A. M. Cucolo

Multiwalled carbon nanotube films as small-sized temperature sensors

We present the fabrication of thick and dense carbon nanotube networks in the form of freestanding films (CNTFs) and the study of their electric resistance as a function of the temperature, from 4 to 420 K. A nonmetallic behavior with a monotonic R(T) and a temperature coefficient of resistance around −7×10−4 K−1 is generally observed. A behavioral accordance of the CNTF conductance with the temperature measured by a solid-state thermistor (ZnNO, Si, or Pt) is demonstrated, suggesting the possibility of using CNTFs as temperature small-sized (freely scalable) sensors, besides being confirmed by a wide range of sensitivity, fast response, and good stability and durability. Concerning electric behavior, we also underline that a transition from nonmetal to metal slightly below 273 K has been rarely observed. A model involving regions of highly anisotropic metallic conduction separated by tunneling barrier regions can explain the nonmetallic to metallic crossover based on the competing mechanisms of the metal...

Epitaxial deposition and properties of Bi2Sr2CaCu2O8+δ/Bi2Sr2YCu2O8+δ/Bi2Sr2CaCu2 O+δ trilayers

We have deposited Bi2Sr2CaCu2O8+δ/Bi2Sr2YCu2O8+δ/Bi2Sr2CaCu2 O+δ (2212/22Y2/2212) heterostructures by an in situ dc sputtering technique at high oxygen pressures on (001) SrTiO3 substrates. The formation of highly c‐axis oriented trilayers with sharp interfaces is demonstrated by x‐ray diffraction and transmission electron microscope (TEM) analysis. Both the top and the bottom 2212 layers are superconducting below 87 K. Tunneling phenomena on junctions fabricated from these trilayers are observed. The conductance versus voltage curves at low temperatures exhibit a change of slope indicative of a gap structure at about 30 mV, a zero‐bias peak, as well as linear background at high voltages.

Leakage and field emission in side-gate graphene field effect transistors

We fabricate planar graphene field-effect transistors with self-aligned side-gate at 100 nm from the 500 nm wide graphene conductive channel, using a single lithographic step. We demonstrate side-gating below 1 V with conductance modulation of 35% and transconductance up to 0.5 mS/mm at 10 mV drain bias. We measure the planar leakage along the SiO2/vacuum gate dielectric over a wide voltage range, reporting rapidly growing current above 15 V. We unveil the microscopic mechanisms driving the leakage, as Frenkel-Poole transport through SiO2 up to the activation of Fowler-Nordheim tunneling in vacuum, which becomes dominant at higher voltages. We report a field-emission current density as high as 1 μA/μm between graphene flakes. These findings are important for the miniaturization of atomically thin devices.We fabricate planar all-graphene field-effect transistors with self-aligned side-gates at 100 nm from the main graphene conductive channel, using a single lithographic step. We demonstrate side-gating below 1V with conductance modulation of 35% and transconductance up to 0.5 mS/mm at 10 mV drain bias. We measure the planar leakage along the SiO2/vacuum gate dielectric over a wide voltage range, reporting rapidly growing current above 15 V. We unveil the microscopic mechanisms driving the leakage, as Frenkel-Poole transport through SiO2 up to the activation of Fowler-Nordheim tunneling in vacuum, which becomes dominant at high voltages. We report a field-emission current density as high as 1μA/μm between graphene flakes. These findings are essential for the miniaturization of atomically thin devices. * Tel: +39.089.969189. E-mail: dibant@sa.infn.it (Antonio Di Bartolomeo)

Metal-insulator transition temperature enhancement in La0.7Ca0.3MnO3 thin films

The effect of oxygen annealing on the structural and transport properties of La0.7Ca0.3MnO3 thin films deposited on SrTiO3 substrates has been investigated by x-ray diffraction analysis and resistive measurements. The as-grown films are fully tensile strained on the substrates and show a depressed metal-insulator transition temperature Tp=131K. As the oxygen content is increased due to longer annealing times, significantly higher Tp are measured, up to 247 K. Correspondingly, an increase of the out-of-plane lattice parameter is observed while the in-plane lattice constants do not change with respect to the as-grown films, which prevents any interpretation of a Tp dependence on the strain. The large increase in Tp is then interpreted in terms of a combined effect of the Mn4+∕Mn3+ ratio variation, the change in the carrier density in the a‐b planes, and the increase in the hydrostatic strain with the oxygen annealing.

Superconducting vortex profile from fixed point measurements the “Lazy Fisherman” tunneling microscopy method

We introduce a mode of operation for studying the vortex phase in superconductors using scanning tunneling microscopy (STM). While in the conventional STM method, the tip is scanned over a sample in which a fixed vortex pattern is prepared, in our “Lazy Fisherman” method the STM tip is kept fixed at a selected location while the vortices are being moved by varying the applied magnetic field. By continuously acquiring the local tunneling conductance spectra, dI∕dV(V), we detect the changes in the local density of states under the tip due to the vortex motion. With no need for scanning, the method permits one to extend the study of vortices to samples in which scanning is difficult or even impossible due to surface nonuniformity and allows one to study vortex dynamics. Using a statistical analysis of the spectra, we reconstruct the single vortex zero bias conductance profile. We apply the method to the c-axis face of a MgB2 single crystal sample and obtain a vortex profile with a coherence length, ξ of 57±2nm.

Single and double side YBCO thin films for x-band microwave filters and devices

The properties of high quality single side and double side YBa2Cu2O7-δ (YBCO) thin films deposited on LaA1O3 substrates by pure oxygen d.c. sputtering are reported. The single side films are highly c-axis oriented with their a (or b) axis parallel to that of the substrate. The critical temperature is higher than 91 K with transition width less than 0.5 K, while the critical current density Jc is higher than 3 MA/cm2 at 77 K. The best double side samples exhibit no appreciable differences in the superconducting properties of the films deposited on the two different sides. X-ray analyses show in both cases highly c-axis oriented thin films with comparable values of the c-axis length (1.169 nm). The microwave properties are tested by a microstrip resonator technique. The surface resistance is lower than 80 μΩ at 77 K and f=5.4 GHz while the penetration depth at low temperature is in the range 150-200 nm.

Morphological and Structural Aspects of the Extremely Halophilic Archaeon Haloquadratum walsbyi

Ultrathin square cell Haloquadratum walsbyi from the Archaea domain are the most abundant microorganisms in the hypersaline water of coastal salterns and continental salt lakes. In this work, we explore the cell surface of these microorganisms using amplitude-modulation atomic-force microscopy in nearly physiological conditions. We demonstrate the presence of a regular corrugation with a periodicity of 16–20 nm attributed to the surface layer (S-layer) protein lattice, striped domains asymmetrically distributed on the cell faces and peculiar bulges correlated with the presence of intracellular granules. Besides, subsequent images of cell evolution during the drying process indicate the presence of an external capsule that might correspond to the giant protein halomucin, predicted by the genome but never before observed by other microscopy studies.

Planar tunnel junctions on 90 K and 60 K YBCO single crystals: Superconducting and normal state properties

Abstract We have studied the tunneling characteristics of planar junctions made on YBCO single crystals with T c =90 K and T c =60 K . Natural barriers of good quality have been obtained on both compounds. In comparison with the 90 K phase, the superconducting conductance curves of the 60 K phase show gap-like structures reduced in amplitude and shifted towards higher energies. For the higher T c material, a gap opening, at about 90 K, is observed. Normal state conductances, measured at T > T c , are highly linear in voltage on both compounds and have steeper slopes on the 60 K phase. The slopes are quite insensitive to temperature variations, and do not depend on the junction resistances nor on the materials used as counterelectrodes.

Zero bias conductance peaks in high-Tc superconductors: clues and ambiguities of two mutually excluding models

Abstract The origin of the zero bias conductance peak (ZBCP) observed in the tunneling characteristics of different high- T c based junctions, is analyzed in the framework of the Anderson–Applebaum theory for Kondo type scattering from magnetic impurities at the barrier or in terms of midgap states induced by proximity effect at the surface of a d x 2− y 2 wave superconductor. The two models mutually exclude each other since they describe a normal state or a superconducting state effect, respectively. We show that the appearance of the anomaly above or below the junction T c , is the main indication that enable us to distinguish between the two cases. In addition to this, in the most controversial situations, the simultaneous presence at low temperatures of dips on both sides of the central structure as well as of conductance maxima at the expected values of the main high- T c superconductor (HTS) energy gap, can be considered as indications of d -wave superconductivity. The case of ZBCPs that for decreasing temperatures merge in conductance minima, is also discussed.

Local tunneling study of three-dimensional order parameter in the π band of Al-doped Mg B 2 single crystals

We have performed an extensive study of the temperature and magnetic field dependencies of local tunneling spectra measured by means of variable temperature scanning tunneling spectroscopy on high quality