Atsushi Okazaki

Bio: Atsushi Okazaki is an academic researcher from Kyushu University. The author has contributed to research in topics: Lattice constant & Crystal. The author has an hindex of 17, co-authored 63 publications receiving 1162 citations.

The Crystal Structures of KMnF 3 , KFeF 3 , KCoF 3 , KNiF 3 and KCuF 3 above and below their Néel Temperatures

Abstract: The crystal structures of the antiferromagnets KMnF 3 , KFeF 3 KCoF 3 , KCNiF 3 and KCuF 3 have been determined above and below their Neel temperatures ( T N ) by X-ray diffraction using single crystals. At room temperature (above T N ) the structures of these compounds are of the ideal perovskite type (cubic) except for that of KCuF 3 Which crystallizes as a tetragonal modification ( a > c ) of the perovskite type. At 78°K (below T N ) the lattice symmetries of KMnF 3 , KFeF 3 and KCoF 3 are monoclinic, rhombohedral (α c ), respectively, while KNiF 3 and KCuF 3 retain their own symmetries at room temperature. The lattice constants have been determined between these temperatures and their changes near the transition temperature were closely followed. This temperature region was extended up to 670°K for KCuF 3 .

The Crystal Structure of Germanium Selenide GeSe

Abstract: The crystal structure of germanium selenide has been analysed by X-ray method. The structure is orthorhombic with the space group D 2 h 16 - Pcmn . The dimensions of the unit cell containing four chemical units, GeSe, are a =4.38, b =3.82 and c =10.79 A. It follows from the positions of all atoms determined by means of the electron-density projection that the structure is of a distorted NaCl type isomorphous with SnS and SnSe, The distances Ge–Se are 2.54, 2.58, 3.30 and 3.39 A.

The Crystal Structure of KCuF3

Abstract: The crystal structure of potassium trifluorocuprate (II) KCuF 3 has been determined by an X-ray analysis. The structure was refined by Fourier method. The crystals are tetragonal, \(a{=}\sqrt{2}a_{0}{=}5.855\) and c =2 c 0 =7.852A; space group D 4 h 18 – I 4/ mcm , with four formula units (KCuF 3 ) in the unit cell, where a 0 and c 0 designate the lattice constants of the fundamental pseudo-perovskite structure. This superstructure is due to a displacement of fluorine ions along the copper-fluorine-copper bonds only in the c plane. The atoms are in the following positions: 4 K + in ( a ): (0, 0, 0; \(\frac{1}{2}\), \(\frac{1}{2}\), \(\frac{1}{2}\))+(0, 0, \(\frac{1}{4}\); 0, 0, \(\frac{3}{4}\)): 4 Cu 2+ in ( d ): (0, 0, 0; \(\frac{1}{2}\), \(\frac{1}{2}\), \(\frac{1}{2}\))+(0, \(\frac{1}{2}\), 0; \(\frac{1}{2}\), 0, 0): 4 F - in ( b ): (0, 0, 0; \(\frac{1}{2}\), \(\frac{1}{2}\), \(\frac{1}{2}\))+(0, \(\frac{1}{2}\), \(\frac{1}{4}\), \(\frac{1}{2}\), 0, \(\frac{1}{4}\)): 8 F - in ( h ): (0, 0, 0; \(\frac{1...

Structural Study of Iron Selenides FeSe x . I Ordered Arrangement of Defects of Fe Atoms

Abstract: The ordered arrangements of the defects of iron atoms in FeSe x have been studied for several compositions by X-ray analysis. These defects tend to assume an ordered arrangement as in the case of pyrrhotite FeS 1.14 . New phases are found in the region ( x =8/7∼4/3) which have been considered as the single phase region. If we choose the orthohexagonal and pseudo-orthohexagonal unit cells for x =8/7 and 4/3, respectively, the unit cell dimensions of the superstructure resulting from this ordering are twice as large as that of the fundamental structure along the a - and b -axes, and three times along the c -axis for x =8/7, and on the other hand, twice only along the c -axis for x =4/3.

Experiments on simple magnetic model systems

Abstract: “…. For the truth of the conclusions of physical science, observation is the supreme Court of Appeal….” (Sir Arthur Eddington, The Philosophy of Physical Science.) In this paper we shall review the theoretical and experimental results obtained on simple magnetic model systems. We shall consider the Heisenberg, XY and Ising type of interaction (ferro and antiferromagnetic), on magnetic lattices of dimensionality 1, 2 and 3. Particular attention will be paid to the approximation of these model systems in real crystals, viz. how they can be realized or be expected to exist in nature. A large number of magnetic compounds which, according to the available experimental information, meet the requirements set by one or the other of the various models are considered and their properties discussed. Many examples will be given that demonstrate to what extent experiments on simple magnetic systems support theoretical descriptions of magnetic ordering phenomena and contribute to their understanding. It will a...

Co-operative Jahn-Teller effects

Abstract: The co-operative Jahn-Teller effect is a phase transition which is driven by the interaction between localized orbital electronic states and the crystal lattice. The possible origins, symmetries and properties of the electron-lattice interactions and how they lead to possible hamiltonians for the coupled system are discussed. The relation of these interactions with quadrupolar interactions and magnetostriction is included. The methods of solution of the hamiltonians lead to an understanding of the electronic states, the phonon spectrum and the mixed normal modes. The wide variety of experimental techniques used on this problem are reviewed in detail and the results are compared with theoretical expressions whenever possible. The application of external stress and magnetic field is of particular significance in the case of the rare earth compounds, because they can product effects which are larger than the low-temperature spontaneous effects.

Electronic and ionic transport properties and other physical aspects of perovskites

Abstract: The perovskites and perovskite-related structures exhibit several features of technical as well as fundamental interest. Technically useful properties include oxide-ion conduction with/without electronic conduction, oxidation catalysis, ferroic displacements in classic and relaxor ferroelectrics, half-metallic ferromagnetism and high-temperature superconductivity. Of more fundamental interest is the ability to tune, by chemical substitution on the large-cation subarray, transition-metal oxides through the crossover on the transition-metal array from localized dn configurations to itinerant d-electron behaviour without/with changing the valence state of that array. The localized-electron configurations may exhibit cooperative Jahn–Teller distortions that introduce anisotropic exchange interactions. At crossover, bond-length fluctuations may segregate into an ordered array of alternating covalent and ionic bonding in a single-valent perovskite; multicentre polarons or correlation bags may replace small polarons in a mixed-valent system. Bond-length fluctuations at crossover give vibronic conduction and suppression of the phonon contribution to the thermal conductivity; the fluctuations may order, to give high-temperature superconductivity, or transform to quantum–critical-point behaviour at lowest temperatures. Crossover of σ-bonding electrons in the presence of localized spins associated with π-bonding electrons gives rise to the colossal magnetoresistance phenomenon above a ferromagnetic Curie temperature.

A topological insulator surface under strong Coulomb, magnetic and disorder perturbations

Abstract: Topological insulators embody a state of bulk matter characterized by spin-momentum-locked surface states that span the bulk bandgap. This highly unusual surface spin environment provides a rich ground for uncovering new phenomena. Understanding the response of a topological surface to strong Coulomb perturbations represents a frontier in discovering the interacting and emergent many-body physics of topological surfaces. Here we present the first controlled study of topological insulator surfaces under Coulomb and magnetic perturbations. We have used time-resolved deposition of iron, with a large Coulomb charge and significant magnetic moment, to systematically modify the topological spin structure of the Bi_2Se_3 surface. We observe that such perturbation leads to the creation of odd multiples of Dirac fermions and that magnetic interactions break time-reversal symmetry in the presence of band hybridizations. We present a theoretical model to account for the observed electron dynamics of the topological surface. Taken collectively, these results are a critical guide in controlling electron mobility and quantum behaviour of topological surfaces, not only for device applications but also in setting the stage for creating exotic particles such as axions or imaging monopoles on the surface.