S. Katano

Crystal and magnetic structure of Ca 3 Ru 2 O 7

The crystal structure of the double-layered Ca{sub 3}Ru{sub 2}O{sub 7} has been studied by convergent beam electron diffraction and powder neutron diffraction. The temperature dependence of the diffraction pattern reveals that all the lattice constants jump at the first-order metal-nonmetal transition at 48 K without a change of the space group symmetry of Bb2{sub 1}m. In the neutron diffraction experiment, an additional magnetic reflection emerges below the Neel temperature, 56 K. A possible model for this antiferromagnetic ordering is proposed, in which model magnetic moments align ferromagnetically within the double layer and antiferromagnetically between the double layers. This model reasonably explains the characteristic field dependence of the magnetoresistance observed at around 6 T.

Soft modes and phonon interactions in studied by means of neutron scattering

This paper presents an inelastic neutron scattering study of the proper ferroelectric and elastic neutron scattering results on the satellite diffraction pattern which characterizes the modulated phase. The temperature dependences of the satellite intensities and modulation wavevector are in fair agreement with results from previous x-ray experiments. Close to the incommensurate-to-ferroelectric transition temperature , an unexpected intensity overshoot is observed, similar to that seen in birefringence and dilatation experiments. The relationship between the lattice dynamics and the observed phase transition sequence is examined. The dispersion of the ferroelectric soft optic phonon (-polarization) and of the acoustic phonons is followed along the and -directions. In the ferroelectric phase, the TO mode shows a considerable softening as the incommensurate phase is approached from below. In the paraelectric and incommensurate phases, the response from the TO (-polarization) and TA ( strain) branches has been investigated via a series of constant-q scans in the -direction (approximately the modulation wavevector direction). The combined inelastic line-shapes, as observed in a number of non-equivalent Brillouin zones, could all be analysed in terms of a coupled-mode damped harmonic oscillator model. In addition, a diverging, resolution-limited, central peak is observed close to . It is suggested that the TO-TA coupling lies at the origin of the incommensurate instability. A phenomenological free energy is developed, in the continuum approximation, in which the TO-TA interaction is included via a pseudo-Lifshitz term of the type .

Crystal structure and critical temperature of RBa2Cu4O8 (R = Tm, Er, Ho, Y, Dy and Gd)

Abstract Bulk superconductors RBa 2 Cu 4 O 8 (R = rare earth element; Tm, Er, Ho, Y, Dy and Gd) were synthesized by hot isostatic pressing (HIP) technique. The highest T c ( = 84 K) was obtained for the substitution of Er. The T c decreases systematically with the increase of the ionic radius of these rare earth ions, and reaches 77 K for the largest ion Gd. The crystal structure of the compounds with R = Tm, Er, Ho and Y has been refined by Rietveld analysis of neutron powder diffraction data at room temperature. Structural changes due to the substitution of these rare earth elements are compared with those of YBa 2 Cu 4 O 8 under high pressure, and they are discussed in connection with the variation of T c . The results indicate that l (Cu(2)-O(2)) and l (Cu(2)-O(3)), the bond lengths between Cu and O in the CuO 2 plane, are the most important parameters to decide the T c of the present systems.

Synthesis and superconducting properties of (Y1−xPrx)Ba2Cu4O8 and (Y1−xPrx)2Ba4Cu7O15−y compounds

Abstract Both (Y1−xPrx)Ba2Cu4O8 (hereafter abbreviated as Y/Pr124) and (Y1−xPrx)2Ba4Cu7O15−y (Y/Pr247) compounds were successfully synthesized over the whole concentration range 0≤x≤1 by means of either hot isostatic pressing (HIP) technique or high pressure oxygen gas technique. We revealed that the superconducting transition temperature disappears at xc=0.8 in both systems in contrast to xc=0.5 for the (Y1−xPrx)Ba2Cu3O7 (Y/Pr123) system reported in the literature and concluded from this that the suppression of superconductivity due to the Pr-doping is weaker in both 124 and 247 systems than that in the 123 system. Furthermore, a combination of the magnetic susceptibility data in the normal state with the Pr–O distance derived from the neutron diffraction experiments led us to conclude that the Pr ion is in the trivalent state for all samples in both 124 and 247 systems. An increase in the critical concentration xc in the Y/Pr124 and Y/Pr247 systems relative to that for the Y/Pr123 system is attributed to an increase in the hybridization of the Pr-4f and O-2p orbitals.

Incommensurate charge ordering in mixed valence system LuFe2O4

Abstract Charge ordering of a mixed valence system LuFe2O4 has been studied by neutron scattering technique, focusing attention on the charge frustration effect on triangular lattice. It is revealed that the system undergoes successive phase transitions following the sequence of disorder → two-dimensional charge density wave state (2D-CDW) → three-dimensional charged density wave state (3D-CDW). In 2D-CDW state, the charge density wave defined by the wave vector ( 1 3 1 3 ) is formed in the hexagonal layer.

Magnetic field effect on the static antiferromagnetism of the electron-doped superconductor Pr1-xLaCexCuO4 (x = 0.11 and 0.15)

Effects of magnetic fields (applied along the c axis) on static spin correlation were studied for the electron-doped superconductors Pr1-xLaCexCuO4 with x=0.11 (T(c)=25 K) and x=0.15 (T(c)=16 K) by neutron-scattering measurements. In the x=0.11 sample, which is located near the antiferromagnetic (AF) and superconducting phase boundary, a commensurate magnetic order develops below around T(c) at zero field. Upon applying a magnetic field up to 9 T both the magnetic intensity and the onset temperature of the order increase with the maximum field effect at approximately 5 T. In contrast, in the overdoped x=0.15 sample any static AF order is neither observed at zero field nor induced by the field up to 8.5 T. Difference and similarity of the field effect between the hole- and electron-doped high-T(c) cuprates are discussed.

Crystal structure and superconductivity of La2−xBaxCuO4 (0.03≤x≤0.24)

Abstract High-resolution neutron powder diffraction has been performed to investigate the crystal structure of La 2− x Ba x CuO 4 as a function of x (0.03≤ x ≤0.24). The crystal parameters at 115 K refined by profile analysis show that the Cu-planar O(1) bond lengths decrease by 0.008 A when x is increased by 0.1. This result indicates that the overlap of Cu and planar O orbitals, and thus the charge transfer, increases with x . The length of the Cu-apical O(2) bonds decreases at a rate of about one half of that for Cu-O(1), suggesting that the apical oxygens O(2) also play an important role in the charge transfer process. The changes of the Cu-O(1) bond lengths of the compound with x =0.125 at 15 K are further studied in connection with the structural phase transition to the low-temperature tetragonal (LTT) phase. These results are discussed in terms of their implication for the electronic structure and superconductivity of this system.

Temperature dependence of order parameters in the antiferroelectric phase of PbZrO3

Abstract Powder X-ray diffraction experiments on the antiferroelectric phase of PbZrO3 were performed between room temperature and 85K. The crystal structure at 100K was also analyzed by the neutron Rietveld method. Temperature dependence of the antiferroelectric superlattice-reflection intensities can be described well by a phenomenological theory for the first-order phase transition. The tetragonal lattice distortion of the perovskite sublattice changes in proportion to the temperature dependence of these intensities. The Pb shifts show the temperature dependence that is proportional to the square root of the intensities. The Pb shifts and the lattice distortion show no anomalies at around 100K, in contrast to the result recently reported in X-ray structure analysis at 100K. The temperature dependence of the birefringence in the ab plane was found to be expressed reasonably by taking the displacements associated with the R25 mode, another order parameter, into account with the phenomenological theory.

Structural phase transition of Sr2CuO2(CO3)

Abstract The structural phase transition of Sr2CuO2(CO3) has been studied by X-ray and neutron powder diffraction and phonon Raman scattering. The crystal structure at room temperature is tetragonal with space group of I 4 (S42) and the lattice parameters a = 7.8045(1) A and c = 14.993(1) A . The structural phase-transition temperature determined by X-ray powder diffraction is found at 490 K. The structure above this transition temperature is tetragonal P4212 (D42), and the lattice parameters are a = 5.54364(8) A and c = 7.53823(11) A at 573 K. The neutron-diffraction data are refined by a Rietveld analysis and these structural parameters for below and above the transition temperature are presented. Raman spectra of Sr2CuO2(CO3) have been measured in the temperature region between 5 K and 593 K. In the temperature region between 5 K and 483 K, a new extra mode was not observed, but the peak intensity of the strongest mode at 480 cm−1 was decreased from room temperature to 373 K. Above 490 K, a new peak has been observed at 160 cm−1 and the weak peaks which are observed at room temperature disappear. Judging from the facts that the spectral shape and line width originating from the CO3 slab change at the transition temperature but a soft mode has not been observed, the type of this phase transition is order-disorder due to the change of the CO3 alignments.

Absence of a magnetic-field effect on static magnetic order in the electron-doped superconductor Nd 1.86 Ce 0.14 CuO 4

Neutron-scattering experiments were performed to study the magnetic-field effect on the electron-doped cuprate superconductor