport of either electron or hole packets can be realized. Prospects for the realization of nanotube based singleelectron pumps are discussed. DOI: 10.1103/PhysRevLett.95.256802 PACS numbers: 85.35.Kt, 72.50.+b, 73.23.Hk, 73.63.Kv The phenomenon of charge pumping has attracted considerable interest in the last two decades from both fundamental and applied points of view [1][2][3][4][5][6][7][8][9][10]. In pumping, a periodic in time and spatially inhomogeneous external perturbation yields a dc current. If a fixed number n of electrons is transferred during a cycle then the pumping current is quantized in units of ef, where e is the electron charge and f is the perturbation frequency. An important aspect of single-electron pumps is their potential to provide an accurate frequency-current conversion which could close the measurement triangle relating frequency, voltage, and current. Previously, a realization of quantized current I nef has been achieved in two different ways: first, using devices comprising charge islands and controlled by a number of phase-shifted ac signals [3,4,7]; and second, using one-dimensional (1D) channels within a GaAs heterojunction where a surface acoustic wave (SAW) produces traveling potential wells which convey packets of electrons along the channel [5]. In the SAW pumps, transport of charge resembles the pumping of water by an Archimedean screw. When this principle is combined with Coulomb blockade it results in the pumping of a fixed number of electrons n per cycle. For metrological applications, the delivered current should be in the range of 1 nA and at present only the SAW single-electron pumps satisfy this requirement. However, the accuracy of the SAW pumps must be improved significantly for them to find metrological applications.A quantum regime of pumping, in which quantum interference plays a key role, was first described by Thouless [1,2]. In the Thouless mechanism, a traveling periodic perturbation induces minigaps in the spectrum of an electronic system, and when the Fermi level lies in a minigap an integer number of electrons n are transferred during a cycle, resulting in a quantized current flowing without dissipation. From a fundamental physics standpoint, this mechanism represents a new macroscopic quantum phenomenon reminiscent of the quantum Hall effect and of superconductivity. Possible applications of charge pumping are not limited to metrology. For example, the ability of the pumps to control the position of single electrons could be used in various quantum information processing schemes [11,12].Recently it has been pointed out that carbon nanotubes have significant advantages over semiconductor and metallic systems in terms of single-electron pumping [8,9]. The typical Cou...
We investigate radio-frequency (rf) reflectometry in a tunable carbon nanotube double quantum dot coupled to a resonant circuit. By measuring the in-phase and quadrature components of the reflected rf signal, we are able to determine the complex admittance of the double quantum dot as a function of the energies of the single-electron states. The measurements are found to be in good agreement with a theoretical model of the device in the incoherent limit. Besides being of fundamental interest, our results present an important step forward towards non-invasive charge and spin state readout in carbon nanotube quantum dots. PACS numbers: 73.63.Fg, 73.63.Kv, 73.23.Hk, 03.67.Lx An important requirement in any quantum information processing scheme is fast manipulation and readout of the quantum system in which the quantum information is encoded. This requires an understanding of the response of the quantum system at finite frequencies which, in the case of an electronic device, involves an understanding of its complex admittance [1,2]. Of particular interest in the context of quantum information processing are double quantum dots which are widely used to define charge and spin qubits [3]. However, while double quantum dots have been investigated in detail over the last decade, experiments to measure and analyze their complex admittance have not yet been performed and this topic has only recently been addressed theoretically [4]. The admittance of quantum dots at finite frequencies is non-trivial as exemplified by recent experiments on single quantum dots [5,6]. The physics is even richer for double quantum dots as internal charge dynamics, i.e. charge transfer between the quantum dots, has to be taken into account. However, the dependence of the admittance on the internal charge dynamics also provides a route towards charge and spin state readout [7].In this work we present a detailed experimental study of the complex admittance of a carbon nanotube double quantum dot which is measured using rf reflectometry techniques. The measurements are compared with a theoretical model of the device where we use a density matrix approach to calculate the double quantum dot admittance. The good quantitative agreement between the experimental and theoretical results allows us to determine the effective conductance and susceptance of the double dot as a function of the energies of the single-electron states. Our measurements thus present a first quantitative analysis of the complex admittance of a double quantum dot. The demonstrated technique also provides the basis for a simple and fast detection scheme for charge and spin state readout in carbon nanotubes -a material with considerable potential for spin-based quantum information processing [8-13] -without the need for a separate charge detector [14].The device we consider is a carbon nanotube grown by chemical vapour deposition on degenerately doped Si ter- minated by 300 nm SiO 2 , see Fig. 1(a). The nanotube is contacted by Au source and drain electrodes which form the outer ...
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