Yusuke Wakabayashi

Direct observation of antiferroquadrupolar ordering: resonant X-Ray scattering study of DyB2C2

Antiferroquadrupolar (AFQ) ordering has been conjectured in several rare-earth compounds to explain their anomalous magnetic properties. No direct evidence for AFQ ordering, however, has been reported. Using the resonant x-ray scattering technique near the Dy L(III) absorption edge, we have succeeded in observing the AFQ order parameter in DyB2C2 and analyzing the energy and polarization dependence. The much weaker coupling between the orbital degrees of freedom and the lattice in 4f electron systems than in 3d compounds makes them an ideal platform to study orbital interactions originating from electronic mechanisms.

Novel Orbital Ordering Induced by Anisotropic Stress in a Manganite Thin Film

A novel structure of orbital ordering is found in a Nd0.5Sr0.5MnO3 thin film, which exhibits a clear first-order transition, by synchrotron x-ray diffraction measurements. Lattice parameters vary drastically at the metal-insulator transition at 170 K (= T(MI)), and superlattice reflections appear below 140 K (= T(CO)). The electronic structure between T(MI) and T(CO) is identified as A-type antiferromagnetic with a d(x2-y2) ferro-orbital ordering. The new type of antiferro-orbital ordering characterized by the wave vector (1/4 1/4 1/2) in cubic notation emerges below T(CO). The accommodation of the large lattice distortion at the first-order phase transition and the appearance of the novel orbital ordering are brought about by the anisotropy in the substrate, a new parameter for the phase control.

Observation of quadrupole helix chirality and its domain structure in DyFe3(BO3)4

Resonant X-ray diffraction (RXD) uses X-rays in the vicinity of a specific atomic absorption edge and is a powerful technique for studying symmetry breaking by motifs of various multipole moments, such as electric monopoles (charge), magnetic dipoles (spin) and electric quadrupoles (orbital). Using circularly polarized X-rays, this technique has been developed to verify symmetry breaking effects arising from chirality, the asymmetry of an object upon its mirroring. Chirality plays a crucial role in the emergence of functionalities such as optical rotatory power and multiferroicity. Here we apply spatially resolved RXD to reveal the helix chirality of Dy 4f electric quadrupole orientations and its domain structure in DyFe3(BO3)4, which shows a reversible phase transition into an enantiomorphic space-group pair. The present study provides evidence for a helix chiral motif of quadrupole moments developed in crystallographic helix chirality.

Resonant X-ray scattering study on the filled skutterudite PrFe4P12

Abstract Resonant X-ray scattering study was carried out to investigate an anomalous ordered state ( T A =6.5 K ) in the filled skutterudite PrFe4P12. At the Pr-LIII absorption edge, we observed resonant features in h+k+l=odd reflections, which are forbidden in the BCC structure above TA. Because these reflections contain the difference of anomalous scattering factor between two Pr atoms in the BCC unit cell, and the ordered state is attributed to the ordering of two different electronic states of Pr.

Orbital Ordered Structure of a Manganite Thin Film Observed by K‐edge Resonant X‐ray Scattering

The low‐temperature orbital ordering structure of Nd0.5Sr0.5MnO3 thin film on SrTiO3 (011) substrate (NSMO/STO011) has been clarified by resonant x‐ray scattering. This thin film is the first example of the manganite thin film that has sharp metal‐insulator transition, and synchrotron x‐ray diffraction reveals the structure of the low‐temperature orbital ordered phase. The orbital order structure was found to be the same as the structure of bulk Nd0.5Sr0.5MnO3, while the tilt/rotation of the MnO6 octahedra were different.

Correlation Of The Superconductivity With The Multi‐Stack Structure In MgB2‐Type Superconductor CaAlSi

CaAlSi is a superconductor with the AlB2‐like structure. In this system, there are two types of multi‐stack structures along c‐axis, 5‐fold structure and 6‐fold structure. The 5‐fold structure sample and the 6‐fold structure sample have TC of 6 K and 8 K, respectively. To investigate the correlation of stack structure with the superconductivity, we determined the crystal structures accurately using synchrotron radiation X‐ray diffraction and neutron diffraction measured at KEK. Al and Si atoms form slightly corrugated honeycomb layers and those are ordered in both stack structures. The difference of stacking structure is caused by phase relation among Al‐Si layers.

Low-temperature zigzag charge-ordering structure of α′-NaV2O5

Abstract The low-temperature (LT) superstructure of NaV2O5 was determined by synchrotron radiation X-ray diffraction. Below the phase transition temperature at 34 K , the LT structure is monoclinic. It was determined that the LT structure is (a−b)×2b×4c with the space group A112, where a, b and c represent the high-temperature orthorhombic unit cell. The valence estimation of V ions shows that the V sites are clearly separated into two groups of V4+ and V5+ with a zigzag charge-ordering pattern.

Charge and orbital ordered states in Nd1−xSr1+xMnO4(x=0.67,0.75)

Abstract The charge and orbital ordered states in the single-layered manganite crystals, Nd 1−x Sr 1+x MnO 4 (x=0.67,0.75) , have been investigated. We have observed the superlattice spots with modulation wave vector, q=(δ,δ,0), δ= 1 8 , 1 4 [ ∼ 1 6 , 1 3 ] in x=0.75[0.67] by resonant and non-resonant X-ray scattering. These results show that not only the atomic displacement pattern but also the charge-orbital structure has the unit cell of 4 2 a× 2 a×c [3 2 a× 2 a×c] .