Nadia Martucciello

Tunable Schottky barrier and high responsivity in graphene/Si-nanotip optoelectronic device

We demonstrate tunable Schottky barrier height and record photo-responsivity in a new-concept device made of a single-layer CVD graphene transferred onto a matrix of nanotips patterned on n-type Si wafer. The original layout, where nano-sized graphene/Si heterojunctions alternate to graphene areas exposed to the electric field of the Si substrate, which acts both as diode cathode and transistor gate, results in a two-terminal barristor with single-bias control of the Schottky barrier. The nanotip patterning favors light absorption, and the enhancement of the electric field at the tip apex improves photo-charge separation and enables internal gain by impact ionization. These features render the device a photodetector with responsivity (3 A/W for white LED light at 3 mW/cm2 intensity) almost an order of magnitude higher than commercial photodiodes. We extensively characterize the voltage and the temperature dependence of the device parameters and prove that the multi-junction approach does not add extra-inhomogeneity to the Schottky barrier height distribution. This work represents a significant advance in the realization of graphene/Si Schottky devices for optoelectronic applications.

Josephson effect in Nb/Al2O3/Al/MgB2 large-area thin-film heterostructures

We report the demonstration of dc and ac Josephson effects in Nb/Al2O3/Al/MgB2 thin-film heterostructure. The heterostructure exhibits moderately hysteretic current–voltage characteristic with a dc Josephson current branch and regular microwave-induced Shapiro steps. From conductance spectrum, a gap of about 2 meV at 7.7 K is estimated for the proximized surface of MgB2 electrode.

Experimental investigation of flux motion in exponentially shaped Josephson junctions

We report an experimental and numerical analysis of exponentially shaped long Josephson junctions with lateral current injection. Quasilinear flux flow branches are observed in the current-voltage characteristic of the junctions in the absence of a magnetic field. A strongly asymmetric response to an applied magnetic field is also exhibited by the junctions. Experimental data are found in agreement with numerical predictions and demonstrate the existence of a geometry-induced potential experienced by the flux quanta in nonuniform width junctions.

Influence of the idle region on the dynamic properties of window Josephson tunnel junctions

The processes developed in the last years for the fabrication of high quality, reliable all‐refractory Josephson tunnel junctions open a window in an insulating region, called the idle region, which provides the passivation of the base electrode all around the junction itself. We report experimental data showing that the phase velocity of electromagnetic waves in the junction barrier can be considerably modified by the presence of the insulating region surrounding the window Josephson tunnel junctions. We have investigated this phenomenon in unidimensional junctions, measuring the asymptotic voltage of the Fiske resonances in the presence of an externally applied magnetic field. We have considered the effect of an idle region: (i) along the long junction dimension and (ii) at the extremities of the junction. When comparing the behavior to that of a bare junction, in the first case the phase velocity has been found to increase and to be affected by dispersion, while in the latter case it decreased and was ...

Timing jitter of cascade switch superconducting nanowire single photon detectors

We investigate the timing jitter in parallel superconducting NbN-nanowire single photon detectors based on a cascade switch mechanism. The measured timing jitter is asymmetric and has an oscillatory dependence on bias current. At the highest bias current the full width at half maximum was 1.5 times larger than an on-chip reference meander NbN nanowire. A physical model of the dynamics occurring during cascade switch is developed, that quantitatively accounts for our observations as a consequence of different nanowire critical currents within the detector.

Pinning mechanism in electron-doped HTS Nd

The electrical transport properties of c-axis oriented Nd1.85Ce0.15CuO4 ? ? superconducting films have been investigated to analyze the pinning mechanism in this material. The samples were grown on SrTiO3 substrates using the dc sputtering high-pressure technique, whereas a detailed analysis of the structure and local composition of the films has been achieved using high-resolution electron microscopy and x-ray microanalysis. Magneto-resistance and current-voltage measurements, in the temperature range from 1.6 to 300 K and in magnetic field up to 9 T, have been reported. In particular, the anisotropic coefficient defined as the ratio between the parallel upper critical field, ab, and the perpendicular one, c, has been evaluated, pointing out the high anisotropy of this compound. Furthermore, the vortex activation energy as a function of the applied magnetic field, parallel and perpendicular to the CuO2 planes, has been derived and compared with the flux-pinning forces to enlighten the peculiar nature of pinning centers in this material.

_{1.85}

We study the effects of low-energy electron beam irradiation up to 10 keV on graphene based field effect transistors. We fabricate metallic bilayer electrodes to contact mono- and bi-layer graphene flakes on SiO

Ce

_2

_{0.15}

, obtaining specific contact resistivity

CuO

\rho_c \simeq 19 k\Omega \mu m^2