Conductivity and Atomic Structure of Isolated Multiwalled Carbon Nanotubes

We report associated high resolution transmission electron microscopy (HRTEM) and transport measurements on a series of isolated multiwalled carbon nanotubes. HRTEM observations, by revealing relevant structural features of the tubes, shed some light on the variety of observed transport behaviors, from semiconducting to quasi-metallic type. Non Ohmic behavior is observed for certain samples which exhibit "bamboo like" structural defects. The resistance of the most conducting sample, measured down to 20 mK, exhibits a pronounced maximum at 0.6 K and strong positive magnetoresistance.Comment: 4 pages, 4 eps figure

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

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

Quantum Noise Measurement of a Carbon Nanotube Quantum Dot in the Kondo Regime

The current emission noise of a carbon nanotube quantum dot in the Kondo regime is measured at frequencies $\nu$ of the order or higher than the frequency associated with the Kondo effect $k_B T_K/h$, with $T_K$ the Kondo temperature. The carbon nanotube is coupled via an on-chip resonant circuit to a quantum noise detector, a superconductor-insulator-superconductor junction. We find for $h \nu \approx k_B T_K$ a Kondo effect related singularity at a voltage bias $eV \approx h \nu$, and a strong reduction of this singularity for $h \nu \approx 3 k_B T_K$, in good agreement with theory. Our experiment constitutes a new original tool for the investigation of the non-equilibrium dynamics of many-body phenomena in nanoscale devices.Comment: 6 pages, 4 figure

Geometrical enhancement of the proximity effect in quantum wires with extended superconducting tunnel contacts

We study Andreev reflection in a ballistic one-dimensional channel coupled in parallel to a superconductor via a tunnel barrier of finite length $L$. The dependence of the low-energy Andreev reflection probability $R_A$ on $L$ reveals the existence of a characteristic length scale $\xi_N$ beyond which $R_A(L)$ is enhanced up to unity despite the low interfacial transparency. The Andreev reflection enhancement is due to the strong mixing of particle and hole states that builds up in contacts exceeding the coherence length $\xi_N$, leading to a small energy gap (minigap) in the density of states of the normal system. The role of the geometry of such hybrid contacts is discussed in the context of the experimental observation of zero-bias Andreev anomalies in the resistance of extended carbon nanotube/superconductor junctions in field effect transistor setups.Comment: 11 pages, 8 figures; minor revisions including added Ref. 7 and inset to Fig. 3b; version as accepted for publication to Phys. Rev.

Superconductive properties of thin dirty SN bilayers

The theory of superconductivity in thin SN sandwiches (bilayers) in the diffusive limit is developed within the standard Usadel equation method, with particular emphasis on the case of very thin superconductive layers, d_S << d_N. The proximity effect in the system is governed by the interlayer interface resistance (per channel) \rho_{int}. The case of relatively low resistance (which can still have large absolute values) can be completely studied analytically. The theory describing the bilayer in this limit is of BCS type but with the minigap (in the single-particle density of states) E_g << \Delta substituting the order parameter \Delta in the standard BCS relations; the original relations are thus severely violated. In the opposite limit of an opaque interface, the behavior of the system is in many respects close to the BCS predictions. Over the entire range of \rho_{int}, the properties of the bilayer are found numerically. Finally, it is shown that the results obtained for the bilayer also apply to more complicated structures such as SNS and NSN trilayers, SNINS and NSISN systems, and SN superlattices.Comment: 15 pages (including 10 EPS figures), REVTeX. Version 2: minor changes; added references, a note is added concerning applicability of our results to SNINS and NSISN systems. To appear in Phys. Rev. B on March 1, 200