Antonio Valletta

Bio: Antonio Valletta is an academic researcher from National Research Council. The author has contributed to research in topics: Thin-film transistor & Polycrystalline silicon. The author has an hindex of 23, co-authored 107 publications receiving 1794 citations. Previous affiliations of Antonio Valletta include Sapienza University of Rome.

Resonant and crossover phenomena in a multiband superconductor: Tuning the chemical potential near a band edge

Abstract: Resonances in the superconducting properties, in a regime of crossover from BCS to mixed Bose-Fermi superconductivity, are investigated in a two-band superconductor where the chemical potential is tuned near the band edge of the second miniband generated by quantum confinement effects. The shape resonances at $T=0$ in the superconducting gaps (belonging to the class of Feshbach-like resonances) is manifested by interference effects in the superconducting gap at the first large Fermi surface when the chemical potential is in the proximity of the band edge of the second miniband. The case of a superlattice of quantum wells is considered and the amplification of the superconducting gaps at the Lifshitz transition of the type neck-collapsing of Fermi surface topology is clearly shown. The results are found to be in good agreement with available experimental data on a superlattice of honeycomb boron layers intercalated by Al and Mg spacer layers.

Superconductivity of a striped phase at the atomic limit

Abstract: The resonant amplification of the superconducting critical temperature, the isotope effect, the change of the chemical potential in a particular 2D striped phase formed by superconducting stripes of width L alternated by separating stripes of width W with a period λ p at the atomic limit is studied. The critical temperature shows a `shape resonance' by tuning the charge density where the chemical potential μ is in the range E n μ E n + ℏ ω 0 , where E n is the bottom of the n th superlattice subband for n >2, and ℏ ω 0 is the energy cutoff for the pairing interaction. The maximum critical superconducting temperature is reached at the cross-over from 2D to 1D behavior. The particular properties of this electronic phase and its similarities with the normal and superconducting properties of doped cuprate perovskites are discussed.

Design and optimization of an ultra thin flexible capacitive humidity sensor

Abstract: In this paper we present the design and fabrication of a humidity sensor on ultra thin (8 μm) flexible polyimide substrate. The ultra thin flexible substrate can be preserved also when a read-out electronic interface is integrated by using Polycrystalline Silicon Thin Film Transistors technology. The sensor device is a capacitor where a thin layer of [bis(benzo cyclobutene)] is used as a dielectric sensitive material between two metal electrodes. The electrode layout has been designed with the aid of numerical simulations in order to optimize the sensor performances. The fabricated sensor has shown sensitivity to relative humidity of 0.38%/RH% and a linearity of 0.996 in the range of 10–90 RH%. Furthermore, measurements regarding the sensor response time, different bending and bias voltage effects have been performed.

Anomalous isotope effect near a 2.5 Lifshitz transition in a multi-band multi-condensate superconductor made of a superlattice of stripes

Abstract: The doping dependent isotope effect on the critical temperature (Tc) is calculated for multi-band multi-condensate superconductivity near a 2.5 Lifshitz transition. We consider a superlattice of quantum stripes with finite hopping between stripes near a 2.5 Lifshitz transition for the appearance of a new sub-band making a circular electron-like Fermi surface pocket. We describe a particular type of BEC (Bose?Einstein Condensate) to BCS (Bardeen?Cooper?Schrieffer condensate) crossover in multi-band/multi-condensate superconductivity at a metal-to-metal transition that is quite different from the standard BEC?BCS crossover at an insulator-to-metal transition. The results show that the isotope coefficient strongly deviates from the standard BCS value 0.5, when the chemical potential is tuned at the 2.5 Lifshitz transition for the metal-to-metal transition. The critical temperature Tc shows a minimum due to the Fano antiresonance in the superconducting gaps and the isotope coefficient diverges at the point where a BEC coexists with a BCS condensate. In contrast Tc reaches its maximum and the isotope coefficient vanishes at the crossover from a polaronic condensate to a BCS condensate in the newly appearing sub-band.

Humidity Sensors Principle, Mechanism, and Fabrication Technologies: A Comprehensive Review

Abstract: Humidity measurement is one of the most significant issues in various areas of applications such as instrumentation, automated systems, agriculture, climatology and GIS. Numerous sorts of humidity sensors fabricated and developed for industrial and laboratory applications are reviewed and presented in this article. The survey frequently concentrates on the RH sensors based upon their organic and inorganic functional materials, e.g., porous ceramics (semiconductors), polymers, ceramic/polymer and electrolytes, as well as conduction mechanism and fabrication technologies. A significant aim of this review is to provide a distinct categorization pursuant to state of the art humidity sensor types, principles of work, sensing substances, transduction mechanisms, and production technologies. Furthermore, performance characteristics of the different humidity sensors such as electrical and statistical data will be detailed and gives an added value to the report. By comparison of overall prospects of the sensors it was revealed that there are still drawbacks as to efficiency of sensing elements and conduction values. The flexibility offered by thick film and thin film processes either in the preparation of materials or in the choice of shape and size of the sensor structure provides advantages over other technologies. These ceramic sensors show faster response than other types.

Superconductivity in the intercalated graphite compounds C6Yb and C6Ca

Abstract: This thesis concerns the discovery of superconductivity in the intercalated graphite compounds C6Yb and C6Ca. A novel technique for synthesis of these intercalates has been developed, and is presented in detail. These two materials are shown to superconduct at 6.5K and 11.5K respectively. The superconductivity is demonstrated by measurements of the magnetisation and resistivity. Initial measurements of the superconducting transition of these materials as a function of pressure shows an increase in the transition with increasing pressure.