Anderson Model out of equilibrium: decoherence effects in transport through a quantum dot

The paper deals with the nonequilibrium two-lead Anderson model, considered as an adequate description for transport through a d-c biased quantum dot. Using a self-consistent equation-of-motion method generalized out of equilibrium, we calculate a fourth-order decoherence rate $\gamma^{(4)}$ induced by a bias voltage $V$. This decoherence rate provides a cut-off to the infrared divergences of the self-energy showing up in the Kondo regime. At low temperature, the Kondo peak in the density of states is split into two peaks pinned at the chemical potential of the two leads. The height of these peaks is controlled by $\gamma^{(4)}$. The voltage dependence of the differential conductance exhibits a zero-bias peak followed by a broad Coulomb peak at large $V$, reflecting charge fluctuations inside the dot. The low-bias differential conductance is found to be a universal function of the normalized bias voltage $V/T_K$, where $T_K$ is the Kondo temperature. The universal scaling with a single energy scale $T_K$ at low bias voltages is also observed for the renormalized decoherence rate $\gamma^{(4)}/T_K$. We discuss the effect of $\gamma^{(4)}$ on the crossover from strong to weak coupling regime when either the temperature or the bias voltage is increased.Comment: 23 pages, 10 figure

A facile method to oxidize carbon nanotubes in controlled flow of oxygen at 350 °C

The functionalization of carbon nanotubes (CNTs) is a very important step in many applications but it is still a very complex and variable task. This work shows an efficient, easily reproducible and optimal method to introduce oxygenated functional groups to CNTs by treating them for 60′ at 350 °C with a mixture of 2% oxygen in argon. The oxidized nanotubes were characterized through FT-IR, Raman and TGA to verify the quality of the oxidation and the lattice integrity of the treated CNTs. The results demonstrate that the treated nanotubes are not damaged even after introducing a significant number of new groups. This methodology could be easily tuned to functionalize other types of graphitic materials

Mechanical properties of mortar containing waste plastic (PVC) as aggregate partial replacement

The purpose of this work is the reuse of polyvinyl chloride (PVC) deriving from waste electrical and electronic equipment (WEEE) used as a partial substitute for the mineral aggregate to produce lightened mortars. PVC was recovered from copper electrical cables, ground and used as replacement of mineral aggregate in 5, 10, 15 and 20 % vol. in mortar. A thermal characterization of the starting material was carried out to understand its composition. The mortar samples were mechanically tested both using class G cement and ordinary Portland cement. The results showed a worsening of the mechanical properties of around 50 % for only 5 % in volume of sand substituted with PVC waste. A likely explanation to this phenomenon was found in the mechanical characteristics of the PVC used and to its poor adhesion with the matrix, that resulted in the creation of porosity. However, the mortar prepared contributes to the conservation of natural resources and maintains mechanical properties adequate for the use in non-structural applications (e.g. screed or substrate)

Recycling of WEEE plastics waste in mortar: The effects on mechanical properties

This work focused on the recycling of WEEE plastic waste as a partial substitute for aggregate in light mortars. The plastic mix, provided by the IREN group, was used as a replacement of aggregate in 15, 30, 45, 60, 75, and 90%vol in mortars. Worsening of the mechanical performance of around 50% was detected already at only 15%vol of mineral aggregate substituted with plastic waste. The explanation of this phenomenon was found in both the scarce mechanical properties of the used plastic and in the poor adhesion between matrix and plastics that resulted in extra-porosity formation, as also demonstrated by comparing the results with several models in the literature. However, the use of plastic waste as a partial replacement of natural aggregate contributes to the preservation of natural resources and, in any case, does not limit the application of these materials in non-structural applications

Polyvinyl butyral-based composites with carbon nanotubes: Efficient dispersion as a key to high mechanical properties

Even if the carbon nanotubes (CNTs) and their derivatives are commonly used as reinforcing phase in composite materials, also in commercial products, their tendency to agglomerate generally determines a scarce dispersion, thus not maximizing the effect due to the second phase. In this article, a perfect dispersion of highly entangled nanotubes was achieved by using a very simple approach: exploiting the dispersing effect of a low-cost polymer, polyvinyl butyral (PVB), coupled with standard ultrasound sonication. Several dispersion approaches were tested in order to develop a consistent and widely applicable dispersion protocol. The tape casting technology was subsequently used to produce 100 to 300 μm thick PVB-matrix composite tapes, reinforced by multiwall CNTs dispersed according to the optimized protocol. Their mechanical properties were evaluated, and a simple model was used to demonstrate that the effective dispersion of CNTs is the key to obtain significantly improved properties

Theoretical analysis of the transmission phase shift of a quantum dot in the presence of Kondo correlations

We study the effects of Kondo correlations on the transmission phase shift of a quantum dot coupled to two leads in comparison with the experimental determinations made by Aharonov-Bohm (AB) quantum interferometry. We propose here a theoretical interpretation of these results based on scattering theory combined with Bethe ansatz calculations. We show that there is a factor of 2 difference between the phase of the S-matrix responsible for the shift in the AB oscillations, and the one controlling the conductance. Quantitative agreement is obtained with experimental results for two different values of the coupling to the leads.Comment: 4 pages, 4 figures, accepted for publication in Physical Review Letter