Spintronics aims to develop electronic devices whose resistance is controlled by the spin of the charge carriers that flow through them 1-3 . This approach is illustrated by the operation of the most basic spintronic device, the spin valve 4-6 , which can be formed if two ferromagnetic electrodes are separated by a thin tunnelling barrier. In most cases, its resistance is greater when the two electrodes are magnetized in opposite directions than when they are magnetized in the same direction 7,8 . The relative difference in resistance, the so-called magnetoresistance, is then positive. However, if the transport of carriers inside the device is spin-or energy-dependent 3 , the opposite can occur and the magnetoresistance is negative 9 . The next step is to construct an analogous device to a field-effect transistor by using this effect to control spin transport and magnetoresistance with a voltage applied to a gate 10,11 . In practice though, implementing such a device has proved difficult. Here, we report on a pronounced gate-field-controlled magnetoresistance response in carbon nanotubes connected by ferromagnetic leads. Both the magnitude and the sign of the magnetoresistance in the resulting devices can be tuned in a predictable manner. This opens an important route to the realization of multifunctional spintronic devices.Early work on spin transport in multiwall carbon nanotubes (MWNTs) with Co contacts showed that spins could propagate coherently over distances as long as 250 nm (ref. 12). The tunnel magnetoresistance (TMR) = (R AP − R P )/R P , defined as the relative difference between the resistances R AP and R P in the antiparallel and parallel magnetization configuration, was found to be positive and amounted to +4% in agreement with Jullière's formula for tunnel junctions 4,13 . A negative TMR of about −30% was reported later for MWNTs contacted with similar Co contacts 14 . In these experiments, the nanotubes did not show quantum dot behaviour. It has been shown, however, that single-wall carbon nanotubes (SWNTs) and MWNTs contacted with nonferromagnetic metals could behave as quantum dots and FabryPérot resonators [15][16][17][18][19] , in which one can tune the position of discrete energy levels with a gate electrode. From this, one can expect to be able to tune the sign and the amplitude of the TMR in nanotubes, in a similar fashion as predicted originally for semiconductor heterostructures 11 .In this letter, we report on TMR measurements of MWNTs and SWNTs that are contacted with ferromagnetic electrodes and capacitively coupled to a back-gate 20 . A typical sample geometry is shown in the inset of Fig. 1. As a result of resonant tunnelling, we observe a striking oscillatory amplitude and sign modulation of the TMR as a function of the gate voltage. We have studied and observed the TMR on nine samples (seven MWNTs and two SWNTs) with various tube lengths L between the ferromagnetic electrodes (see the Methods section). We present here results for one MWNT device and one SWNT device.We first discu...
We report electrical transport measurements through a semiconducting single-walled carbon nanotube (SWNT) with three additional top-gates. At low temperatures the system acts as a double quantum dot with large inter-dot tunnel coupling allowing for the observation of tunnel-coupled molecular states extending over the whole double-dot system. We precisely extract the tunnel coupling and identify the molecular states by the sequential-tunneling line shape of the resonances in differential conductance.Comment: 5 pages, 4 figure
We report on electrical transport measurements in a carbon nanotube quantum dot coupled to a normal and a superconducting lead. Depending on the ratio of Kondo temperature T K and superconducting gap , the zero bias conductance resonance either is split into two side-peaks or persists. We also compare our data with a simple model of a resonant level-superconductor interface.At low temperatures carbon nanotubes act as quantum dots. Different transport regimes, depending on the transparency of the contacts, such as the Coulomb blockade and Kondo effect, can be realized [1,2]. Recently it has also been possible to couple a carbon nanotube quantum dot in the Kondo regime to superconducting leads, demonstrating a rich interplay of these two many-particle phenomena [3]. In this paper we consider a slightly different geometry, namely a quantum dot connected to both a normal and a superconducting lead. These hybrid systems are interesting for two reasons. First, the interplay of the Kondo effect and superconductivity can be examined on a different basis. Various predictions have been made for this scenario, e.g. suppression or enhancement of the conductance [4], side-peaks at the position of the superconducting gap [5] and excess Kondo resonances [6]. Second, the structure mentioned above is the basic building block of proposed Andreev entanglers making use of either the zero-dimensional quantum dot charging energy U C [7] or the one-dimensional Luttinger repulsion energy of a nanotube in order to spatially separate pairs of entangled electrons [8]. In the following we will focus on the interplay of the Kondo effect and the superconducting lead.Here we report on electrical transport measurements of a multi-wall carbon nanotube (MWNT) quantum dot connected to a normal and a superconducting lead. The sample is prepared as follows. First MWNTs are spread on a degenerately doped silicon substrate, in the experiment serving as a backgate, which is covered by a 400 nm insulating layer of SiO 2 . Then single nanotubes are contacted by means of standard electron-beam-lithography and e-gun-evaporation. Similar to reference [3] the superconducting contact is a 45 nm Au/160 nm Al proximity bilayer. However, by using tilt-angle evaporation for the Al layer one obtains a structure such as the one sketched in figure 1(a). Whereas the left-hand side of the MWNT is coupled to the superconducting Au/Al bilayer, the right-hand electrode is formed simply by the 45 nm gold layer. There will also be Al deposited on this side, but the spatial separation of the nanotube-gold contact and the Al film is fairly long (approximately 1 µm). To check quantitatively whether also on this side of the sample proximity effects have to be taken into account, one can estimate the Thoules energy [11]. The Thoules energy represents an upper limit in energy for observing superconducting correlations (assuming perfect barriers). One obtains E T =h D/L 2 ≈ 3 µeV ≈ 10 mK using a gold diffusion constant D = 5 × 10 −3 m 2 s −1(corresponding to an estimated...
We report the experimental realization of double quantum dots in singlewalled carbon nanotubes. The device consists of a nanotube with source and drain contact, and three additional top-gate electrodes in between. We show that, by energizing these top-gates, it is possible to locally gate a nanotube, to create a barrier, or to tune the chemical potential of a part of the nanotube. At low temperatures we find (for three different devices) that in certain ranges of top-gate voltages our device acts as a double quantum dot, evidenced by the typical honeycomb charge stability pattern.
In this letter we present an experimental realization of the quantum mechanics textbook example of two interacting electronic quantum states that hybridize forming a molecular state. In our particular realization, the quantum states themselves are fabricated as quantum dots in a molecule, a carbon nanotube. For sufficient quantum-mechanical interaction (tunnel coupling) between the two quantum states, the molecular wavefunction is a superposition of the two isolated (dot) wavefunctions. As a result, the electron becomes delocalized and a covalent bond forms. In this work, we show that electrical transport can be used as a sensitive probe to measure the relative weight of the two components in the superposition state as a function of the gate-voltages. For the field of carbon nanotube double quantum dots, the findings represent an additional step towards the engineering of quantum states. In the quantum world particles such as electrons behave as extended objects with the character of a wave. The combination of wave-like behaviour on the one hand and interactions on the other hand leads to 'localized' electron waves, also called quantum states. When two quantum states overlap, quantum interference results in a superposition state with new qualities. In particular, a molecular bond may emerge from the constructive interference of the quantum states. Engineered double quantum dots provide an experimental platform enabling one to control this prototype of molecular formation [1][2][3][4][5][6][7]. In this letter, the molecular states are realized in a double quantum dot device gate-defined in a single-walled carbon nanotube (SWCNT) [8][9][10]. We show that, by analyzing the electrical conductance through the device, it is actually possible to map the electron delocalization of the covalent chemical bond formed between the two quantum dots.In Fig. 1(a-c) we show the experimental realization of our carbon nanotube double quantum dot device. A scanning electron micrograph of such a device, showing source and drain contacts and three top-gate electrodes in between can be seen in Fig. 1(c). The three top-gates can be used to adjust the electrostatic potential landscape within the nanotube as depicted in Fig. 1(a), yielding the double quantum dot structure. Using the outer two gate electrodes the difference of the level energies in the left and right dot, the detuning = (E 1 − E 2 )/2 can be adjusted as well. Transport takes place through a molecular state as indicated in Fig. 1(b), provided the detuning is not much larger than the tunnel coupling t, which in turn is proportional to the overlap of the dot wavefunctions. One can say that the detuning determines the degree of localization of the electron on the double quantum dot. Only for ≈ 0 the probability of finding the electron on the left or the right dot will be comparable, for | | t the electron will mainly be localized on one of the two dots.
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