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. The Anderson model of a localized magnetic impurity yields both the Kondo regime and the closely related Mixed Valence regime [8]. In the conventional Kondo regime the charge of an impurity or a nanostructure is an integer while the spin state alternates. In the Mixed Valence regime the charge is not quantized and also fluctuates [5]. In the Coulomb Blockade systems, the Mixed Valence regime has been previously realized in a narrow range of gate voltages close to the charge degeneracy points (i.e. in the vicinity of conductance peaks) [9]. However, if the coupling to the contacts is increased, the regions of well-defined charge should gradually disappear and the Mixed Valence behavior should spread over the conductance valleys. Our paper is devoted to studying this regime.We study the evolution [10] from the Kondo to the Mixed Valence regime by controlling the contact transparency within the same semiconducting nanotube. The quantum-mechanical orbitals originating in two elec-

We study the SU͑4͒ Kondo effect in carbon nanotube quantum dots, where doubly degenerate orbitals form four-electron "shells." The SU͑4͒ Kondo behavior is investigated for one, two, and three electrons in the topmost shell. While the Kondo state of two electrons is quenched by a magnetic field, in the case of an odd number of electrons two types of SU͑2͒ Kondo effect may survive. Namely, the spin SU͑2͒ state is realized in a magnetic field parallel to the nanotube ͑inducing primarily orbital splitting͒. Application of the perpendicular field ͑inducing Zeeman splitting͒ results in the orbital SU͑2͒ Kondo effect.At low temperatures, a variety of nanoscale Coulomb blockade 1 systems with degenerate ground states exhibit the Kondo effect. 2 This many-body phenomenon has now been observed in semiconductor quantum dots, molecules, carbon nanotubes, and magnetic addatoms on metallic surfaces ͑see Ref. 3 for a review͒. In high-quality nanotubes the quantummechanical orbitals originating in two electronic subbands are doubly degenerate, forming four-electron "shells" 4,5 ͑see also Ref. 6 for additional references͒. In each shell, the Kondo behavior develops in the valleys with one, two, and three electrons. 4,7 The Kondo effect with one electron in a shell is expected 8 to obey the SU͑4͒ symmetry, 9,10 as studied recently in Ref. 11. In this paper, we investigate the SU͑4͒ Kondo effect in the one-, two-, and three-electron valleys in a magnetic field.The nanotubes are grown on a Si/ SiO 2 substrate by chemical vapor deposition using CO as a feedstock gas. 12 This method was verified to produce mostly single-wall nanotubes with diameters of about 2 nm. Cr/ Au electrodes separated by 200 nm ͑Figs. 1-3͒ or 600 nm ͑Fig. 4͒ are deposited on top of the nanotubes. All the measurements are conducted at temperatures between 1.2 K and 2 K. We choose to work with several small-gap semiconducting nanotubes, 13 which demonstrate high p-type conductance at negative gate voltages. At positive gate voltages, the middle section of the nanotube fills with electrons. The part of the nanotube adjacent to the electrodes stays p-type ͑"leads"͒. Therefore, a quantum dot is formed within a nanotube, defined by p-n and n-p junctions. As a result, a Coulomb blockade sets in at low temperatures ͑Fig. 1͒. Figure 1 shows conductance map of a 200-nm-long nanotube quantum dot measured as a function of the source-drain bias V SD and gate voltage V gate . The "Coulomb diamonds" 1 demonstrate clear four-electron shell filling. The p-n junction transparency grows with V gate , resulting in an enhancement of the Kondo effect in each successive shell. The zero-bias Kondo ridge appears in Coulomb diamonds with one, two, and three electrons ͑visible for V gate Ͼ 10 V͒.The ambipolar semiconductor nanotubes as studied here are uniquely suited for observation of the SU͑4͒ Kondo effect: the electrons are reflected adiabatically from the p-n junctions at the ends of the quantum dot, resulting in little mode mixing. Therefore, the level mismatch between the two orbi...

Single Walled carbon nanotubes (SWNTs) were grown directly on flat substrates using chemical vapor deposition (CVD) method with carbon monoxide and hydrogen mixture as feeding gas. Comparing with other CVD methods, this new approach yields higher efficiency and more reproducible results in obtaining high quality SWNTs separated as individual nanotubes on substrates. Such samples can be used to fabricate nanodevices directly with no further purification or dispersion. Important factors that affect the nanotube growth and possible mechanisms are also discussed.Since their discovery, single-walled carbon nanotubes (SWNTs) have been heavily studied owing to their outstanding physical and chemical properties. 1 They are widely considered as ideal candidates for interconnections and active components in nanoscale electronic devices. Recent research has demonstrated SWNT-based nanodevices, such as quantum wires, 2 field-effect transistors, 3-5 field emitters, 6 etc. Until recently, most of the SWNT-based devices used materials synthesized by either the laser ablation method 7 or the arc method, 8 where the as-grown samples have to be purified and suspended in solvent before deposition on surfaces for device fabrication. The processes of purification and suspending nanotubes in solvent involve the use of highly oxidative chemicals and ultrasonication that are known to create defects on nanotubes and alter their electronic properties. For nanoscale electronics, defect-free SWNTs on surfaces are especially desirable. An alternative and successful way to fabricate nanotube devices is to use nanotubes grown directly on surfaces to avoid the undesired purification steps. However, up to now, surface growth of SWNTs has been limited to chemical vapor decomposition (CVD) of methane on metal nanoparticles supported on surfaces. [9][10][11][12] The growth conditions are critical for the preparation of nanotubes with high purity and efficiency. Here we report an improved CVD method for the growth of SWNTs on surfaces using monodispersed Fe/Mo nanoparticles as the catalyst and a mixture of carbon monoxide (CO) and hydrogen (H 2 ) as feed gas. In contrast to CVD of methane, 9-12 this method allows higher efficiency for nanotube growth and easier control of the growth conditions, thus offering more reproducible results.In our experiments, the catalysts are monodispersed Fe/ Mo nanoparticles (4.2 nm) prepared according to reference 13. The nanoparticles can be dissolved in nonpolar solvents such as hexane. They were deposited on the substrate by slow evaporation of the solution. The substrate was then annealed in air at 700°C for 10 min to remove the organic coating on the particles formed during their preparation steps. 13 Si/SiO 2 substrates were purchased from MEMC Electronic Materials with 500 nm SiO 2 layer. Alumina (sapphire A plane) and magnesium oxide substrates (Superconductive Components, Inc.) were also used. To grow nanotubes, we used a two-furnace setup (Figure 1). The substrates were loaded into the second furnac...

The quantum-mechanical orbitals in carbon nanotubes are doubly degenerate over a large number of states in the Coulomb blockade regime. We argue that this experimental observation indicates that electrons are reflected without mode mixing at the nanotube-metal contacts. Two electrons occupying a pair of degenerate orbitals (a "shell") are found to form a triplet state starting from zero magnetic field. Finally, we observe unexpected low-energy excitations at complete filling of a four-electron shell.

We find that electrons in single-wall carbon nanotubes may propagate substantial distances ͑tens of nanom-eters͒ under metal contacts. We perform four-probe transport measurements of the nanotube conductance and observe significant deviations from the standard Kirchhoff's circuit rules. Most noticeably, injecting current between two neighboring contacts on one end of the nanotube induces a nonzero voltage difference between two contacts on the other end.