M. Arai

Local structural instability of high-T c oxide superconductors studied by inelastic neutron scattering

The scattering function,S(Q, E), of superconducting La1.85Sr0.15CuO4 and YBa2Cu3O7 was measured over a wide range ofQ-E space by inelastic neutron scattering. Fine structures in their phonon density of states as well as theQ-dependences ofS(Q, E) were observed. TheQ-dependence ofS(Q, E) is very susceptive atTc, with an anomalous behavior similar to a critical phenomenon. We have found that the anomalous behavior is related to the local structural instability, i.e., the 〈110〉 buckling motion of the Cu-O plane and the shortening of the distance between the apical oxygen and the copper atom in the plane.

Dynamical Magnetic Structure of the Spin Density Wave State in Cr

Inelastic neutron scattering experiments were carried out on the spin density wave (SDW) state of a single- Q chromium crystal. We made systematic investigations in a wide energy range up to ω∼100 meV by combining the triple axis and time-of-flight neutron spectroscopy. The q profile of the inelastic scattering is of a three-peaked structure; the commensurate scattering (CMS) at the antiferromagnetic superlattice and the incommensurate scattering (ICMS) near the wave vector points of the static SDW. In the longitudinal SDW state the dynamical susceptibility for the CMS increases with increasing ω and eventually overwhelms that for the ICMS. The ICMS, on the other hand, reaches a maximum at ω∼20 meV. This cross-over phenomenon is reflected in the temperature dependence of the magnetic excitations so that the CMS dominates at temperatures above 200 K.

Quantum effect in the high-field phase of CsCuCl3

Abstract A small magnetization jump at Bc = 12.5 T in CsCuCl3 found in 1978 has been a mystery for a long time. In this compound, the magnetization was supposed to be linear because of smooth tilt of the magnetic moments toward the field direction for B || c-axis. Then a jump was completely unexpected by the molecular field model. Recently, Shiba proposed a model where the 120°-structure at low field changes to the coplanar type above Bc due to a quantum effect. This is the first example which shows a quantum effect in high fields very clearly in a magnetic system with strong frustration. This structure is confirmed by high field neutron diffraction and high field submillimeter wave ESR. But the plateau observed in the magnetization for B ⊥ c is still an open question.

Neutron diffraction of CsCuCl3 in high fields

Abstract Neutron diffraction measurements of triangular lattice CsCuCl 3 have been performed in a pulsed high magnetic field up to 14T. For an applied field parallel to the c -axis and below B c = 10T at 7 K, Bragg reflection is observed at (1/3 1/3 0.085) showing a long period helical spin alignment along the c -axis and a triangular umbrella-like structure in the c -plane. The intensity of this reflection drastically changes at B c and a new reflection at (1/3 1/3 0) appears. These results well explain a new type of magnetic phase transition which cannot be understood by the classical theory. When B is perpendicular to the c -axis more complicated results are observed.

Anomalous breaking of phonon softening in the superconducting state of La1.85Sr0.15CuO4

Abstract Neutron inelastic scattering measurements on single crystals of La 2− x Sr x CuO 4− y have been performed to study the relation between the soft phonon and the superconductivity. The phonon softening of the Z-point mode due to the incipient structural phase transition to the low-temperature tetragonal (P4 2 /ncm) or orthorhombic (Pccn) phase appeared to be frozen below T c . The narrowing of the phonon line-width upon cooling was saturated below T c , suggesting the appearance of the extra line broadening in the superconducting state.

Cu-Ag alloy Bitter type magnet for repeating pulsed field

Cu-Ag alloy is used for the repeating pulsed field magnets. It is found that fields up to 22 T or more will be available for this purpose instead of 16 T which is obtained with normal copper magnet used at present. This result is a big advantage for neutron diffraction experiments.

Small-angle neutron diffraction from the helical magnet Fe0.8Co0.2Si

The preferred direction of the helical axis in Fe0.8Co0.2Si has been studied by small-angle neutron diffraction. In the virgin state, the helical axis points along all directions of the crystal. The analysis of the shape of the Bragg scattering indicates that the propagation vector tends to lie along the 〈0 0 1〉 axis in Fe0.8Co0.2Si.

A dynamic correlation function from inelastic neutron scattering data

Abstract For a non-dispersive vibrational mode in a solid the Q -dependence of the scattering function S ( Q , E ) at the energy of the mode contains information relating to the atomic displacements of the mode. An empirical approach to the analysis of such data has recently been employed whereby a Fourier transform is used to obtain a ‘dynamic correlation function’ in real-space. This technique is demonstrated using new data on amorphous boron. The theoretical basis of the method is discussed and it is shown that, whilst the result of the Fourier transform is not a true correlation function, the method is capable of providing information concerning the atomic distances which participate in a particular vibration and the relative phase involved.

Dynamic correlation function studies of the medium-range order in materials

Abstract This paper discusses the extraction of structural and dynamical information from the Q -dependence of the dynamical structure factor, S ( Q , E ), measured by inelastic neutron scattering. The analysis of data for vitreous SiO 2 is considered, and it is shown that the Q -dependence of S ( Q , E ) may be investigated either in real space by use of a dynamic correlation function (DCF) or in reciprocal space by modelling. The modelling approach provides a new means to extract the bond angle distribution, and the results obtained agree well with previous information on the bond angle distribution. The application of the DCF approach to data for a (crystalline) high T c superconductor is used to show that the DCF may provide partial structure factor information without isotope substitution when the material contains elements with different masses - a result with important potential for the study of amorphous materials.

Magnetic structure of DyAg in high magnetic field studied by neutron diffraction

We have carried out neutron-diffraction experiments on the metamagnetic compound DyAg which has two phase transitions at 7.6 T and 10.2 T in a field along the [001] direction. Some models have been proposed according to the magnetization process. By observing the intensity variation of magnetic Bragg peaks, we finally reached a unique model, and this differs from all of the proposed models so far.