Localization, Coulomb interactions and electrical heating in single-wall carbon nanotubes/polymer composites

Low field and high field transport properties of carbon nanotubes/polymer composites are investigated for different tube fractions. Above the percolation threshold f_c=0.33%, transport is due to hopping of localized charge carriers with a localization length xi=10-30 nm. Coulomb interactions associated with a soft gap Delta_CG=2.5 meV are present at low temperature close to f_c. We argue that it originates from the Coulomb charging energy effect which is partly screened by adjacent bundles. The high field conductivity is described within an electrical heating scheme. All the results suggest that using composites close to the percolation threshold may be a way to access intrinsic properties of the nanotubes by experiments at a macroscopic scale.Comment: 4 pages, 5 figures, Submitted to Phys. Rev.

Minimum Conductivity and Evidence for Phase Transitions in Ultra-clean Bilayer Graphene

Bilayer graphene (BLG) at the charge neutrality point (CNP) is strongly susceptible to electronic interactions, and expected to undergo a phase transition into a state with spontaneous broken symmetries. By systematically investigating a large number of singly- and doubly-gated bilayer graphene (BLG) devices, we show that an insulating state appears only in devices with high mobility and low extrinsic doping. This insulating state has an associated transition temperature Tc~5K and an energy gap of ~3 meV, thus strongly suggesting a gapped broken symmetry state that is destroyed by very weak disorder. The transition to the intrinsic broken symmetry state can be tuned by disorder, out-of-plane electric field, or carrier density

Bulk and boundary zero-bias anomaly in multi-wall carbon nanotubes

We compute the tunneling density of states of doped multi-wall nanotubes including disorder and electron-electron interactions. A non-conventional Coulomb blockade reflecting nonperturbative Altshuler-Aronov-Lee power-law zero-bias anomalies is found, in accordance with recent experimental results. The presence of a boundary implies a universal doubling of the boundary exponent in the diffusive limit.Comment: 4 pages, to appear in PRL (revised version

Paraconductivity in Carbon Nanotubes

We report the calculation of paraconductivity in carbon nanotubes above the superconducting transition temperature. The complex behavior of paraconductivity depending upon the tube radius, temperature and magnetic field strength is analyzed. The results are qualitatively compared with recent experimental observations in carbon nanotubes of an inherent transition to the superconducting state and pronounced thermodynamic fluctuations above $T_{c}$. The application of our results to single-wall and multi-wall carbon nanotubes as well as ropes of nanotubes is discussed.Comment: 7 pages, 1 figur

Quantized Adiabatic Charge Transport in a Carbon Nanotube

The coupling of a metallic Carbon nanotube to a surface acoustic wave (SAW) is proposed as a vehicle to realize quantized adiabatic charge transport in a Luttinger liquid system. We demonstrate that electron backscattering by a periodic SAW potential, which results in miniband formation, can be achieved at energies near the Fermi level. Electron interaction, treated in a Luttinger liquid framework, is shown to enhance minigaps and thereby improve current quantization. Quantized SAW induced current, as a function of electron density, changes sign at half-filling.Comment: 5 pages, 2 figure

Quantum Interference Effects in Electronic Transport through Nanotube Contacts

Quantum interference has dramatic effects on electronic transport through nanotube contacts. In optimal configuration the intertube conductance can approach that of a perfect nanotube ($4e^2/h$). The maximum conductance increases rapidly with the contact length up to 10 nm, beyond which it exhibits long wavelength oscillations. This is attributed to the resonant cavity-like interference phenomena in the contact region. For two concentric nanotubes symmetry breaking reduces the maximum intertube conductance from $4e^2/h$ to $2e^2/h$. The phenomena discussed here can serve as a foundation for building nanotube electronic circuits and high speed nanoscale electromechanical devices

Spin-orbit coupling in interacting quasi-one-dimensional electron systems

We present a new model for the study of spin-orbit coupling in interacting quasi-one-dimensional systems and solve it exactly to find the spectral properties of such systems. We show that the combination of spin-orbit coupling and electron-electron interactions results in: the replacement of separate spin and charge excitations with two new kinds of bosonic mixed-spin-charge excitation, and a characteristic modification of the spectral function and single-particle density of states. Our results show how manipulation of the spin-orbit coupling, with external electric fields, can be used for the experimental determination of microscopic interaction parameters in quantum wires.Comment: 5 pages including 4 figures; RevTeX; to appear in Phys.Rev.Let

Shot noise of a quantum dot with non-Fermi liquid correlations

The shot noise of a one-dimensional wire interrupted by two barriers shows interesting features related to the interplay between Coulomb blockade effects, Luttinger correlations and discrete excitations. At small bias the Fano factor reaches the lowest attainable value, 1/2, irrespective of the ratio of the two junction resistances. At larger voltages this asymmetry is power-law renormalized by the interaction strength. We discuss how the measurement of current and these features of the noise allow to extract the Luttinger liquid parameter.Comment: 4 pages, 3 figures,to be published in Phys. Rev. B. For high resolution image of Fig.1 see http://server1.fisica.unige.it/~braggio/doc.ht

Superconductivity in Ropes of Single-Walled Carbon Nanotubes

We report measurements on ropes of Single Walled Carbon Nanotubes (SWNT) in low-resistance contact to non-superconducting (normal) metallic pads, at low voltage and at temperatures down to 70 mK. In one sample, we find a two order of magnitude resistance drop below 0.55 K, which is destroyed by a magnetic field of the order of 1T, or by a d.c. current greater than 2.5 microA. These features strongly suggest the existence of superconductivity in ropes of SWNT.Comment: Accepted for publication in Phys. Rev. Let