Journal of Neutron Research

The status of the ISIS Facility is reviewed and new developments are discussed.

We present an analysis of the experimental data obtained by means of inelastic neutron scattering (INS) experiments for diluted Heisenberg antiferromagnet RbMn_0.39Mg_0.61F₃ (H. Ikeda et al., J. Phys. Soc. Jpn. 67, 3376 (1998)). Calculated results recover well INS results. These results confirm the observation of antiferromagnetic fractons in RbMn_0.39Mg_0.61F₃ by Ikeda et al.

Results on the spin dynamics in one-dimensional (1D) and percolating systems obtained under the UK–Japan Collaboration Program on Neutron Scattering are summarized.

Dynamical structures in the vicinity of Q = (π, π) of optimally-doped YBa₂Cu₃O_6.9 were measured by means of an inelastic neutron scattering technique utilizing pulsed neutron source. Obtained incommensurability δ of energy transfer at E = 36 meV was found to be ∼0.1 [rlu] which is not the value to be expected from the formation of spin-charge stripes. In higher energy above resonance energy (42 meV), a ring structure centered at (π, π) was observed up to 54 meV. Combined with the low energy part of the dynamical structure, the obtained results provide a total picture of the dynamical aspects in the high-T_c superconductor YBa₂Cu₃O_6.9.

Feasibility of various types of pulsed magnetic field generators for neutron scattering and X-ray scattering is discussed. Both, a conventional single shot type pulsed field and repeating type pulsed field generators are considered. As a new approach, a miniature type generator is also proposed. The characteristics of possible magnetic fields such as time structure, intensity and interval between shots are compared by considering the requests for the spectrometer in which a high field coil is installed.

We report on progress of the development of neutron spin filters based on polarized gaseous ³He at KENS (Neutron Science Laboratory, KEK). ³He polarizer cells made of quartz glass and sapphire are prepared and tested. Quartz glass cells with spin relaxation times of more than 100 h are routinely produced. A ³He polarization of 63% has been achieved with a sapphire cell of a ³He pressure of 3 atm, and with this ³He cell, a neutron polarization of about 86% was obtained at thermal energy.

Incoherent inelastic neutron scattering spectra of antiferroelectric Cu(HCOO)₂·4H₂O and its isomorphous Cu(HCOO)₂·4D₂O were measured. Comparing the spectra between these crystals, we extracted the H₂O-related modes from many other modes. We find new peaks in the energy range of the H₂O librational modes. The spectra in this range become broad above the phase transition temperature indicating that the antiferroelectric dynamics are dominated by the order–disorder of the H₂O molecules induced by the out-of-plane hydrogen motion.

The static spin structure factor S(Q) of CuGeO₃ was determined above its spin-Peierls transition temperature by inelastic neutron scattering. At 20 K, S(Q) has a Lorentzian line shape centered at the antiferromagnetic (AF) zone center. With increasing temperature, however, it splits into two peaks with their centers at incommensurate wave vectors at higher temperatures (70 and 120 K). The observed results are consistent with the prediction by recent numerical studies on the S = 1/2 Heisenberg AF chain with competing spin interactions [I. Harada and Y. Aoyama, (1999) “Static spin structure factor of CuGeO₃ above spin-Peierls transition temperature”, J. Phys. Chem. Solids 60, 1105; I. Harada, Y. Nishiyama, Y. Aoyama and S. Mori, (2000) “Spin correlation in the one-dimensional Heisenberg antiferromagnet with competing interactions”, J. Phys. Soc. Jpn. 69, (Suppl. A), 339].

Inelastic neutron scattering measurements have been performed on superconducting Na_xHfNCl powder samples with x = 0.27 and 0.38 (T_c = 22 K): We have observed several temperature dependent phonon anomalies, e.g. at E ∼ 15, 29, 76, and 81 meV for the former sample. A phonon peak with E = 14.5 meV at Q ∼ 6Å⁻¹ increases for both samples with decreasing temperature below T = 40 K; which is much higher than T_c. The phonon peak is assigned to an ab-plane vibrational mode, E_u, of chlorine and hafnium atoms, which strongly modifies the electronic band structure. The phonon anomaly is discussed in terms of carrier inhomogeneity.

We carried out inelastic neutron scattering (INS) measurements to study the dynamic structure of liquid (l-) Se and Te₅₀Se₅₀. The bond-stretching modes for both liquids were found to lie in higher energies than those forming a trigonal phase, showing that the chainlike molecules are more isolated than in the trigonal phase. The Q-dependence of the intensity of the dynamic structure factor was examined at the region of the stretching modes, and found to be explained by a simple model where we assumed completely out-of-phase motions of two atoms. The result shows the possibilities of extracting short-range structural information from such Q-dependences for these kinds of basically isolated molecules.

Neutron diffraction measurements with H/D isotopic substitution have been performed for hexafluoro-iso-propanol (HFIP; 1, 1, 1, 3, 3, 3-hexafluoro -2-propanol)–water mixtures at 0.1, 0.2 and 0.4 of HFIP mole fractions (x_HFIP). The nine partial structure factors obtained were subjected to an empirical potential structure refinement (EPSR) method to derive all site–site pair correlation functions. It has been found that with increasing HFIP concentration the degree of hydrogen bonding between water–water, HFIP–water, HFIP–HFIP increases as have previously been found for aliphatic alcohols–water mixtures. There was no evidence that water structure is well defined around the hydrophobic groups of HFIP. At x_HFIP = 0.1, the hydrophobic groups of HFIP contact with each other, whereas a zigzag chain structure due to hydrogen bonds via the hydroxyl groups grows in the more concentrated HFIP mixtures.