Seiichi Kagoshima

X-Ray Scattering Study of Phonon Anomalies and Superstructures in TTF-TCNQ

By the X-ray scattering measurements of TTF-TCNQ, two anomalies at q b =0.29 b * and 0.41 b * are studied, where q b is the component of the wave vectors of phonons and superstructures along the b * -axis. The anomaly at 0.29 b * appears in the TA phonon mode below 150 K and becomes the superstructure below 54 K. The anomaly at 0.41 b * appears in the LA phonon mode below room temperature and it also forms the superstructure below 54 K. It is concluded that the wave vector of the modulation of the superstructure at 0.41 b * (which is equivalent to 0.59 b * in the reduced zone scheme) is twice the wave vector of that at 0.29 b * (including the temperature dependent a * -component). Further, it was found that the wave number of the phonon anomaly at 0.41 b * varies from 0.41 b * to 0.45 b * with increasing temperature from 150 K to room temperature.

Semiclassical Interpretation of the Angular-Dependent Oscillatory Magnetoresistance in Quasi-Two-Dimensional Systems

In an attempt to understand angular-dependent oscillatory magnetoresistance phenomena recently discovered in organic conductors, calculations of magnetoresistance in quasi-two-dimensional systems were carried out in the framework of the Boltzmann transport theory. Calculated magnetoresistance curves show the angular-dependent oscillations reminiscent of those found experimentally. It is argued that the essential physics underlying the resistance oscillations lies in the angular dependence of the high field asymptotic behavior (saturation vs divergence) of semiclassical magnetoresistance which arises from Fermi surface topology.

High Transition Temperature Superconductor: Y-Ba-Cu Oxide

New material with high temperature superconductivity is discovered. This material is an oxide of the composition of yttrium, barium and copper. The observation of the superconductivity begins at 85K and completely vanishing resistance occurs at 45 K.

Crystal structure analysis under uniaxial strain at low temperature using a unique design of four-axis x-ray diffractometer with a fixed sample

For the purpose of crystal structure analysis under uniaxial strain at low temperatures, we developed a pressure cell for uniaxial compression and a unique design of an x-ray diffractometer wherein both the x-ray source and the detector are capable of two-axial rotation with a fixed sample. This arrangement is advantageous to crystal structure analyses under extreme conditions that require a large and heavy apparatus. Using the present diffractometer, we performed the crystal structure analyses of the organic conductor, α-(BEDT–TTF)2I3 (BEDT–TTF denotes bis(ethylene)dithio-tetrathiafulvalene), under uniaxial strain and ambient pressure, and at room and low temperatures, and obtained results that were qualitatively consistent with those of resistivity measurements.

Recent Topics of Organic Superconductors

Recent developments in research into superconductivity in organic materials are reviewed. In the epoch-defining quasi-one-dimensional TMTSF superconductors with T c ∼1 K, T c decreases monotonically with increasing pressure, as do signatures of spin fluctuations in the normal state, providing good evidence for magnetically-mediated pairing. Upper critical fields exceed the Zeeman-limiting field by several times, suggesting triplet pairing or a transition to an inhomogeneous superconducting state at high magnetic fields, while triplet pairing is ruled out at low fields by NMR Knight-shift measurements. Evidence for a spatially inhomogeneous superconducting state, Fulde–Ferrel–Larkin–Ovchinnikov state, which has long been sought in various superconducting systems, is now captured by thermodynamic and transport measurements for clean and highly two-dimensional BEDT-TTF and BETS superconductors. Some of the layered superconductors also serve as model systems for Mott physics on anisotropic triangular lattice....

Quenching effect of the anion ordering in the organic superconductor (TMTSF)2ClO4: An X-ray study

Abstract In the title compound it is found directly by an X-ray diffuse scattering study that the orientational ordering of ClO4 anions is suppressed when a sample is quenched from higher temperature to 15K. Also the lattice parameter is found to be frozen on quenching. Possible interrelation between these phenomena and the onset of superconductivity is discussed briefly.

X-ray evidence for structural changes in the organic conductors, α-(BEDT-TTF)2I3, α-(BEDT-TTF)2IBr2 and ß-(BEDT-TTF)2I3

Abstract Interrelations are investigated between the electrical and structural properties of organic conductors, BEDT-TTF (bis(ethylenedithiolo)tetrathiafulvalene) trihalides. In α-)BEDT-TTF)2I3 evidence is obtained by X-ray diffraction for BEDT-TTF molecular dimerization at the metal-insulator transition temperature of 135 K. Another BEDT-TTF molecular dimerization is found in α-(BEDT-TTF)2IBr2, which has semiconducting properties and an activation energy that increases with decreasing temperature. These dimerizations are considered to be responsible for the changes in the electrical properties. Superconducting s-(BEDT-TTF)2I3 is found to have a superstructure below 175 K, above which short-range fluctuations are observed up to about 200 K.

Electronic states and anti-ferromagnetic order in mixed-stack charge-transfer compound (BEDT-TTF)(F2TCNQ)

Abstract Crystal structure, optical and magnetic properties are reported for a unique class of donor-acceptor charge-transfer (CT) complex (BEDT-TTF)(F 2 TCNQ). Charge-transfer from the donor to the acceptor is complete, forming an ionic mixed-stack CT crystal. “Side-by-side” interaction between BEDT-TTF molecules results in a one-dimensional (1D) correlated electronic system that is perpendicular to the donor-acceptor mixed-stack column. Consequently, F 2 TCNQ radical spins are strongly isolated from neighboring molecules, showing Curie-like behavior in magnetic susceptibility, followed by an anti-ferromagnetic order at around T N = 30 K.

X-ray evidence for a deformation of CDW during the sliding motion in K0.30MoO3

Abstract The wave vector and the correlation length of the CDW ordering in K 0.30 MoO 3 are studied by x-ray diffraction as functions of electric fields applied along the one-dimensional axis. Several samples change their CDW-ordering above a threshold field. The change of the wave vector occurs mainly in the (2a ∗ -c ∗ ) - direction, which is perpendicular to the one-dimensional b ∗ - axis. It depends also on the field direction and magnitude. Time resolved x-ray experiments show that characteristic time of this change is of the order of 1 ms. The correlation length becomes longer along the (2a ∗ -c ∗ )-direction in the transient period.

Structural Phase Transition in β-MxV2O5 (M=Na, Li)

Satellite reflections, which indicate the occurrence of the structural phase transition, were found in β-Na 0.33 V 2 O 5 and β-Li 0.36 V 2 O 5 by X-ray diffraction measurements. In the case of Na 0.33 V 2 O 5 , the satellite with the wave vector of (0, 0.5, 0) grows below 200 K. In the case of Li 0.36 V 2 O 5 , there are three types of satellites; (0, 0.53, 0), (0, 0.57, 0.125), (0, 0.5, 0.5). In both cases of Na 0.33 V 2 O 5 and Li 0.36 V 2 O 5 , the b * component of the wave vector of the superstructure may be explained by the orclering of V 4+ -V 4+ pairs.