Modeling Magnetic Anisotropy of Single Chain Magnets in ∣d/J∣≥1|d/J| \geq 1 Regime

Single molecule magnets (SMMs) with single-ion anisotropies $\mathbf d$, comparable to exchange interactions J, between spins have recently been synthesized. In this paper, we provide theoretical insights into the magnetism of such systems. We study spin chains with site spins, s=1, 3/2 and 2 and on-site anisotropy $\mathbf d$ comparable to the exchange constants between the spins. We find that large $\mathbf d$ leads to crossing of the states with different $M_S$ values in the same spin manifold of the $\mathbf d = 0$ limit. For very large $\mathbf d$'s we also find that the $M_S$ states of the higher energy spin states descend below the $M_S$ states of the ground state spin manifold. Total spin in this limit is no longer conserved and describing the molecular anisotropy by the constants $D_M$ and $E_M$ is not possible. However, the total spin of the low-lying large $M_S$ states is very nearly an integer and using this spin value it is possible to construct an effective spin Hamiltonian and compute the molecular magnetic anisotropy constants $D_M$ and $E_M$. We report effect of finite sizes, rotations of site anisotropies and chain dimerization on the effective anisotropy of the spin chains

Modeling Molecular Magnets with Large Exchange and On-Site Anisotropies

Spins in molecular magnets can experience both anisotropic exchange interactions and on-site magnetic anisotropy. In this paper we study the effect of exchange anisotropy on the molecular magnetic anisotropy both with and without on-site anisotropy. When both the anisotropies are small, we find that the axial anisotropy parameter $D_M$ in the effective spin Hamiltonian is the sum of the individual contributions due to exchange and on-site anisotropies. We find that even for axial anisotropy of about $15\%$, the low energy spectrum does not correspond to a single parent spin manifold but has intruders states arising from other parent spin. In this case, the low energy spectrum can not be described by an effective Hamiltonian spanning the parent spin space. We study the magnetic susceptibility, specific heat as a function of temperature and magnetization as a function of applied field to characterize the system in this limit. We find that there is synergy between the two anisotropies, particularly for large systems with higher site spins.Comment: 30 pages, 11 figures and 3 tables. Supporting information included after the main articl

ON POTENTIALIZED PARTIAL FINITE DIFFERENCE EQUATIONS: ANALYZING THE COMPLEXITY OF KNOWLEDGE-BASED SPATIAL ECONOMIC DEVELOPMENTS

Knowledge-based regional and urban studies are plentiful; some systematics might be in order at this junction, so first the different links between economic production units in geographical space have to be clearly defined. Then a tool to represent the dynamics of those links should be selected; potentialized partial differential equations (PPDEs) are an appropriate tool to represent space-time relations in pre-geographical space. In practice, however, only discrete data are available, hence the question of how finite differences could generate PPFDEs (potentialized partial finite difference equations). A case has been worked out and simulated, showing a high degree of spatio-temporal complexity. Spatial econometric estimation is possible, as other work has shown; so an application to empirical data for France could be presented. Different versions of the latter have been worked out; they are presented in succession, followed by a last exercise on US data.COMPLEXITY, SPATIAL ECONOMETRICS, POTENTIAL, FINITE DIFFERENCES

Malononitrile, a Synthetic Tool for New Chromophoric Systems

New near-IR-absorbing dyes were synthesized by condensation of the dicyanovinyl derivative of 1-acetyl-3-oxo-2,3-dihydroindole with nitroso compounds. The resulting products were found to be mixtures (ca. 9:1) of the (E)- and (Z)-isomers. The more bathochromic (E)-form could be converted into the (Z)-form by heating in methanol. At higher temperature new cyclization products were obtained. A mechanism of the isomerisation and cyclization is proposed

From ZIF-8@Al2O3Composites to Self-Supported ZIF-8 One-Dimensional Superstructures

Efficient preparation of composite materials consisting of ZIF-8 nanocrystals embedded inside the channels of macroporous anodic aluminum oxide membranes is reported. 1-D self-supported ZIF-8 superstructures are recovered through matrix dissolution

Microscopic Model for High-spin vs. Low-spin ground state in [Ni2M(CN)8][Ni_2{M(CN)_8]} (M=MoV,WV,NbIVM=Mo^V, W^V, Nb^{IV}) magnetic clusters

Conventional superexchange rules predict ferromagnetic exchange interaction between Ni(II) and M (M=Mo(V), W(V), Nb(IV)). Recent experiments show that in some systems this superexchange is antiferromagnetic. To understand this feature, in this paper we develop a microscopic model for Ni(II)-M systems and solve it exactly using a valence bond approach. We identify the direct exchange coupling, the splitting of the magnetic orbitals and the inter-orbital electron repulsions, on the M site as the parameters which control the ground state spin of various clusters of the Ni(II)-M system. We present quantum phase diagrams which delineate the high-spin and low-spin ground states in the parameter space. We fit the spin gap to a spin Hamiltonian and extract the effective exchange constant within the experimentally observed range, for reasonable parameter values. We also find a region in the parameter space where an intermediate spin state is the ground state. These results indicate that the spin spectrum of the microscopic model cannot be reproduced by a simple Heisenberg exchange Hamiltonian.Comment: 8 pages including 7 figure

The poxviral scrapin MV-LAP requires a myxoma viral infection context to efficiently downregulate MHC-I molecules

AbstractDownregulation of MHC class I molecules is a strategy developed by some viruses to escape cellular immune responses. Myxoma virus (MV), a poxvirus causing rabbit myxomatosis, encodes MV-LAP that is known to increase MHC-I endocytosis and degradation through a C4HC3 motif critical for an E3 ubiquitin ligase activity. Here, we performed a functional mapping of MV-LAP and showed that not only the C4HC3 motif is necessary for a marked downregulation of MHC-I but also a conserved region in the C-terminal part of the protein. We also showed that the putative transmembrane domains are responsible for a specific subcellular localization of the protein: they retain MV-LAP in the ER in transfected cells and in the endolysosomal compartments in infected cells. We observed that a specific MV infection context is necessary for a fully efficient downregulation of MHC-I. Our data suggest that the functionality of viral LAP factors, inherited by herpes- and poxviruses from mammalian cells, is more complex than anticipated

Corrosion of Pure iron and Hydrogen Permeation in the Presence of H 2 S with O 2 contamination

International audienceThis paper examines the influence of traces of oxygen on corrosion and hydrogen charging of steel in an H 2 S containing environment. It is well known that H 2 S promotes hydrogen entry into steels, that may result in many types of steel failures such as Hydrogen Induced Cracking (HIC), Sulfide Stress Cracking (SSC), and Stress-Oriented Hydrogen Induced Cracking (SOHIC). Since it is a huge concern for oil and gas industries, standard test methods have been developed and published as NACE technical methods (e.g. NACE TM0284 and NACE TM0177). Though it is recognized that oxygen pollution should be avoided during H 2 S cracking tests, there is still a lack of experimental data to illustrate the potential impacts of a small oxygen pollution. The aim of the present study is to check if oxygen traces can modify the mechanisms of corrosion and hydrogen charging of steel in H 2 S containing medium. Experiments consisted of hydrogen permeation measurements through a thin pure iron membrane. They were performed at free potential circuit in order to ensure more realistic environmental conditions. The corrosion rate was also evaluated and test solutions analyzed

Electrochemical study of oxygen impact on corrosion and hydrogen permeation of Armco iron in the presence of H 2 S

International audienceH 2 S corrosion of mild steel is a recurrent issue in the oil and gas industry. Many studies related to the corrosion and hydrogen permeation of steel in an H 2 S containing environment have been made during the past decades with the intent of improving the knowledge and the prevention of economic loss. Since H 2 S is also a hydrogen entry promoter, lots of studies are also dedicated to the understanding of H 2 S cracking. Although it is generally accepted to avoid oxygen contamination in such a medium, there is a lack of research concerning its effect on the corrosion and hydrogen charging of steel. In this study, the effect of oxygen on corrosion and hydrogen charging of steels in an H 2 S containing environment is studied using Electrochemical Impedance Spectroscopy (EIS). An equivalent electrical circuit has been built according to SEM observations, literature research and experimental results. Using this equivalent electrical circuit, experimental data was analyzed and the average corrosion rates were deduced and found to be in good agreement with corrosion rates obtained by weight loss measurements. Furthermore, the evolution of fitting parameters (double layer capacity, charge transfer resistance, diffusion impedance, etc.) was found to be in good agreement with the real physical meaning of such parameters in the given conditions. This research contributes to the explanation of the mechanism behind the high corrosion rate observed in an H 2 S environment polluted with traces of oxygen

EIS study of iron and steel corrosion in aqueous solutions at various concentrations of dissolved H2S : impact of oxygen contamination.

International audienceMildly acidic water containing dissolved H 2 S presents a strong risk in the cracking of low-carbon steels. Several studies on H 2 S cracking mechanisms have shown that the main driving force is linked to the ability of H 2 S to promote hydrogen entry into the bulk material. Standard test methods have been developed and published as NACE technical standards (e.g. NACE TM0284 and NACE TM0177) to aid materials selection in the oil and gas sector. Though it is recognized that oxygen pollution should be avoided during H 2 S cracking tests, there is a lack of experimental data to illustrate the effects of a small oxygen pollution. Dissolved oxygen concentrations greater than the recommended upper limit (50 parts per billion) can easily be obtained in the case of poor laboratory practices. This paper will focus on the interactions between oxygen and H 2 S on electrochemical behavior of unalloyed steel. A continuous O 2 injection at a level corresponding to 500 ppb is applied, together with H 2 S bubbling in our test solutions, for periods lasting the same order as SSC standard tests. Steel surface reaction phenomena/corrosion rates in H 2 S saturated solution, with or without oxygen pollution, are studied using electrochemical impedance spectroscopy. The evolution of corrosion rates obtained from impedance analysis was compared to two other independent methods: i/ weight loss measurements and, ii/ hydrogen permeation. Without O 2 pollution, a permeation efficiency of 100% was obtained, as expected. Permeation current density was thus found to match precisely with the corrosion current density determined by impedance analysis at different times. On the other hand, when a continuous O 2 pollution was added in the system, significantly higher corrosion rates were observed, associated with test solution acidification. At the same time, permeation efficiency was decreased by up to one order of magnitude