K. Ubukata

An Overall Energy Spectrum of Magnetic Fluctuations in the Superconducting La1.85Sr0.15CuO4

High-energy pulsed neutron inelastic scattering experiments have been performed using single crystals of La 1.85 Sr 0.15 CuO 4 to study magnetic fluctuations in the superconducting phase. A peak centered at the reciprocal point (π,π) is well defined below around 120 meV and rapidly damped beyond this energy. However, the \lq\lqbackground-subtracted” scattering intensities indicate that magnetic signals remain up to around 300 meV. These data combined with the results obtained by triple-axis neutron scattering experiments provide the first overall energy spectrum of the dynamical susceptibility in La 1.85 Sr 0.15 CuO 4 . The obtained spectrum is much wider than that of YBa 2 Cu 3 O 7- d ( d ∼0).

Neutron Scattering Studies on Spin-Peierls Material CuGeO3

Crystal structure and phonon dynamics up to 150 meV of the spin-Peierls material CuGeO 3 have been investigated by neutron scattering experiments. We found a large lattice anomaly in the a-b plane, vertical to the one-dimensional antiferromagnetic spin chain, associated with additional diffraction peaks in the b * -axis at temperatures much higher than the spin-Peierls transition temperature. The phonon modes of the chain oxygens at 100 meV show consistent evolution with diffraction. Therefore, we conclude that the structural evolution of the chain oxygens in the a-b plane, which mediates the spin-spin interaction along the c -axis, drives the spin-Peierls transition.

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.

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.

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.

Lattice anomaly of La1.85Sr0.15CuO4 studied by neutron scattering

Abstract Inelastic neutron scattering studies were performed on a powder and assembly of single crystals of La1.85Sr0.15CuO4. Phonon density of states and the Q-dependence of dynamic structure factor as well as the details of phonon scattering function were investigated as a function of temperature. We found specific phonon modes at the Brillouin zone center and the zone boundary showed an anomalous behavior.

Neutron diffraction study of DyAg in high magnetic field

Abstract We have performed neutron diffraction experiments in high magnetic fields up to 13 T on DyAg to investigate the complicated magnetic structures, which has multi-stepwise jumps in the magnetization curve, for instance, at Hc1 = 7.4 T and Hc2 = 10.2 T for H parallel to the [001] crystal axis. Several models were proposed according to the magnetization measurement. Our results, however, clearly found an inconsistency with these models, and we finally obtained two probable models consistent with the observed diffraction data.

Magnetic structure of DyAg in high magnetic fields

Abstract Neutron diffraction on metamagnetic DyAg, a cubic rare earth intermetallic compound, has been performed in pulsed high magnetic fields up to 13T. We have observed prominent intensty decreases at critical fields. These magnetic phase transitions cannot be explained by any structural models proposed before accordin to the magnetization process.

Lattice anomaly and dynamics of CuGeO3

We have performed neutron scattering on both single crystal and powder samples of spin-Peierls system CuGeO3. Below the transition temperature Tsp, we found spontaneous strains in all crystal axes. Temperature dependence of the lattice constants and phonon density of states change drastically well above Tsp, which is apparently linked with the transition. Thus, we concluded that inherent structural instability makes a favorable condition for the SP transition and also continuous transformation affects magnetic behavior above Tsp.