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Structure and magnetic properties of the weak ferromagnet Sr2-xLaxIrO4

The 5d electron-based Sr2?xLaxIrO4 system (0?x?0.2) has been synthesized by a solid-state route. The x = 0 composition is a nonmetallic weak ferromagnet with a Curie temperature at about 240?K. The crystal structure behaviour and magnetic properties exhibited by this Sr2?xLaxIrO4 system can be explained on the basis of the extended character of the 5d electrons of the Ir cation and its valence states. Rietveld analysis of x-ray powder diffraction on Sr2IrO4 agrees well with previous structural neutron experiments. Furthermore, density functional theory (DFT) calculations predict that I41/acd represents a more stable crystal structure than K2NiF4 (I4/mmm). Electrical resistivity and magnetic properties of Sr2?xLaxIrO4 are strongly dependent on the Ir3+ content. The Sr2?xLaxIrO4 magnetic behaviour in the range of 2?240?K can be ascribed to a weak ferromagnet, produced by an array of canted antiferromagnetically ordered Ir4+ magnetic moments.

Lithiated rare-earth thiospinels and selenospinels

Abstract The spinels A[Ln 2 X 4 with A =Cd or Mg and X =S or Se were prepared for Ln =Ho, Er, Tm, Yb, andY 2− x Yb x (x = 0.0, 1.0,or2.0). Lithiation with n -butyllithium was attempted in the hope of obtaining both mixed-valent and intermediate-valent compounds as well as a continuous variation of the Fermi energy from below to within a localized4 f n configuration. Formation of the mixed-valent compoundsLi x Mg[Yb 3+ 2−x Yb 2+ x X 4 , X =S or Se, with the spinelYb 2 X 4 framework was accomplished, but lithium displaced cadmium from the cadmium spinels and magnesium from Mg[Tm 2 Se 4 , which frustrated attempts to obtain an intermediate-valent compound of variable electron/atom ratio.

Effect of Scale on the Far-Field Pressure Skewness and Kurtosis of Heated Supersonic Jets

In heated supersonic jets, Mach wave radiation and crackle have been identified as dominant noise components that propagate to the downstream region of the jet, in a direction noted as the Mach wave angle. At certain conditions, the Mach waves coalesce in the near field causing steepening of the wavefront, which exceeding a certain level produces a noise feature called “crackle.” The skewness and kurtosis of the pressure and its time derivative (dP/dt) have been widely studied as a measurement of crackle. In this paper, we investigate the impact of different test conditions and different nozzle exit diameters on the far-field high-order statistics of the pressure and dP/dt signals of three converging-diverging conical nozzles, with a design Mach number of 1.5 and jet exit diameters of 0.542, 0.813, and 1.085. Results are compared to a smooth contoured nozzle designed by the Method of Characteristics, with the same design Mach number. For all nozzles, cold and heated jets, TR=1.0 to 3.0, are tested at over, design, and under-expanded conditions. Second, third, and fourth order statistics are examined in three far-field arrays positioned at a nondimensionalized constant radial distance of r=40De. The OASPL, skewness, and kurtosis magnitudes and their propagation angles are proportional to the jet temperature and the NPR, and have peak amplitudes near the Mach wave angle. The pressure skewness and kurtosis plots collapsed for all three scaled nozzles when the pressure signals were not filtered. The dP/dt statistics collapsed when the signals were downsampled proportional to the nozzle exit diameters, applying beforehand a low-pass filter at a proportional cutoff frequency, to avoid aliasing effects.

Electronic structure of the ferromagnetic superconductor UCoGe from first principles

The superconductor UCoGe is analysed with electronic structure calculations using a linearized augmented plane wave method based on density functional theory. Ferromagnetic and antiferromagnetic calculations with and without correlations (via LDA+UH) were done. In this compound the Fermi level is situated in a region where the main contribution to DOS comes from the U-5f orbital. The magnetic moment is mainly due to the Co-3d orbital with a small contribution from the U-5f orbital. The possibility of fully non-collinear magnetism in this compound seems to be ruled out. These results are compared with the isostructural compound URhGe; in this case the magnetism comes mostly from the U-5f orbital.

Anisotropic normal-state properties of the MgB2 superconductor

Based on the experimentally-found existence of two superconducting gaps in MgB2 (one gap associated to the boron σ-states and the other to the boron π-states), the different contributions to the transport properties, electrical conductivity and Hall coefficient were studied using the full potential-linearized augmented plane wave method and the generalized gradient approximation. Four different relaxation times were needed to adjust the electrical conductivity and Hall coefficient to experimental values. MgB2 doping was analysed in the rigid band approximation; this permitted a detailed study of the partial substitution of magnesium for aluminium (Mg1−xAlxB2). Other substitutions such as AB2 (A = Be, Sc, Zr, Nb and Ta) are also discussed. The MgB2 σ-bands (boron σ-states), which are associated to the large gap, are very anisotropic at EF, while the π bands have very little anisotropic character. In Mg1−xAlxB2, Tc diminishes with Al content; the other compounds are not superconductors or have a low Tc. In this work it was found that with electron doping, such as Al substitution, the σ-band conductivity decreases and the corresponding bands become less anisotropic. The σ-band contribution for BeB2 and ScB2 at EF is very small and the anisotropy is much lower. For Zr, Nb and Ta there are no σ-bands at EF. These results give a clear connection between superconductivity and the character of the σ-band, band conductivity, and band anisotropy. This gives a plausible explanation for the diminution of Tc with different doping of MgB2.

The loss of anisotropy in MgB2 with Sc substitution and its relation with the critical temperature

The electrical conductivity anisotropy of the σ-bands is calculated for the (Mg,Sc)B2 system using a virtual crystal model. Our results reveal that the anisotropy drops with relatively little scandium content (<30%); this behaviour coincides with the lowering of Tc and the reduction of the Kohn anomaly. This anisotropy loss is also found in the Al and C doped systems. In this work it is argued that the anisotropy, or 2D-character, of the σ-bands is an important parameter for the understanding of the high Tc found in MgB2.

Flow Statistics and Noise of Ideally Expanded Supersonic Rectangular and Circular Jets

Simulations of an ideally expanded supersonic jet for two operating temperatures are conducted with a low aspect ratio of two rectangular nozzle, and an equivalent diameter circular nozzle of the same design Mach number. The emphasis is on accurately resolving the near-field data, capturing the far-field acoustics, and examining the flow asymmetry of the rectangular jets. The simulated acoustic data of the cold and heated jets are validated against experimentally recorded sound-pressure-level spectra with very good agreement. Rectangular nozzle metrics are calculated in two orthogonal, axial spanning planes to determine the impact of the nozzle exit asymmetry, and then they are compared with the circular jet. The shock cell structure, the jet core length, and the axial distribution of the flow at similar radial distances (normalized by the major, minor, or circular radial lengths) are observed to be different between the two planes of the rectangular jet and relative to the circular jet. The dynamics of t...

Synthesis and Crystal Structure of Oxygen-deficient Bilayer Ruthenate Sr3Ru2O7–δ

The structural properties of oxygen-deficient Ruddlesden–Popper-type Sr3Ru2O7–δ compounds are presented. Sr3Ru2O7–δ compounds (δ = 0.17, 0.23, 0.28, 0.40, and 0.47) were obtained by hydrogen reduction of the parent Sr3Ru2O7 ruthenate. Rietveld structure refinements were performed to determine the crystal structure of the reduced compounds. Oxygen deficiency in the samples was studied by redox titrations and the Ru3+ content was confirmed by electron paramagnetic resonance. Magnetisation measurements were performed to study the magnetic response of the reduced phases. Removal of the oxygen atoms from the parent compound resulted in the decrease of the c-lattice parameter and increase of the a-lattice parameter that is related to partial reduction of Ru4+, in Sr3Ru2O7, to Ru3+. Rietveld analyses showed that the apical oxygen atoms of the RuO6 octahedra were partially lost during reduction. Redox titration experiments showed a linear correlation between reduction of the compounds and the annealing time under H2.

A Multiband Model for LaO1-xFxFeAs

Based on electronic structure calculations using WIEN2k code for the iron oxypnictide LaO1-xFxFeAs a multi-band model is proposed. Within the BCS framework a generalized Fermi surface with overlapping bands is introduced. s-wave pairing symmetry and different doping values are considered. This model is used to describe some properties of iron-based oxypnictide superconductors as function of the coupling parameter as well as other relevant parameters of the model. In order to get numerical results the experimental data of LaO1-xFxFeAs with several doping concentrations provide the input of this work.

Magnetic stripes in the UCoGe superconductor

The magnetic superconductor UCoGe is analyzed with an electronic structure package, spin-orbit coupling and intra-atomic repulsion (via Hubbard UH) were included. The possibility of an antiferromagnetic configuration is studied, but it is found to be unstable, also the non-collinear magnetization seems to be ruled out. The magnetization is given mainly by the Co-atoms with M = 0.6 ?B/f.u. The U-atoms have two magnetic moments; M = 0.07 and 0.32 ?B/f.u. arranged in an alternated planes along the b-direction forming magnetic stripes.