Masayoshi Ohashi

Bio: Masayoshi Ohashi is an academic researcher from Yamagata University. The author has contributed to research in topics: Neutron diffraction & Antiferromagnetism. The author has an hindex of 22, co-authored 100 publications receiving 2288 citations. Previous affiliations of Masayoshi Ohashi include Tohoku University.

Hole-concentration-induced transformation of the magnetic and orbital structures in Nd 1 − x Sr x MnO 3

Abstract: We have peformed neutron diffraction measurements on melt-grown polycrystalline samples of ${\mathrm{Nd}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ $(0.49l~xl~0.75).$ A systematic transformation of the crystalline and magnetic structures of this system was observed. This can be consistently explained by the change of the character of the Mn ${e}_{g}$ orbitals. When increasing X, the ${\mathrm{Nd}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ system exhibits an evolution from a metallic ferromagnetic state, to a metallic A-type antiferromagnetic (AFM), and then to an insulating C-type AFM state. The CE-type charge-ordered AFM state was observed only in the vicinity of $x=1/2$ and it coexists with the A-type AFM state for $x\ensuremath{\gtrsim}1/2,$ indicating that the energy difference between these two states is very small. We also found that the ${\mathrm{MnO}}_{6}$ octahedra are apically compressed in the CE-type and A-type AFM states due to the ${d(3x}^{2}\ensuremath{-}{r}^{2}{)/d(3y}^{2}\ensuremath{-}{r}^{2})$ or ${d(x}^{2}\ensuremath{-}{y}^{2})$ orbital ordering, whereas they are apically elongated by the rod-type ${d(3z}^{2}\ensuremath{-}{r}^{2})$ orbital ordering in the C-type AFM state. Finally, a selective broadening of Bragg peaks was observed in the C-type AFM phase and its x dependence strongly suggests the onset of charge ordering for either $x=4/5$ or $x=3/4.$

The new neutron powder diffractometer with a multi-detector system for high-efficiency and high-resolution measurements

Abstract: We describe a multipurpose neutron powder diffractometer for high-efficiency and high-resolution measurements, HERMES, which has been installed at the JRR-3M reactor in Japan Atomic Energy Research Institute. To obtain high-quality data for a short counting time, or with a small sample amount, HERMES has a multi-detector system which consists of 150 3He neutron detectors and simple window type collimation. The best resolution is Δ2θ of 18' and Δd/d of 3 ×10-3, where 2θ and d are scattering-angle and d-spacing, respectively. The duration of an ordinary powder diffraction experiment is 1–5 h under the present conditions in JRR-3M. Some typical results of powder diffraction experiments and performance are given in this paper. We also show a result of 2D intensity mapping of magnetic diffuse scattering from a single crystal of a spin glass material.

Antiferroquadrupolar ordering and magnetic properties of the tetragonal DyB2C2 compound

Abstract: Magnetic properties were investigated on a single-crystalline DyB 2 C 2 compound with the tetragonal structure. Spontaneous magnetizations appear along the a- and [1 1 0]-directions below T C =15.3 K. A very small shoulder is observed only along the c -axis at T Q =24.7 K, although large λ-type anomalous specific heats are observed at T C and T Q . There exists a large magnetic anisotropy in the tetragonal basal plane which is observed in magnetization processes below T C . Neutron powder diffraction experiments reveal that magnetic reflections are observed only below T C . In the magnetically ordered phase, the Dy moments are arranged to be perpendicular to neighbors along the c -axis. The results are well interpreted by postulating that the phase between T C and T Q is an antiferroquadrupolar ordered one. DyB 2 C 2 is a novel compound with a high antiferroquadrupolar ordering temperature of 24.7 K.

Observation of modulated magnetic long-range order in la1.88sr0.12cuo4

Abstract: Magnetic superlattice peaks are observed in single-crystal neutron-diffraction measurements on orthorhombic ${\mathrm{La}}_{1.88}{\mathrm{Sr}}_{0.12}{\mathrm{CuO}}_{4}$ at reciprocal points of (1/2\ifmmode\pm\else\textpm\fi{}\ensuremath{\epsilon},1/2,0) and (1/2,1/2\ifmmode\pm\else\textpm\fi{}\ensuremath{\epsilon},0) in the tetragonal notation where $\ensuremath{\epsilon}=$0.126$\ifmmode\pm\else\textpm\fi{}$0.003. The La NMR measurement reveals a broadening of the field-swept spectrum below $\ensuremath{\sim}$45 K corresponding to the existence of magnetic order. The remarkable softening of longitudinal sound waves along [110] is observed in the same crystal. The features observed in the neutron diffraction, NMR, and ultrasonic measurements suggest that the dynamical incommensurate spin correlation is pinned by a lattice instability toward the low-temperature tetragonal phase.

Angle-resolved photoemission studies of the cuprate superconductors

Abstract: The last decade witnessed significant progress in angle-resolved photoemission spectroscopy (ARPES) and its applications. Today, ARPES experiments with 2-meV energy resolution and $0.2\ifmmode^\circ\else\textdegree\fi{}$ angular resolution are a reality even for photoemission on solids. These technological advances and the improved sample quality have enabled ARPES to emerge as a leading tool in the investigation of the high-${T}_{c}$ superconductors. This paper reviews the most recent ARPES results on the cuprate superconductors and their insulating parent and sister compounds, with the purpose of providing an updated summary of the extensive literature. The low-energy excitations are discussed with emphasis on some of the most relevant issues, such as the Fermi surface and remnant Fermi surface, the superconducting gap, the pseudogap and $d$-wave-like dispersion, evidence of electronic inhomogeneity and nanoscale phase separation, the emergence of coherent quasiparticles through the superconducting transition, and many-body effects in the one-particle spectral function due to the interaction of the charge with magnetic and/or lattice degrees of freedom. Given the dynamic nature of the field, we chose to focus mainly on reviewing the experimental data, as on the experimental side a general consensus has been reached, whereas interpretations and related theoretical models can vary significantly. The first part of the paper introduces photoemission spectroscopy in the context of strongly interacting systems, along with an update on the state-of-the-art instrumentation. The second part provides an overview of the scientific issues relevant to the investigation of the low-energy electronic structure by ARPES. The rest of the paper is devoted to the experimental results from the cuprates, and the discussion is organized along conceptual lines: normal-state electronic structure, interlayer interaction, superconducting gap, coherent superconducting peak, pseudogap, electron self-energy, and collective modes. Within each topic, ARPES data from the various copper oxides are presented.

Orbital Physics in Transition-Metal Oxides

Abstract: An electron in a solid, that is, bound to or nearly localized on the specific atomic site, has three attributes: charge, spin, and orbital. The orbital represents the shape of the electron cloud in solid. In transition-metal oxides with anisotropic-shaped d-orbital electrons, the Coulomb interaction between the electrons (strong electron correlation effect) is of importance for understanding their metal-insulator transitions and properties such as high-temperature superconductivity and colossal magnetoresistance. The orbital degree of freedom occasionally plays an important role in these phenomena, and its correlation and/or order-disorder transition causes a variety of phenomena through strong coupling with charge, spin, and lattice dynamics. An overview is given here on this "orbital physics," which will be a key concept for the science and technology of correlated electrons.

Slater-Pauling behavior and origin of the half-metallicity of the full-Heusler alloys

Abstract: Using the full-potential screened Korringa-Kohn-Rostoker method we study the full-Heusler alloys based on Co, Fe, Rh, and Ru. We show that many of these compounds show a half-metallic behavior; however, in contrast to the half-Heusler alloys the energy gap in the minority band is extremely small due to states localized only at the Co (Fe, Rh, or Ru) sites which are not present in the half-Heusler compounds. The full-Heusler alloys show a Slater-Pauling behavior and the total spin magnetic moment per unit cell ${(M}_{t})$ scales with the total number of valence electrons ${(Z}_{t})$ following the rule ${M}_{t}{=Z}_{t}\ensuremath{-}24.$ We explain why the spin-down band contains exactly 12 electrons using arguments based on group theory and show that this rule holds also for compounds with less than 24 valence electrons. Finally we discuss the deviations from this rule and the differences compared to the half-Heusler alloys.