Effect of Spin Dilution on the Magnetic State of Delafossite CuFeO2 with an S = 5/2 Antiferromagnetic Triangular Sublattice

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Simulation of the d.c. critical current in superconducting sintered ceramics

The new superconducting high-Tc sintered ceramics can be described in some case as a lattice of interconnected rods, in other cases as a more or less random packing of parallelepiped crystallites ; their size is about a few microns. The d.c. critical current at zero voltage of such a material is not related to the critical current of the bulk material, but to its granular structure. Indeed, the critical current between two adjacent cells is governed by the critical current of the weak link between them ; this link behaves within some limits as a Josephson junction, the critical current of which is known. For our present problem, the system can be modeled as a lattice of Josephson junctions. We present here results for the d.c. critical current at zero voltage of lattices of identical Josephson junctions in two dimensions. The influence of the finiteness of size of the sample is examined. The relationship with normal conductivity simulations and percolation is discussed

X-ray magnetic circular dichroism in (Ge,Mn) compounds: experiments and modeling

X-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectra at the L$_{2,3}$ edges of Mn in (Ge,Mn) compounds have been measured and are compared to the results of first principles calculation. Early \textit{ab initio} studies show that the Density Functional Theory (DFT) can very well describe the valence band electronic properties but fails to reproduce a characteristic change of sign in the L$_{3}$ XMCD spectrum of Mn in Ge$_3$Mn$_5$, which is observed in experiments. In this work we demonstrate that this disagreement is partially related to an underestimation of the exchange splitting of Mn 2$p$ core states within the local density approximation. It is shown that the change in sign experimentally observed is reproduced if the exchange splitting is accurately calculated within the Hartree-Fock approximation, while the final states can be still described by the DFT. This approach is further used to calculate the XMCD in different (Ge,Mn) compounds. It demonstrates that the agreement between experimental and theoretical spectra can be improved by combining state of the art calculations for the core and valence states respectively.Comment: 8 page

Neutron diffraction and magnetocaloric effect studies of MnFe 1-x Co x P series of solid solutions

International audienceMnFe 1-x Co x P intermetallic series of solid solutions (0.4<x<0.6) have been studied by means of powder neutron diffraction in 10−320 K temperature range. Rietveld analysis pointed out that Co 2 P-type orthorhombic crystal structure (SG: Pnma) presents for all series. Helicoidal incommensurate antiferromagnetic structure with propagation vector q = [0, 0, q] were evidenced for all compounds at low temperature range. The q value decreases with cobalt content and the second order polynomial q(x) it was evidenced, that is found well correlated with this dependence. Magnetic moments values of µ Mn =3.34 µ B and µ (Fe,Co) =0.06 µ B were determined from neutron diffraction refinements for x=0.4 at 10 K. In addition, magnetic interactions in relations with electronic band structure calculations of MnFe 1-x Co x P were presented and discussed reference to previous published data. Finally, magnetocaloric properties for selected compounds of the MnFe 1-x Co x P and MnFe 0.45 Co 0.45 P 0.9 Ge 0.1 series of compounds are presented

Synthesis of recyclable tire additives via equilibrium ring-opening metathesis polymerization

Linear <i>trans</i>-polypentenamers are highly desired materials among synthetic tire additives due to their comparable physical properties to natural rubber. <i>trans</i>-Polypentenamer can be prepared by equilibrium ring-opening metathesis polymerization (ROMP) using well-defined ruthenium catalyst systems. This unique feature of the equilibrium polymerization reaction opens a way for the synthesis of durable, environmentally benign elastomers where polymers including synthetic tire additives can be synthesized and readily recycled using the same transition metal catalyst system. The addition of silica fillers significantly improves the physical properties of the composite materials in comparison to the use of polymeric material. It is also known that the structural effects and the polymer–filler surface interaction are of prime importance. Herein, we report on the synthesis of silica filler compatible recyclable polypentenamer copolymers via equilibrium ROMP of cyclopentene <b>1</b> and 4-(triethoxy)­siloxy cyclopentene <b>11</b>. It has been demonstrated that polypentenamer tire additives can be synthesized via equilibrium ROMP affording polymers with high yields (>80%) at 0 °C and can be readily depolymerized at 40 °C and/or under diluted conditions using the same metathesis catalyst systems. Furthermore, the polypentenamer can also be synthesized in neat at room temperature and at very low (10⁵) monomer/catalyst ratio. This methodology is based on the synthesis of polyolefins utilizing a ruthenium-based metathesis catalyst via equilibrium ROMP of cyclopentenes and their silylated derivatives

Transitions dipolaires induites par collisions sur un faisceau de formaldéhyde. Effet d'un champ électrique

Using a H2CO molecular beam, a method is first described which permits the observation of rotational transitions ΔJ = ± n (n ≽ 1), induced by collisions with the molecules of a target gas. In the case H2CO → H 2CO, selection rules are defined and rotational resonance effects are observed on the differential cross sections for some ΔJ = — 1 transitions. It appears that a direct electric field E considerably modifies the H2CO cross sections σJ. Using the pair H 2CO → NH3, the study of σJ variations with E shows that the ΔJ = 0 transitions become forbidden, when E is intense. The experimental results are compared with the theoretical transition probabilities variations, due to E and calculated in the Born approximation. An experimental method is deduced which permits, by application of an electric field, to select AJ = 0 transitions on a molecular beam.Nous présentons d'abord une méthode expérimentale qui permet d'observer les transitions rotationnelles ΔJ = ± n (n ≽ 1) induites par collisions entre les molécules d'un faisceau de formaldéhyde et un gaz tampon. En prenant H2CO comme gaz tampon, nous précisons les règles de sélection et, sur les sections de collisions correspondant à plusieurs transitions ΔJ = — 1, nous mettons en évidence des effets de résonance rotationnelle. Nous constatons qu'un champ électrique continu modifie considérablement les sections de collisions du formaldéhyde. Sur le système dipolaire H2 CO-NH3, l'étude des variations caractéristiques de celles-ci en fonction du champ appliqué montre que les transitions ΔJ. = 0 de H2CO deviennent interdites en champ fort. Nous comparons les résultats expérimentaux aux variations de probabilité de transition dues au champ électrique et calculées dans l'approximation de Born. On déduit une méthode expérimentale permettant de trier par effet de champ les transitions ΔJ = 0 sur le faisceau moléculaire

Critical Behavior of La0.8Ca0.2Mn1−xCoxO3 Perovskite (0.1 ≤ x ≤ 0.3)

The critical properties of La0.8Ca0.2Mn1−xCoxO3 (x = 0, 0.1, 0.2 and 0.3) compounds were investigated by analysis of the magnetic measurements in the vicinity of their critical temperature. Arrott plots revealed that the paramagnetic PM-ferromagnetic (FM) phase transition for the sample with x = 0 is a first order transition, while it is a second order transition for all doped compounds. The critical exponents β, γ and δ were evaluated using modified Arrott plots (MAP) and the Kouvel-Fisher method (KF). The reliability of the evaluated critical exponents was confirmed by the Widom scaling relation and the universal scaling hypothesis. The values of the critical exponents for the doped compounds were consistent with the 3D-Heisenberg model for magnetic interactions. For x = 0.1, the estimated critical components are found inconsistent with any known universality class. In addition, the local exponent n was determined from the magnetic entropy change and found to be sensitive to the magnetic field in the entire studied temperature range.This work has been supported by the Tunisian Ministry of Scientific Research and Technology and Institute Neel at Grenobl

Ab Initio Study of the Electronic and Energy Properties of Diamond Carbon

In this chapter, we present a study on the electronic properties of diamond carbon, using band structure and density of states calculations. The calculations are based on the use of the grid-based projector-augmented wave (GPAW) and atomic simulation environment (ASE) methods. The main results of our work are the optimization of diamond energy (to −17.57 eV) and the calculation of the gap with the PBE (Perdew, Burke, and Ernzerhof) and the functional hybrid PBE0 hybrid functional, which is about 5.368 eV (the closest value to the value found in the literature). We were also able to reproduce the experimental value of the lattice constant of diamond to within 0.2% for PBE0 and 0.4% for PBE. Our results contribute to the study of the electronic properties of diamond using GPAW and ASE simulation, which is a set of Python modules, designed to facilitate the setup, execution, and analysis of atomic/electronic calculations. This tight integration of ASE and GPAW should be exploited in future research of the electronic properties of diamond, which is one of the most promising materials for the integrated electronic and photonic, radio, optoelectronic, and quantum devices industry. This chapter provides interesting information for the theoretical and experimental communities working in this field

Magnetocaloric properties of Fe_{2-x}T_xP (T = Ru and Rh) from electronic structure calculations and magnetisation measurements

An analysis of the magnetocaloric properties of the pure and substituted Fe2P compounds is made based on KKR-CPA electronic structure calculations and magnetisation M(H,T) measurements. The computed electronic densities of states and magnetic moments are used to calculate both the values of the electronic and magnetic entropies, which fairly agree with the experimental findings. To enlighten the magnetic properties above Curie temperature, the paramagnetic state behaviours are simulated using the disordered local moments (DLM) concept. The KKR-CPA computations show, that in Fe2P, the Fe magnetic moment of the (3f) site disappears in the DLM state, while the moment of the (3g) site is only little lowered, comparison made with the low temperature ferromagnetic state.Comment: 17 pages, 8 figures, Submitted to J. Phys.