Effect of Sr substitution on superconductivity in Hg2(Ba1-ySry)2YCu2O8-d (part2): bond valence sum approach of the hole distribution

The effects of Sr substitution on superconductivity, and more particulary the changes induced in the hole doping mechanism, were investigated in Hg2(Ba1-ySry)2YCu2O8-d by a "bond valence sum" analysis with Sr content from y = 0.0 to y = 1.0. A comparison with CuBa2YCu2O7-d and Cu2Ba2YCu2O8 systems suggests a possible explanation of the Tc enhancement from 0 K for y = 0.0 to 42 K for y = 1.0. The charge distribution among atoms of the unit cell was determined from the refined structure, for y = 0.0 to 1.0. It shows a charge transfer to the superconducting CuO2 plane via two doping channels pi(1) and pi(2), i.e. through O2(apical)-Cu and Ba/Sr-O1 bonds respectively.Comment: 13 pages, 5 figures, accepted for publication in Journal of Physics: Condensed Matte

Mesures de vies moyennes par effet Doppler

Des vies moyennes de niveaux nucléaires ont été déterminées par la méthode de l'effet Doppler. Les rayonnements gamma de désexcitation ont été détectés dans des compteurs Ge(Li). Des vies moyennes ont été obtenues pour les niveaux 0,953 MeV de 12B et 6,44 MeV de 14N

High pressure x-ray diffraction study of the volume collapse in Ba24Si100 clathrate

International audienceThe high pressure stability of the silicon type-III clathrate Ba24Si100 has been studied by x-ray diffraction (XRD) up to a maximum pressure of 37.4 GPa. The high pressure behavior of this Si type-III clathrate appears to be analogous to the structural type-I parent Ba8Si46. An isostructural volume collapse is observed at ~23 GPa, a value higher than for Ba8Si46 (13-15 GPa). The crystallinity of the structure is preserved up to the maximum attained pressure without amorphization, which appears to be in contradiction with the interpretation given in a Raman spectroscopy study [Shimizu et al., Phys. Rev. B 71, 094108 (2005)]. Nevertheless, the XRD analysis shows the appearance of a type-III disordered nanocaged-based crystalline structure after the volume collapse. Moreover, we find that the volume collapse transformation is (quasi)reversible after pressure release. Additionally, a low pressure transition first evidenced by Raman spectroscopy is also observed in our XRD study at 5 GPa: The variation of the isotropic thermal factors of Ba atoms shows a clear discontinuity at this pressure while the average positions of Ba atoms remain identical

Direct observation of the influence of the As-Fe-As angle on the Tc of superconducting SmFeAsO1−x_{1-x}Fx_{x}

The electrical resistivity, crystalline structure and electronic properties calculated from the experimentally measured atomic positions of the compound SmFeAsO$_{0.81}$F$_{0.19}$ have been studied up to pressures ~20GPa. The correlation between the pressure dependence of the superconducting transition temperature (Tc) and crystallographic parameters on the same sample shows clearly that a regular FeAs$_{4}$ tetrahedron maximizes Tc, through optimization of carrier transfer to the FeAs planes as indicated by the evolution of the electronic band structures.Comment: 15pages, 4 figure

Dopant-dependent impact of Mn-site doping on the critical-state manganites: R0.6Sr0.4MnO3 (R=La, Nd, Sm, and Gd)

Versatile features of impurity doping effects on perovskite manganites, $R_{0.6}$Sr$_{0.4}$MnO$_{3}$, have been investigated with varying the doing species as well as the $R$-dependent one-electron bandwidth. In ferromagnetic-metallic manganites ($R$=La, Nd, and Sm), a few percent of Fe substitution dramatically decreases the ferromagnetic transition temperature, leading to a spin glass insulating state with short-range charge-orbital correlation. For each $R$ species, the phase diagram as a function of Fe concentration is closely similar to that for $R_{0.6}$Sr$_{0.4}$MnO$_{3}$ obtained by decreasing the ionic radius of $R$ site, indicating that Fe doping in the phase-competing region weakens the ferromagnetic double-exchange interaction, relatively to the charge-orbital ordering instability. We have also found a contrastive impact of Cr (or Ru) doping on a spin-glass insulating manganite ($R$=Gd). There, the impurity-induced ferromagnetic magnetization is observed at low temperatures as a consequence of the collapse of the inherent short-range charge-orbital ordering, while Fe doping plays only a minor role. The observed opposite nature of impurity doping may be attributed to the difference in magnitude of the antiferromagnetic interaction between the doped ions.Comment: 7 pages, 6 figure

INTERACTION BETWEEN ARGON AND DOPANTS IN SPUTTERED a-Si : H

The concentrations of As, B, H, Ar and Si in sputtered a-Si : H are measured by helium Rutherford backscattering and nuclear reactions analysis. Excess or deficit of hydrogen and argon by comparison with intrinsic a-Si : H are found in presence of dopants at high deposition rate. This is related to the plasma deposition method and would suggest micro grain structure in the deposited layer

Local electronic structure and magnetic properties of LaMn0.5Co0.5O3 studied by x-ray absorption and magnetic circular dichroism spectroscopy

We have studied the local electronic structure of LaMn0.5Co0.5O3 using soft-x-ray absorption spectroscopy at the Co-L_3,2 and Mn-L_3,2 edges. We found a high-spin Co^{2+}--Mn^{4+} valence state for samples with the optimal Curie temperature. We discovered that samples with lower Curie temperatures contain low-spin nonmagnetic Co^{3+} ions. Using soft-x-ray magnetic circular dichroism we established that the Co^{2+} and Mn^{4+} ions are ferromagnetically aligned. We revealed also that the Co^{2+} ions have a large orbital moment: m_orb/m_spin ~ 0.47. Together with model calculations, this suggests the presence of a large magnetocrystalline anisotropy in the material and predicts a non-trivial temperature dependence for the magnetic susceptibility.Comment: 8 pages, 7 figure

3D atom probe tomography of swift heavy ion irradiated multilayers

International audienceNanometer scale layered systems are well suited to investigate atomic transport processes induced by high-energy electronic excitations in materials, through the characterization of the interface transformation. In this study, we used the atom probe technique to determine the distribution of the different elements in an (amorphous-Fe$_2$Tb 5 nm/hcp-Co 3 nm)$_{20}$ multilayer before and after irradiation with Pb ions in the electronic stopping power regime. Atom probe tomography is based on reconstruction of a small volume of a sharp tip evaporated by field effect. It has unique capabilities to characterize internal interfaces and layer chemistry with sub-nanometer scale resolution in three dimensions. Depth composition profiles and 3D element mapping have been determined, evidencing for asymetric interfaces in the as-deposited sample, and very efficient Fe-Co intermixing after irradiation at the fluence $7\times10^{12}$ ion cm$^{-2}$. Estimation of effective atomic diffusion coefficients after irradiation suggests that mixing results from interdiffusion in a molten track across the interface in agreement with the thermal spike model