Exchange Interactions and High-Energy Spin States in Mn_12-acetate

We perform inelastic neutron scattering measurements on the molecular nanomagnet Mn_12-acetate to measure the excitation spectrum up to 45meV (500K). We isolate magnetic excitations in two groups at 5-6.5meV (60-75K) and 8-10.5meV (95-120K), with higher levels appearing only at 27meV (310K) and 31meV (360K). From a detailed characterization of the transition peaks we show that all of the low-energy modes appear to be separate S = 9 excitations above the S = 10 ground state, with the peak at 27meV (310K) corresponding to the first S = 11 excitation. We consider a general model for the four exchange interaction parameters of the molecule. The static susceptibility is computed by high-temperature series expansion and the energy spectrum, matrix elements and ground-state spin configuration by exact diagonalization. The theoretical results are matched with experimental observation by inclusion of cluster anisotropy parameters, revealing strong constraints on possible parameter sets. We conclude that only a model with dominant exchange couplings J_1 ~ J_2 ~ 5.5meV (65K) and small couplings J_3 ~ J_4 ~ 0.6meV (7K) is consistent with the experimental data.Comment: 17 pages, 12 figure

Competition between phonon superconductivity and Kondo screening in mixed valence and heavy fermion compounds

We consider competition of Kondo effect and s-wave superconductivity in heavy fermion and mixed valence superconductors, using the phenomenological approach for the periodic Anderson model. Similar to the well known results for single-impurity Kondo effect in superconductors, we have found principal possibility of a re-entrant regime of the superconducting transition temperature, $T_c$, in heavy fermion superconductors in a narrow range of model parameters and concentration of f-electrons. Suppression of $T_c$ in mixed valence superconductors is much weaker. Our theory has most validity in the low-temperature Fermi liquid regime, without re-entrant behavior of $T_c$. To check its applicability, we performed the fit for the $x$-dependence of $T_c$ in Ce$_{1-x}$La$_x$Ru$_3$Si$_2$ and obtained an excellent agreement with the experimental data, although no re-entrance was found in this case. Other experimental data are discussed in the light of our theoretical analysis. In particular, we compare temperatures of the superconducting transition for some known homologs, i.e., the analog periodic lattice compounds with and without f-elements. For a few pairs of homologs superconductivity exists only in the heavy fermion materials, thus confirming uniqueness of superconductivity mechanisms for the latter. We suggest that for some other compounds the value of $T_c$ may remain of the same order in the two homologs, if superconductivity originates mainly on some light Fermi surface, but induces sizable superconducting gap on another Fermi surface,for which hybridization or other heavy fermion effects are more significant.Comment: 11 pages, 4 figures, pd

Butterfly Hysteresis and Slow Relaxation of the Magnetization in (Et4N)3Fe2F9: Manifestations of a Single-Molecule Magnet

(Et4N)3Fe2F9 exhibits a butterfly--shaped hysteresis below 5 K when the magnetic field is parallel to the threefold axis, in accordance with a very slow magnetization relaxation in the timescale of minutes. This is attributed to an energy barrier Delta=2.40 K resulting from the S=5 dimer ground state of [Fe2F9]^{3-} and a negative axial anisotropy. The relaxation partly occurs via thermally assisted quantum tunneling. These features of a single-molecule magnet are observable at temperatures comparable to the barrier height, due to an extremely inefficient energy exchange between the spin system and the phonons. The butterfly shape of the hysteresis arises from a phonon avalanche effect.Comment: 18 pages, 5 eps figures, latex (elsart

Understanding the NH3 adsorption mechanism on a vanadium-based SCR catalyst: A data-driven modeling approach

Ammonia adsorption is a precondition for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) to take place and it influences catalyst performance under transient conditions. For a vanadium-based SCR catalyst NH3 adsorption takes place on multiple adsorption sites over the catalyst surface with different behaviours depending on temperature, gas concentration and catalyst oxidation state. In this study, a mechanistic NH3 adsorption model within the framework of Langmuir adsorption models was developed for describing the NH3 adsorption isotherms obtained with a gas flow reactor for a vanadium-based SCR. The model was created by a data-driven modeling process, which involves different steps. First, a large set of candidate models was created systematically by combining multiple feasible adsorption mechanisms. Then, a parameter estimation workflow was performed using three different objective functions with increased complexity. Finally, a model reconciliation step was executed and a quality assessment was done for creating a unified robust model with a high degree of validity. As a result of this method, an NH3 adsorption model with five adsorption sites with different mechanisms was obtained that captures the main features from the experimental data. Furthermore, the model parameters have physical significance and relate to the adsorption strength and spatial arrangement for NH3 and water molecules. The proposed model can be used in the development of transient models with increased validity over a wide experimental region

A Modeling Framework to Assess Strategies Alignment based on Collaborative Network Emotions

[DE] The Collaborative Networks (CN) discipline has been largely studied in last decades, addressing different problems and proposing solutions for the robust establishment of collaborative processes, within the enterprises willing to collaborate. The main aim of CN research is, therefore, to generate approaches that enable creating effective relationships in the long term, to achieve stable and agile alliances. The concept of alignment among the CN partners has been considered since the beginning of CN research. Nevertheless, novel perspectives of study in CN, such as the consideration of collaborative emotional states, within the CN, have been introduced in recent years. This paper connects the research area of strategies alignment and the CN emotion models. Accordingly, a modelling framework to assess strategies alignment considering the emotional environment within the CN is proposed. The modelling framework allows representing how the enterprises emotions affect in the selection and alignment of formulated enterprises¿ strategiesAndres, B.; Ferrada, F.; Poler, R.; Camarinha-Matos, L. (2018). A Modeling Framework to Assess Strategies Alignment based on Collaborative Network Emotions. IFIP Advances in Information and Communication Technology. 534:349-361. https://doi.org/10.1007/978-3-319-99127-6_30S349361534Camarinha-Matos, L.M.: Collaborative networks in industry and the role of PRO-VE. Int. J. Prod. Manag. Eng. 2(2), 53–57 (2014)Andres, B., Poler, R.: Models, guidelines and tools for the integration of collaborative processes in non-hierarchical manufacturing networks: a review. Int. J. Comput. Integr. Manuf. 2(29), 166–201 (2016)Bititci, U., Martinez, V., Albores, P., Parung, J.: Creating and managing value in collaborative networks. Int. J. Phys. Distrib. Logist. 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Microstructure of Bentonite in Iron Ore Green Pellets

Sodium-activated calcium bentonite is used as a binder in iron ore pellets and is known to increase strength of both wet and dry iron ore green pellets. In this article, the microstructure of bentonite in magnetite pellets is revealed for the first time using scanning electron microscopy. The microstructure of bentonite in wet and dry iron ore pellets, as well as in distilled water, was imaged by various imaging techniques (e.g., imaging at low voltage with monochromatic and decelerated beam or low loss backscattered electrons) and cryogenic methods (i.e., high pressure freezing and plunge freezing in liquid ethane). In wet iron ore green pellets, clay tactoids (stacks of parallel primary clay platelets) were very well dispersed and formed a voluminous network occupying the space available between mineral particles. When the pellet was dried, bentonite was drawn to the contact points between the particles and formed solid bridges, which impart strength to the solid compac

Electron energy relaxation times from ballistic-electron-emission spectroscopy

Using a Green’s-function approach that incorporates band-structure effects, and a complementary k-space Monte-Carlo analysis, we show how to get a theoretically consistent determination of the inelastic mean free path λee(E) due to electron-electron interaction from ballistic electron emission spectroscopy. Exploiting experimental data taken at T=77K on a thin-Au film (ee(E) predicted by the standard Fermi-liquid theory provides excellent agreement between theoretical and experimental I(V) spectra. In agreement with theories for real metals, an enhancement of λee(E) by a factor of two with respect to its electron-gas value is found

Characterization Method for Gas Flow Reactor Experiments - NH3Adsorption on Vanadium-Based SCR Catalysts

In this study, NH3 adsorption isotherms for a commercial vanadium-based SCR catalyst coated on a monolith substrate were obtained using a gas flow reactor over a wide range of parameters which have not been performed before in a single study. The isotherms were obtained under different conditions, where adsorption temperature, NH3 concentration, water concentration, washcoat loading, and catalyst oxidation state were varied. For this purpose, a systematic data processing method was developed, which characterizes the dispersion and delay effects in the experimental setup using a residence time distribution model, and artifacts such as NH3 adsorbed in the experimental setup and uncertainties in the washcoat loading were removed. As a result, data from different catalyst samples were integrated, and adsorption isotherms with low data spread and well-defined regions were obtained. This allows the identification of the complex nature of the catalyst and dynamics, where multiple types of adsorption sites are present. For instance, the oxidized catalyst has 50% higher NH3 storage capacity compared to the reduced state of the sample. Moreover, water reduces the NH3 storage capacity at high concentrations (5.0%), whereas at low concentration (0.5%), water increases the NH3 adsorption capacity for an oxidized catalyst. The proposed data processing method can be extended for the analysis of further phenomena in catalysts studied using gas flow reactors, complementing current methods and providing information for models with extended validity and lower parameter correlations