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

Direct observation of the ground state of a 1/3 quantum magnetization plateau in SrMn3_3P4_4O14_{14} using neutron diffraction measurements

We can directly investigate the ground state in magnetization-plateau fields (plateau ground state) using neutron diffraction measurements. We performed neutron diffraction measurements on the spin-5/2 trimer substance SrMn$_3$P$_4$O$_{14}$ in magnetization-plateau fields. The integrated intensities of magnetic reflections calculated using an expectation value of each spin in a plateau ground state of an isolated-trimer model agree well with those obtained experimentally in the magnetization-plateau fields. We succeeded in direct observation of a plateau ground state in SrMn$_3$P$_4$O$_{14}$

Anomalous pressure dependence of the atomic displacements in the relaxor ferroelectric PbMg1/3_{1/3}Ta2/3_{2/3}O3_3

The crystal structure of the PbMg$_{1/3}$Ta$_{2/3}$O$_3$ (PMT) relaxor ferroelectric was studied under hydrostatic pressure up to $\sim 7$ 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 $\sim$ 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

Candidate Quantum Spin Liquid in the Ce\textsuperscript{3+} Pyrochlore Stannate Ce2_2Sn2_2O7_7

We report the low temperature magnetic properties of Ce$_2$Sn$_2$O$_7$, a rare-earth pyrochlore. Our susceptibility and magnetization measurements show that due to the thermal isolation of a Kramers doublet ground state, Ce$_2$Sn$_2$O$_7$ has Ising-like magnetic moments of $\sim1.18$ $\mu_\mathrm{B}$. The magnetic moments are confined to the local trigonal axes, as in a spin ice, but the exchange interactions are antiferromagnetic. Below 1 K the system enters a regime with antiferromagnetic correlations. In contrast to predictions for classical $\langle 111 \rangle$-Ising spins on the pyrochlore lattice, there is no sign of long-range ordering down to 0.02 K. Our results suggest that Ce$_2$Sn$_2$O$_7$ features an antiferromagnetic liquid ground state with strong quantum fluctuations.Comment: 6 pages, 4 figure

Absence of Long Range Magnetic Order in the La1.4Sr0.8Ca0.8Mn2O7 Bilayered Manganite

In this work we studied, by means of high-resolution neutron diffraction as a function of temperature, the La1.4Sr0.8Ca0.8Mn2O7 bilayered manganite for two different annealing treatments. Out data allowed us to shown, for the first time, the absence of long-range magnetic order in this optimally doped bilayered manganite where the A-site of the structure is doped with equal proportions of different isovalent cations (Ca and Sr). The system, however, presents defined IM transitions which suggest that the transport properties are not linked to the evolution of long-range order and that two dimensional spin ordering in the layers of the perovskite blocks may be sufficient to 'assist' the hole hopping. Possible reason for the suppression of magnetic order induced by the Ca doping is a size effect coupled to the cation size mismatch between the Sr and Ca ions.Comment: 24 pages, 7 figure

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