Coupling between 4f and itinerant electrons in SmFeAsO1-xFx (0.15 < x < 0.2) superconductors: an NMR study

$^{19}$F NMR measurements in SmFeAsO$_{1-x}$F$_x$, for $0.15\leq x\leq 0.2$, are presented. The nuclear spin-lattice relaxation rate $1/T_1$ increases upon cooling with a trend analogous to the one already observed in CeCu$_{5.2}$Au$_{0.8}$, a quasi two-dimensional heavy-fermion intermetallic compound with an antiferromagnetic ground-state. In particular, the behaviour of the relaxation rate either in SmFeAsO$_{1-x}$F$_x$ or in CeCu$_{5.2}$Au$_{0.8}$ can be described in the framework of the self-consistent renormalization theory for weakly itinerant electron systems. Remarkably, no effect of the superconducting transition on $^{19}$F $1/T_1$ is detected, a phenomenon which can hardly be explained within a single band model.Comment: 4 figure

Life cycle assessment of a floating offshore wind farm in Italy

Mitigation of climate change requires consistent actions toward the reduction of emissions from the energy sector: in the last years, renewable energy technologies, such as wind power, have become a cost-effective option to pursue the transition to low emission systems for power generation. Offshore wind energy can provide access to additional wind resources, also overcoming some issues related to onshore wind deployments such as land-use competition and social acceptability. The Life Cycle Assessment (LCA) methodology can be used to gain insight into the environmental performances of different technologies, e.g. renewable energy generation technologies, along the lifecycle stages and across a number of impact categories. This paper reports the cradle-to-grave LCA of a floating offshore wind farm, consisting of 190 wind turbines with 14.7 MW rated power, intended to be deployed in the Mediterranean Sea. The employed technology is represented by the IEA 15 MW reference wind turbine supported by the reference semi-submersible platform. The selected functional unit is the delivery of 1 GWh of electricity to the onshore grid and the impact assessment method is the EPD (version 2018), which is usually used for the creation of Environmental Product Declarations (EPDs) and considers 8 impact categories. The results of the analysis show that the supply of raw materials, especially steel, for aerogenerators and floaters is the most significant contributor to the overall potential impacts in all the impact categories, except for abiotic depletion of elements, where power cables are the hotspot. In the perspective of decarbonisation, the estimated carbon intensity is 31 g CO2eq/kWh and so it results competitive with other low emissions electricity generation technologies. To compare the estimated global warming impacts to other studies, some harmonisations efforts on capacity factor and lifetime of turbines are made. Moreover, the wind farm performance has been evaluated in terms of carbon and energy payback time, estimated in 2 and 3 years respectively, showing a substantial benefit when compared to the expected 30-year lifetime. As a conclusion, despite the number of approximations and conservative assumptions, floating offshore wind power, represented by the modelled case study, can be considered a promising technology and has been found to be already competitive with other renewable electricity generation technologies. Future research should address the uncertainty rooted to the data: repeating the analysis relying on the executive project, and therefore on a more detailed modelling, would help to get more accurate results

Ex-ante life cycle assessment of FineFuture flotation technology: case study of Grecian Magnesite

Purpose This study aims at evaluating the environmental performance of a novel froth flotation technology in mining industry from a life cycle perspective. The technology is being developed under EU Horizon 2020 project titled "FineFuture" (FF) with the aim of saving valuable materials in fine particles that are currently wasted due to lack of technology.Methods FF relies on chemically enhancing the physical characteristics of particles allowing it to float and concentrate. Prospective life cycle assessment (pLCA) was conducted for two possible industrial applications of FF flotation technology in the case study of Grecian Magnesite (GM) which is a main magnesium oxide producer in Europe. Each application can be perceived as a standalone comparative LCA study comparing current system with future system incorporating FF technology on industrial scale.Results and discussion The future scenarios did not decisively support FF technology in neither of the two applications from an environmental point of view. When applied to fines of &lt; 4 mm granular size with the aim of material recovery, the future scenario performed better than the current situation only in 5 out of 16 impact categories. The main issue is the added burden of calcination phase. When the technology was tested to upgrade the existing magnesite concentrate before calcination, it introduced some gains in most of the impact categories, but the difference compared to the current situation is not very considerable. Testing improved scenarios showed a great benefit to the overall performance of the scenarios by introducing cleaner fuels and burners in calcination phase.Conclusion and recommendations Overall, the results tend to favour applying FF technology to upgrade low quality concentrates rather than beneficiating &lt; 4 mm fines. However, and in any case, if FF technology is to be applied, combining it with cleaner fuels and burners in calcination should be prioritized. Furthermore, it was found that improving the purity (i.e. quality) in the flotation tank output is a key factor from an environmental view. The results also showed little impact of the added electric energy demand from the new units. As any pLCA, the study has limitations mainly originating from the low technology readiness level (TRL) when data collection activities were carried out. Further studies should start from pilot-scale data and adopting more accurate upscaling approaches to calculate the impacts of a full industrial deployment of the technology

Kohn-Sham Exchange Potential for a Metallic Surface

The behavior of the surface barrier that forms at the metal-vacuum interface is important for several fields of surface science. Within the Density Functional Theory framework, this surface barrier has two non-trivial components: exchange and correlation. Exact results are provided for the exchange component, for a jellium metal-vacuum interface, in a slab geometry. The Kohn-Sham exact-exchange potential $V_{x}(z)$ has been generated by using the Optimized Effective Potential method, through an accurate numerical solution, imposing the correct boundary condition. It has been proved analytically, and confirmed numerically, that $V_{x}(z\to \infty)\to - e^{2}/z$; this conclusion is not affected by the inclusion of correlation effects. Also, the exact-exchange potential develops a shoulder-like structure close to the interface, on the vacuum side. The issue of the classical image potential is discussed.Comment: Phys. Rev. Lett. (to appear

Novel properties of the Kohn-Sham exchange potential for open systems: application to the two-dimensional electron gas

The properties of the Kohn-Sham (KS) exchange potential for open systems in thermodynamical equilibrium, where the number of particles is non-conserved, are analyzed with the Optimized Effective Potential (OEP) method of Density Functional Theory (DFT) at zero temperature. The quasi two-dimensional electron gas (2DEG) is used as an illustrative example. The main findings are that the KS exchange potential builds a significant barrier-like structure under slight population of the second subband, and that both the asymptotic value of the KS exchange potential and the inter-subband energy jump discontinuously at the one-subband (1S) -> two-subband (2S) transition. The results obtained in this system offer new insights on open problems of semiconductors, such as the band-gap underestimation and the band-gap renormalization by photo-excited carriers.Comment: 7 pages, 3 figures, uses epl.cls(included), accepted for publication in Europhysics Letter

Superconducting phase fluctuations in SmFeAsO0.8_{0.8}F0.2_{0.2} from diamagnetism at low magnetic field above TcT_{c}

Superconducting fluctuations (SF) in SmFeAsO$_{0.8}$F$_{0.2}$ (characterized by superconducting transition temperature $T_{c} \simeq 52.3$ K) are investigated by means of isothermal high-resolution dc magnetization measurements. The diamagnetic response to magnetic fields up to 1 T above $T_{c}$ is similar to what previously reported for underdoped cuprate superconductors and it can be justified in terms of metastable superconducting islands at non-zero order parameter lacking of long-range coherence because of strong phase fluctuations. In the high-field regime ($H \gtrsim 1.5$ T) scaling arguments predicted on the basis of the Ginzburg-Landau theory of conventional SF are found to be applicable, at variance with what observed in the low-field regime. This fact enlightens that two different phenomena are simultaneously present in the fluctuating diamagnetism, namely the phase SF of novel character and the conventional SF. High magnetic fields (1.5 T $\lesssim H \ll H_{c2}$) are found to suppress the former while leaving unaltered the latter one.Comment: 7 pages, 5 figure

Solid-state Nonlinear Optical Properties of Mononuclear Copper(II) Complexes with Chiral Tridentate and Tetradentate Schiff Base Ligands

Salen-type metal complexes have been actively studied for their nonlinear optical (NLO) properties, and push-pull compounds with charge asymmetry generated by electron releasing and withdrawing groups have shown promising results. As a continuation of our research in this field and aiming at solid-state features, herein we report on the synthesis of mononuclear copper(II) derivatives bearing either tridentate N2O Schiff bases L(a-c)- and pyridine as the forth ancillary ligand, [Cu(La-c)(py)](ClO4) (1a-c), or unsymmetrically-substituted push-pull tetradentate N2O2 Schiff base ligands, [Cu(5-A-5'-D-saldpen/chxn)] (2a-c), both derived from 5-substituted salicylaldehydes (sal) and the diamines (1R,2R)-1,2-diphenylethanediamine (dpen) and (1S,2S)-1,2-diaminocyclohexane (chxn). All compounds were characterized through elemental analysis, infrared and UV/visible spectroscopies, and mass spectrometry in order to guarantee their purity and assess their charge transfer properties. The structures of 1a-c were determined via single-crystal X-ray diffraction studies. The geometries of cations of 1a-c and of molecules 2a-c were optimized through DFT calculations. The solid-state NLO behavior was measured by the Kurtz-Perry powder technique @1.907 µm. All chiral derivatives possess non-zero quadratic electric susceptibility (χ(2)) and an efficiency of about 0.15-0.45 times that of standard urea