172 research outputs found
Synthesis, crystal structure and chemical stability of the superconductor FeSe_{1-x}
We report on a comparative study of the crystal structure and the magnetic
properties of FeSe1-x (x= 0.00 - 0.15) superconducting samples by neutron
powder diffraction and magnetization measurements. The samples were synthesized
by two different methods: a 'low-temperature' one using powders as a starting
material at T =700 C and a "high-temperature' method using solid pieces of Fe
and Se at T= 1070 C. The effect of a starting (nominal) stoichiometry on the
phase purity of the obtained samples, the superconducting transition
temperature Tc, as well as the chemical instability of FeSe1-x at ambient
conditions were investigated. It was found that in the Fe-Se system a stable
phase exhibiting superconductivity at Tc~8K exists in a narrow range of
selenium concentration (FeSe0.974(2)).Comment: 7 pages, 7 figures, 1 tabl
Anomalous pressure dependence of the atomic displacements in the relaxor ferroelectric PbMgTaO
The crystal structure of the PbMgTaO (PMT) relaxor
ferroelectric was studied under hydrostatic pressure up to GPa by
means of powder neutron diffraction. We find a drastic pressure-induced
decrease of the lead displacement from the inversion centre which correlates
with an increase by 50 % of the anisotropy of the oxygen temperature
factor. The vibrations of the Mg/Ta are, in contrast, rather pressure
insensitive. We attribute these changes being responsible for the previously
reported pressure-induced suppression of the anomalous dielectric permittivity
and diffuse scattering in relaxor ferroelectrics
Magnetic excitations in the spin-trimer compounds Ca3Cu3-xNix(PO4)4 (x=0,1,2)
Inelastic neutron scattering experiments were performed for the spin-trimer
compounds Ca3Cu3-xNix(PO4)4 (x=0,1,2) in order to study the dynamic magnetic
properties. The observed excitations can be associated with transitions between
the low-lying electronic states of linear Cu-Cu-Cu, Cu-Cu-Ni, and Ni-Cu-Ni
trimers which are the basic constituents of the title compounds. The exchange
interactions within the trimers are well described by the Heisenberg model with
dominant antiferromagnetic nearest-neighbor interactions J. For x=0 we find
JCu-Cu=-4.74(2) meV which is enhanced for x=1 to JCu-Cu=-4.92(6) meV. For x=1
and x=2 we find JCu-Ni=-0.85(10) meV and an axial single-ion anisotropy
parameter DNi=-0.7(1) meV. While the x=0 and x=1 compounds do not exhibit
long-range magnetic ordering down to 1 K, the x=2 compound shows
antiferromagnetic ordering below TN=20 K, which is compatible with the
molecular-field parameter 0.63(12) meV derived by neutron spectroscopy.Comment: 22 pages (double spacing), 1 table, 9 figures, Submitted to Phys.
Rev. B (2007
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