The connection of electrical leads to wire-like molecules is a logical step in the development of molecular electronics, but also allows studies of fundamental physics. For example, metallic carbon nanotubes are quantum wires that have been found to act as one-dimensional quantum dots, Luttinger liquids, proximity-induced superconductors and ballistic and diffusive one-dimensional metals. Here we report that electrically contacted single-walled carbon nanotubes can serve as powerful probes of Kondo physics, demonstrating the universality of the Kondo effect. Arising in the prototypical case from the interaction between a localized impurity magnetic moment and delocalized electrons in a metallic host, the Kondo effect has been used to explain enhanced low-temperature scattering from magnetic impurities in metals, and also occurs in transport through semiconductor quantum dots. The far greater tunability of dots (in our case, nanotubes) compared with atomic impurities renders new classes of Kondo-like effects accessible. Our nanotube devices differ from previous systems in which Kondo effects have been observed, in that they are one-dimensional quantum dots with three-dimensional metal (gold) reservoirs. This allows us to observe Kondo resonances for very large electron numbers (N) in the dot, and approaching the unitary limit (where the transmission reaches its maximum possible value). Moreover, we detect a previously unobserved Kondo effect, occurring for even values of N in a magnetic field.

Manipulation of the spin states of a quantum dot by purely electrical means is a highly desirable property of fundamental importance for the development of spintronic devices such as spin filters, spin transistors and single spin memories as well as for solid-state qubits [1][2][3][4][5][6] . An electrically gated quantum dot in the Coulomb blockade regime can be tuned to hold a single unpaired spin-1/2, which is routinely spin polarized by an applied magnetic field 7 . Using ferromagnetic electrodes, however, the quantum dot becomes spin polarized by the local exchange field [8][9][10][11] . Here, we report on the experimental realization of this tunnelling-induced spin splitting in a carbon-nanotube quantum dot coupled to ferromagnetic nickel electrodes with a strong tunnel coupling ensuring a sizeable exchange field. As charge transport in this regime is dominated by the Kondo effect, we can use this sharp many-body resonance to read off the local spin polarization from the measured bias spectroscopy. We demonstrate that the exchange field can be compensated by an external magnetic field, thus restoring a zero-bias Kondo resonance, and we demonstrate that the exchange field itself, and hence the local spin polarization, can be tuned and reversed merely by tuning the gate voltage.Since their discovery, carbon nanotubes (CNTs) have been intensively studied for their unique electrical properties. Their high Fermi velocity and low content of nuclear spins make them particularly well suited for spintronics applications using the transformation of spin information into electrical signals. Spin-valve effects in nanojunctions with a CNT spanning two ferromagnetic electrodes have already been observed [12][13][14][15] , and a strong gate dependence of the tunnel magnetoresistance has been demonstrated for CNT quantum dots in both the Coulomb blockade and the Fabry-Perot regime 13 . In the intermediate coupling regime, odd-numbered quantum dots exhibit the Kondo effect, seen as a pronounced zero-bias conductance peak at temperatures below a characteristic Kondo temperature [16][17][18] , T K . This effect relies on the conduction electrons being able to flip the spin of the dot during successive cotunnelling events and is therefore expected to be sensitive to spin polarization of the electrodes. As pointed out by Martinek et al. 9 , quantum charge fluctuations, the electrons ability to tunnel on and off the dot, will renormalize the single-particle energy levels. In the case of ferromagnetic electrodes, this energy renormalization will be spin dependent and break the spin degeneracy on the dot, causing the spin states and thereby the zero-bias Kondo peak to split in two. This tunnelling-induced exchange-field splitting of the Kondo peak was observed by Patsupathy et al. 8 in an ungated electromigrated Ni gap holding a C 60 molecule. In contrast, the measurements here were made on a CNT quantum dot coupled to ferromagnetic Ni electrodes and, most importantly, with a back gate that enables us to tune the energy levels on...

We report on quantum dot based Josephson junctions designed specifically for measuring the supercurrent. From high-accuracy fitting of the current-voltage characteristics, we determine the full magnitude of the supercurrent (critical current). Strong gate modulation of the critical current is observed through several consecutive Coulomb blockade oscillations. The critical current crosses zero close to, but not at, resonance due to the so-called 0-pi transition in agreement with a simple theoretical model.

We report measurements of the nonlinear conductance of InAs nanowire quantum dots coupled to superconducting leads. We observe a clear alternation between odd and even occupation of the dot, with sub-gap-peaks at $|V_{sd}|=\Delta/e$ markedly stronger(weaker) than the quasiparticle tunneling peaks at $|V_{sd}|=2\Delta/e$ for odd(even) occupation. We attribute the enhanced $\Delta$-peak to an interplay between Kondo-correlations and Andreev tunneling in dots with an odd number of spins, and substantiate this interpretation by a poor man's scaling analysis

We review transport measurements on singlewalled carbon nanotubes contacted by metal electrodes. At room temperature some devices show transistor action similar to that of p-channel field effect transistors, while others behave as gate-voltage independent wires. At low temperatures transport is usually dominated by Coulomb blockade. In this regime the quantum eigenstates of the finite-length tubes can be studied. At higher temperatures power law behaviour is observed for the temperature and bias dependence of the conductance. This is consistent with tunneling into a onedimensional Luttinger liquid in a nanotube. We also discuss recent developments in contacting nanotubes which should soon allow study of their intrinsic transport properties.

We fabricated reproducible high transparency superconducting contacts consisting of superconducting Ti/Al/Ti trilayers to gated single-wall carbon nanotubes. The reported semiconducting single-wall carbon nanotubes have normal state differential conductance up to 3e2/h and exhibit clear Fabry-Perot interference patterns in the bias spectroscopy plot. We observed subharmonic gap structure in the differential conductance and a distinct peak in the conductance at zero bias, which is interpreted as a manifestation of the supercurrent. The gate dependence of this supercurrent as well as the excess current are examined and compared to the coherent theory of superconducting quantum point contacts with good agreement.

We have observed the Kondo effect in strongly coupled semiconducting nanowire quantum dots. The devices are made from indium arsenide nanowires, grown by molecular beam epitaxy, and contacted by titanium leads. The device transparency can be tuned by changing the potential on a gate electrode, and for increasing transparencies the effects dominating the transport changes from Coulomb Blockade to Universal Conductance Fluctuations with Kondo physics appearing in the intermediate region.Comment: 4 pages, 4 figure

We present two-terminal magnetotransport measurements on single-wall carbon nanotube devices, where one or two of the terminals are ferromagnetic. Both ferromagnetic semiconductor ͓͑Ga,Mn͒As͔ and metal ͑Fe͒ contact materials have been investigated. In both types of devices we have observed strong hysteretic magnetoresistance below 30 K. The magnetoresistance features develop into large peaks and dips at subkelvin temperatures and they are present even with only one ferromagnetic terminal.