Ferromagnetic transition in Au- based Approximants

The structure of Tsai-type magnetic quasicrystals and its related compounds (called approximants) are characterized by the space-filling of an icosahedral cluster which has a rare-earth icosahedron [1]. From an experimental point of view, such compounds have been known to show the spin glass like behavior without exception [2]. However, the discovery of the antiferromagnetic phase transition in the Cd-Tb approximant [3] gives a counterexample to this trend. Moreover, ferromagnetic transitions were observed in the Au-based approximant recently. In this paper, magnetic phase transitions in Au-Si-R (R= Gd ,Tb, Dy and Ho) approximants are discussed. In all the systems, the temperature dependence of magnetization show ferromagnetic transition at Tc. On the other hand, the magnetization curves below Tc are different between Gd-compound and non-Gd compounds. The difference in the magnetization may be attributed to the existence of the CEF effect in the non-Gd compounds which have non-zero orbital angular momentum.

Magnetic structure of the chiral triangular magnet MnSb₂O₆

Incommensurate helical (or cycloidal) magnetic structure may have left- and right-wound states (helicity), which are in principle equally populated in a magnet with inversion symmetry. In addition, for a Heisenberg triangular antiferromagnet, clockwise and counter-clockwise rotations of the 120 degree spin structure provide another intriguing degree of freedom. Hence, a triangular magnet that has incommensurate helical ordering along the stacking direction will show intriguing interplay of the helicity (of the helical structure) and chirality (in the triangular plane). Such phenomenon is, however, rarely studied in the past since only one example, the Ba3NbFe3Si2O14 langathite, has been known to date [1]. In this work, we study MnSb2O6, which consists of distorted triangular lattice stacking along the c-axis [2,3]. MnSb2O6 belongs to the space group P321, and hence lacks inversion symmetry. Due to this fact, unique selection of the helicity and chirality may be expected. However, the earlier studies were carried out using unpolarized neutron diffraction with mostly the powder sample, and thus helicity and chirality selection cannot be concluded. Here, we have performed single-crystal diffraction experiment using polarized neutrons in addition to the unpolarized ones, and have succeeded in determination of the magnetic structure of MnSb2O6. The resulting magnetic structure is nearly cycloidal with the magnetic modulation vector q = (0, 0, 0.182) (see figure below). The spin rotation plane is, however, inclined from the ac-plane toward the b-axis for approximately 30 degrees. Polarization analysis indicates that both the helicity of the (nearly-) cycloidal structure and chirality of the in-plane 120 degree structure are uniquely selected. The 30 degree inclination from the ac-plane is a key finding of this work, allowing new kind of multiferroicity in this material.

Structural studies of myo-inositol kinase

The TK2285 protein from a hyperthermopilic archaeon Thermococcus kodakarensis is a myo-inositol kinase. Only two myo-inositol kinases have been identified so far. One is the TK2285 protein and the other is an enzyme from Zea mays. Both of them synthesize myo-inositol monophosphate that shows enantiomerism. Because it is too difficult to discriminate enantiomers by NMR or chromatography analysis, it has not been identified which of the six hydroxyls is phosphorylated by these enzymes. Also, little is known about the substrate recognition of myo-inositol kinase, since only the unliganded crystal structure of TK2285 has been reported. In order to reveal the substrate-binding mechanism of myo-inositol kinase and identify the phosphorylated hydroxyl group of the product, we determined the crystal structures of TK2285 as the substrate-complex and the product-complex. The substrate-complex of TK2285 was prepared by using the TK2285, myo-inositol and AMP-PCP, and the products-complex was prepared by incubating the TK2285 with myo-inositol and ATP. The substrate-complex structure showed that all of the six hydroxyls of myo-inositol interacted with TK2285. This coincides with the fact that the Km value for myo-inositol is 100-1000 fold lower than those for other sugars. Also 3-hydroxyl group of myo-inositol, which the gamma-phosphate of AMP-PCP was nearest to, was thought to be phosphorylated by this enzyme. This was proved by the product-complex structure that had ADP and myo-inositol 3-phosphate. Site-directed mutagenesis and structure comparison with TK2285 homologs also provided information about the substrate-binding mechanism of myo-inositol kinase.

Capturing structural dynamics of materials by picosecond X-ray pulses

Picosecond time-resolved X-ray techniques, such as time-resolved X-ray diffraction, scattering, and spectroscopy, utilize the pulsed nature of synchrotron radiation from storage rings, and are becoming general and powerful tools to explore structural dynamics in various materials. This method enables to produce "atomic structural movies" at picosecond temporal resolution. It will be fascinating to apply such capability to capture ultrafast structural dynamics in advanced materials of strongly-correlated electron systems, photochemical catalytic reaction dynamics in liquid or on solid surface, light-induced response of photosensitive proteins, etc. Photon Factory Advanced Ring (PF-AR) at the High Energy Accelerator Research Organization (KEK), Tsukuba, Japan is a 6.5-GeV electron storage ring dedicated for single-bunch operation and is suitable for the picosecond time-resolved X-ray studies. An in-vacuum undulator beamline NW14A at the PF-AR was designed and constructed to conduct a wide variety of time-resolved X-ray measurements, such as time-resolved X-ray diffraction, scattering and spectroscopy [1]. Successful examples of time-resolved X-ray studies applied to materials science will be presented in the talk.

Macroscopic and microscopic strain of SiO2 glass under uniaxial compression

Although SiO2 glass is brittle due to its covalency and the lack of dislocation movement seen in crystals, it can deform without fracturing when compressed to high pressures. The phenomenon may be attributable to the well-known permanent densification by the reconstruction of the network structure consisting of SiO4 tetrahedra. To explore so-called plastic deformation without permanent densification, we measured the change in size (macroscopic strain) of uniaxially-compressed disk-shaped SiO2 glass by an optical microscope [1]. Also, to understand the anisotropy in structure (microscopic strain), we measured the azimuth-angle dependence of the position of the first sharp diffraction peak (FSDP) of uniaxially-compressed SiO2 glass with a radial X-ray diffraction technique [2]. In the microscope observation, the glass was found to deform largely without fracturing up to at least 20 GPa from 6-8 GPa, where uniaxial conditions were achieved. In the X-ray diffraction observation, a large anisotropy was found in the FSDP which corresponds to the intermediate-range network structure of the glass. The recovered glass was examined by the radial X-ray diffraction up to a high-Q range and was found to remain largely anisotropic (equivalent to about 2 GPa in differential stress) in the intermediate-range network structure and not to remain anisotropic in the short-range SiO4 tetrahedral structure. It seems intuitive that the residual anisotropy is due to the anisotropic reconstruction of the network structure during permanent densification. However, the macroscopic strain measured in the microscope observation was an order of magnitude larger than the microscopic strain in the X-ray diffraction observation, and therefore it cannot be explained solely by the anisotropic permanent densification. The permanent densification may also enhance the reconstruction of the network structure and therefore plastic deformation.

Magnetism and its interplay with lattice in the iron-based ladder compounds

Since the discovery, research on iron-based superconductivity (SC) has become one of the main streams in condensed matter physics [1]. The interplay between structure, magnetism and SC is one of most intriguing subjects of this field. The common structural feature is the presence of square planar sheets of Fe atoms coordinated tetrahedrally by pnictogens or chalcogens. They have been, at the early stage, realized in the ZrCuSiAs (1111), ThCr2Si2 (122), anti-PbO (11) and Cu2Sb (111) structures. To gain further insight into the mechanism of the SC and variation of magnetic orders, investigation of Fe-based compounds with a separate spatial dimension is important. This is because the dimensionality should influence magnetism and can control itinerancy of electrons by changing Fermi surface topology. As spin ladders in copper oxides shed a new light on the mechanism of SC, a study on an analogue with ladder geometry among Fe-based compounds is highly desired. Here we report our recent studies of iron-based ladder compounds AFe2X3 (A = K, Rb, Cs, Ba; X = S, Se, Te) [2,3]. Crystal structure is novel, comprising of FeX4 tetrahedra with channels which host A atoms, and four-fold coordinated Fe2+ ions form two-leg ladder geometry. Unlike most of parent compounds of the Fe-based SCs, the ladder compounds are insulating down to the lowest measured temperature. Through bulk properties and neutron diffraction measurements, a variety of magnetic structures and low dimensional characteristics were elucidated. These would provide a clue of the SC realized in a separate dimension systems. The description of theory that accounts for the observed magnetic structures will be also presented.