Experimental Study of Spectral Properties of a Frenkel-Kontorova System.

We report on microwave emission from linear parallel arrays of underdamped Josephson junctions, which are described by the Frenkel-Kontorova (FK) model. Electromagnetic radiation is detected from the arrays when biased on current singularities (steps) appearing at voltages V(n)=Φ(0)(nc̅/L), where Φ(0)=2.07×10(-15) Wb is the magnetic flux quantum, and c̅, L, and n are, respectively, the speed of light in the transmission line embedding the array, L its physical length, and n an integer. The radiation, detected at fundamental frequency c̅/2L when biased on different singularities, indicates shuttling of bunched 2π kinks (magnetic flux quanta). Resonance of flux-quanta motion with the small-amplitude oscillations induced in the arrays gives rise to fine structures in the radiation spectrum, which are interpreted on the basis of the FK model describing the resonance. The impact of our results on design and performances of new digital circuit families is discussed.

Characterization of anomalous pair currents in Josephson junction networks.

Measurements performed on superconductive networks shaped in the form of planar graphs display anomalously large currents when specific branches are biased. The temperature dependences of these currents evidence that their origin is due to Cooper pair hopping through the Josephson junctions connecting the superconductive islands of the array. The experimental data are discussed in terms of theoretical models which predict, for the system under consideration, an inhomogeneous Cooper pair distribution on the superconductive islands of the network as a consequence of a Bose-Einstein condensation phenomenon.

Superconducting anisotropy in (CaCuO2)n/(SrTiO3)m superlattices.

The superconducting properties of (CaCuO2)n/(SrTiO3)m artificial superlattices have been investigated via transport measurements under the application of external magnetic fields. The coherence lengths in the plane of the substrate and in the direction perpendicular to it (ξab and ξc, respectively) have been measured while varying m, the thickness of the SrTiO3 block. The results show that with increasing m, i.e. with increasing structural anisotropy, the superconducting anisotropy γ = ξab/ξc decreases. This apparent anomalous relation between the structural and the superconducting anisotropies suggests that γ is more affected by local doping at the interface rather than by the separation between the superconducting blocks. This interpretation of the experimental results has been confirmed by both the irreversibility lines and the magnetic field dependence of the activation energy for fluxon motion.

Low temperature conductivity of carbon nanotube aggregates.

We compare, over wide temperature ranges, the transport properties of single-wall carbon nanotubes arranged in the form of aligned arrays or in the form of fibres. The experimental data show that both the forms of aggregates present a crossover in the transport mechanism from three-dimensional hopping of the electrons between localized states at high temperature to fluctuation-induced tunnelling across potential barriers at low temperature. The role of the junctions formed between the bundles in the array and between the nanotubes inside the fibres is discussed on the basis of the experimental results.

Charge transport and tunneling in single-walled carbon nanotube bundles.

We investigate experimentally the transport properties of single-walled carbon nanotube bundles as a function of temperature and applied current over broad intervals of these variables. The analysis is performed on arrays of nanotube bundles whose axes are aligned along the direction of the externally supplied bias current. The data are found consistent with a charge transport model governed by the tunneling between metallic regions occurring through potential barriers generated by a nanotube's contact areas or bundle surfaces. Based on this model and on experimental data, we describe quantitatively the dependencies of the height of these barriers upon bias current and temperature.