Magnetization curves for general cylindrical samples in a transverse field

Using the recent results for the surface current density on cylindrical surfaces of arbitrary cross-section producing uniform interior magnetic field we propose a method for obtaining solutions of Bean’s critical state model for general cylindrical samples. The method uses the technique of conformal mapping to express the sample surface and the flux-fronts in terms of a set of coefficients that depend on a parameter. The flux-fronts are to be determined by solving a system of nonlinear ordinary differential equations for the coefficients. Retaining only a certain finite number of leading coefficients we get an approximate solution. The procedure is illustrated by considering two cyclindrical samples — one with an elliptical cross-section and the other with a non-elliptical cross-section. The virgin curve and small and large magnetization hysteresis loops for the two samples are obtained.     

Low-field vortex dynamics in various high-T c thin films

We present a novel ac susceptibility technique for the study of vortex creep in superconducting thin films. With this technique we study the dynamics of dilute vortices in c-axis oriented Y-123, Hg-1212, and Tl-1212 thin films, as well as a axis oriented Hg-1212 thin films. Results on the Hg-1212 and Tl-1212 thin films indicate that dislocation-mediated plastic flux creep of single vortices dominates at low temperatures and fields. As the temperature (or the field) is increased, the increasing vortex-vortex interactions promote a collective behavior, which can be characterized by elastic creep with a non-zero μ exponent. Also, in some of these samples effects of thermally assisted quantum creep are visible up to 45 K in some of these samples. In Y-123 thin films, creep is found to be collective down to the lowest temperatures and fields investigated, while the quantum creep persists only up to 10–11 K.     

EXAFS analysis of Zr-based quasicrystals

The character of transformation of the local environment of zirconium and palladium atoms in Zr₇₀Pd₃₀ binary alloy at a transition from the amorphous state to the quasicrystalline and crystalline states and in Zr₇₀Be₃₀ and Zr₈₀Pt₂₀ binary alloys at a transition from the amorphous to the quasicrystalline state have been investigated by extended X-ray absorption fine-structure (EXAFS) spectroscopy and X-ray diffraction. The parameters of the zirconium, palladium, and platinum local environment are determined, and the general features and differences in the local atomic rearrangement during formation of icosahedral clusters around zirconium atoms in these three compounds are established.     

Study of the formation and stability of the Pd and Pt metallic nanoparticles on carbon support

Catalysts containing Pd and Pt on a Sibunit carbon support were studied by the temperature-programmed reduction, in situ X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy (XAFS). The reduction of Pd and Pt species in samples 2%Pd/C and 2%Pt/C calcined in an air flow at 370°C was studied. Reduction of the 2%Pd/C sample begins at 50—60 °C and is completed at 250—300°C. Particles of various dispersion are formed during reduction. Long-distance peaks observed in the EXAFS spectra point to the presence of a fraction of relatively large crystallites. The average Pd—Pd coordination number (5) at 200 °C gives evidence that a number of very small Pd nanoparticles, oligomeric clusters, is present. Reduction at T > 200°C results in sintering of a small fraction of the Pd particles. Reduction of Pt in 2%Pt/C sample begins at 120—150 °C and is completed at 300—350°C. The sintering-resistant monodispersed Pt particles are formed under these conditions.     

Crystal-quasicrystal local structural transition in Al-Cu-Fe

The rearrangement of the local environments of copper and iron in the ternary alloy Al₆₅Cu₂₂Fe₁₃ in transition from a crystalline phase to a quasicrystalline phase has been studied by combined extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) analysis. It has been found that the nearest environment of copper retains symmetry characteristic of a crystal; however, a turn and small shifts of copper matrix atoms causes a considerable rearrangement of aluminum atoms around iron. As a result, icosahedral clusters with pentagonal symmetry are formed around iron atoms, and translational symmetry breaking is accompanied by the transition of Al₆₅Cu₂₂Fe₁₃ to a quasicrystalline state.     

Effect of alkali and alkaline-earth cations on the neodymium environment in a rare-earth rich aluminoborosilicate glass

The local structure around neodymium in an aluminoborosilicate glass bearing 3.6 mol% Nd₂O₃ is studied by optical absorption spectroscopy and EXAFS at the Nd L_III- and K-edges. The influence of the nature of alkalis (M⁺ = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) and alkaline-earths (M²⁺ = Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) on the coordination sphere of Nd³⁺ ions in the glass is particularly investigated. The Nd³⁺ sites are well-defined with NdO mean distances of 2.46 ± 0.03 Å, whatever the alkali and alkaline-earth ion type except Li⁺ and Mg²⁺, for which glasses exhibit slightly more disordered Nd sites and longer NdO distances (2.49 ± 0.03 Å). Using bond valence considerations, a model is proposed for the Nd site, and consists in 7–8 non-bridging oxygens (NBO), every NBO being charge compensated by 2–3 alkalis and alkaline-earths. The NdO mean distance is adjusted according to the mean field strength of these cations, to avoid overbonding of the NBO’s. A glass series with varying Ca²⁺/Na⁺ concentration ratio shows that Nd³⁺ cations are able to maintain this average coordination site even at high alkaline-earth content.     

Pressure‐induced Oxidation State Change of Ytterbium in YbGa2

The crystal structure and the electronic properties of YbGa₂ realising a CaIn₂ type atomic arrangement were characterised at ambient conditions using single crystal X‐ray diffraction data and magnetic susceptibility measurements at ambient pressure. Pressure‐induced changes of structural and electronic properties of YbGa₂ were measured by means of angle‐dispersive X‐ray powder diffraction and XANES at the Yb L_III threshold. At pressures above 22(2) GPa, YbGa₂ undergoes a structural phase transition into a high pressure modification with a UHg₂ type crystal structure. Parallel to the pressure‐induced structural alterations, ytterbium in YbGa₂ undergoes an increase of the oxidation state from +2 at ambient conditions to +3 in the high‐pressure phase. Quantum chemical calculations of the Electron‐Localisation‐Function confirm that the phase transition is associated with a conversion of the three‐dimensional gallium network of the low‐pressure crystal structure into two‐dimensional gallium layers in the high‐pressure modification.     

Magnetization curves for non-elliptic cylindrical samples in a transverse field

Using recent results for the surface current density on cylindrical surfaces of arbitrary cross-section producing uniform interior magnetic field and an assumed set of flux-fronts, solutions of Bean’s critical state model for cylindrical samples with non-elliptic cross-section are presented. Magnetization hysteresis loops for two cross-sections with different aspect ratios are obtained. A comparison with some exact results shows the limitations of this approach.     

Low temperature features of the local structure of Sm<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Y<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>S

The particular features of the local electronic and local crystal structures of the mixed-valence compound Sm_{1 ? x} Y _x S are studied by the XAFS spectroscopy methods in the temperature range 20–300 K for the yttrium concentration x = 0.17, 0.25, 0.33, and 0.45. The temperature behavior of the valence of Sm, as well as of the lengths and the Debye-Waller factors of the bonds Sm-S, Sm-Sm(Y), Y-S, and Y-Sm(Y), has been determined. The violation of the Vegard law has been observed. A model for the estimation of the energy width of the 4f level and of its position with respect to the Fermi level is proposed.