Noriaki Hanasaki

Variable magnetotransport properties in the TPP[Fe(Pc)L2]2 system (TPP = tetraphenylphosphonium, Pc = phthalocyaninato, L = CN, Cl, and Br)

A series of one-dimensional conductors, TPP[FeIII(Pc)L2]2 with L = CN, Cl, and Br, have been prepared. These compounds show giant negative magnetoresistance due to the interaction between conduction π-electrons and localized Fe-centered d-spins, but the magnitude is found to vary based on the axial ligand, suggesting that the strength of the π–d interactions is sensitively influenced by the axial substituent. The magnetic susceptibility also shows the difference in the magnetic interactions by the axial substituent. The ab initio calculation of the isolated molecules has revealed that the Jahn–Teller effect in this system induces splitting of dxz and dyz orbitals, and that the energy shift of the d-orbital accommodating an unpaired electron is varied by the axial substituent. The axial-ligand dependence of the intramolecular π–d interaction is found to correlate with the magnitude of the magnetoresistance.

Field-Induced Discommensuration in Charge Density Waves in o-TaS3

We report a synchrotron X-ray study of charge density waves (CDWs) in an o-TaS3 crystal. We found that two independent CDWs coexist in the temperature range of 130 - 50 K. These waves are incommensurate and commensurate CDWs with longitudinal wave vectors qc ¼ 0:252cand 0:250c � , respectively. The temperature and electric current dependences of the intensity of the two CDW satellites were measured. We found that the commensurate CDW was converted to the incommensurate CDW at 80 K by inducing current flow. Our observation was interpreted in terms of the dynamics of topological defects. We determined the edge dislocation configuration from the electric current dependence of the intensity of the two CDWs. The result implies for the first time that discommensurations are induced in the commensurate CDW by applying an electric field.

Stoichiometric Study of the Dielectric and Magnetic Properties in Charge Frustrated System LuFe2O4

We report dielectric and magnetic properties of a single crystal of LuFe 2 O 4 with different degrees of oxygen deficiency. The oxygen stoichiometry was monitored with the gas mixture ratio of CO 2 and CO during the crystal growing process. A sample having high resistivity of about 10 kΩ cm sample was found at room temperature. The relation between the control of the charge frustration and the completeness of the charge ordering is discussed.

Charge Order Competition Leading to Nonlinearity in Organic Thyristor Family

Simultaneous synchrotron radiation and transport measurements for the organic thyristor family compound θ-(BEDT-TTF) 2 RbZn(SCN) 4 have revealed the presence of microscopic competing domains between the twofold and 3 ×4 charge orderings in a triangular lattice. In contrast to conventional correlated electron systems, charge ordering is a unique critical parameter of phase competition. With external current, only the twofold ordering melts, which is responsible for the emergence of a nonlinear response.

Giant magnetoresistance response by the π–d interaction in an axially ligated phthalocyanine conductor with two-dimensional π–π stacking structure

Electrochemical oxidation of [MIII(Pc)(CN)2]− (M = Co and Fe, Pc = phthalocyaninato) and peri-xanthenoxanthene (PXX) in acetonitrile yields partially oxidized salts of [PXX]2[M(Pc)(CN)2]·CH3CN. In the crystal, the Pc ring is formally oxidized by 0.5 e and forms a two-dimensional double-sheet layer. These crystals show semiconducting behavior with small activation energy under ambient pressure, but show metallic behavior under high pressures. When M = Co, the metallic behavior is maintained down to 5 K. On the other hand, in the π–d system with M = Fe, the metallic behavior is disrupted by a steep increase in the resistivity at low temperatures. In this high resistivity state, negative differential resistance appears in the measurements of current–voltage characteristics, suggesting the development of charge disproportionation due to the π–d interaction. The magnetic susceptibility for M = Fe reflects anisotropic molecular magnetism of the Fe(Pc)(CN)2 unit, and shows an anomaly due to antiferromagnetic interaction between the Fe d-spins. The increase in the resistivity at low temperature under high pressures is suppressed by the application of a magnetic field. The magnitude of the negative magnetoresistance effect is extremely large; −99.8% was recorded at a field of 15 T at 3 K under 12 kbar.

Nonlinear Transport Phenomena in Highly One-Dimensional MIII(Pc)(CN)2 Chains with π–d Interaction (M = Co and Fe and Pc = Phthalocyaninato)

The charge transport properties of a partially oxidized salt composed of Co(Pc)(CN) 2 units with a typical one-dimensional electronic system have been suggested to be determined by charge disproportionation. The current–voltage ( I – V ) characteristics show nonlinear behavior at low temperature, which is suppressed by applying pressure. The observed nonlinearity is considered to result from the electric-field-induced delocalization of carriers in the charge disproportionation state. In the isomorphous magnetic Fe(Pc)(CN) 2 system, the nonlinear behavior is observed at higher temperatures, suggesting that charge disproportionation is more developed in a system with local moments. The threshold voltage of negative differential resistance decreases when an external magnetic field is applied, confirming that the localization of charge carriers is released by magnetic field application.

Successive Transition in Rare-earth Intermetallic Compound GdNiC2

In GdNiC2, the magnetic state is investigated under the high magnetic fields and the low temperatures. We find the intermediate phases, in which the magnetization value is close to a half of the full moment, and the successive transition in the phase boundary between the antiferromagnetic, the metamagenetic, and the paramagnetic phases. The mechanism of the intermediate phase is discussed.

Magneto-electric effect in charge-ordered LuFe2O4

A magneto-electric (ME) effect was observed in a charge-ordered ferroelectric LuFe2O4. The dielectric response in a three-dimensional charge-ordered phase is strongly influenced by magnetic field, especially around the ferri-magnetic ordering temperature. We also show the relation between the oxygen stoichiometry and the ME signal character above the magnetic transition temperature.

Hydrothermal Synthesis of Hexagonal Vanadium Oxide

We synthesize the hexagonal vanadium oxide containing the alkaline-metal ion (Li, Na, and K). The crystal belongs to the magnetoplumbite-type structure. The title compound, in which the vanadium valance is not an integer, shows the metallic and the paramagnetic behavior. The electronic specific heat coefficient γ is 34.7 mJ/K2mol, which is perhaps due to the spin fluctuation.

X-ray Absorption Spectra in Pyrochlore Niobates

The X-ray absorption spectra reveal that the valence of the Nb ion, and the displacement of the Nb ions from the center of the oxygen octahedra in the pyrochlore niobates YCaNb2O7 and NdCaNb2O7, which is consistent with the previous report of the neutron diffraction.