K. Hiraki

Synthesis, Structural Transformation, Thermal Stability, Valence State, and Magnetic and Electronic Properties of PbNiO3 with Perovskite- and LiNbO3-Type Structures

We synthesized two high-pressure polymorphs PbNiO(3) with different structures, a perovskite-type and a LiNbO(3)-type structure, and investigated their formation behavior, detailed structure, structural transformation, thermal stability, valence state of cations, and magnetic and electronic properties. A perovskite-type PbNiO(3) synthesized at 800 °C under a pressure of 3 GPa crystallizes as an orthorhombic GdFeO(3)-type structure with a space group Pnma. The reaction under high pressure was monitored by an in situ energy dispersive X-ray diffraction experiment, which revealed that a perovskit-type phase was formed even at 400 °C under 3 GPa. The obtained perovskite-type phase irreversibly transforms to a LiNbO(3)-type phase with an acentric space group R3c by heat treatment at ambient pressure. The Rietveld structural refinement using synchrotron X-ray diffraction data and the XPS measurement for both the perovskite- and the LiNbO(3)-type phases reveal that both phases possess the valence state of Pb(4+)Ni(2+)O(3). Perovskite-type PbNiO(3) is the first example of the Pb(4+)M(2+)O(3) series, and the first example of the perovskite containing a tetravalent A-site cation without lone pair electrons. The magnetic susceptibility measurement shows that the perovskite- and LiNbO(3)-type PbNiO(3) undergo antiferromagnetic transition at 225 and 205 K, respectively. Both the perovskite- and LiNbO(3)-type phases exhibit semiconducting behavior.

High-Pressure Synthesis and Correlation between Structure, Magnetic, and Dielectric Properties in LiNbO3-Type MnMO3 (M = Ti, Sn)

LiNbO(3)-type MnMO(3) (M = Ti, Sn) were synthesized under high pressure and temperature; their structures and magnetic, dielectric, and thermal properties were investigated; and their relationships were discussed. Optical second harmonic generation and synchrotron powder X-ray diffraction measurements revealed that both of the compounds possess a polar LiNbO(3)-type structure at room temperature. Weak ferromagnetism due to canted antiferromagnetic interaction was observed at 25 and 50 K for MnTiO(3) and MnSnO(3), respectively. Anomalies in the dielectric permittivity were observed at the weak ferromagnetic transition temperature for both the compounds, indicating the correlation between magnetic and dielectric properties. These results indicate that LiNbO(3)-type compounds with magnetic cations are new candidates for multiferroic materials.

Charge ordering in θ-(BEDT-TTF)2RbZn(SCN)4

The electronic state of θ-(BEDT-TTF) 2 RbZn(SCN)4 has been investigated by the precise analyses of the angular dependence of 13 C-NMR lineshape. It is found that the metal-insulator transition observed at 190 K at ambient pressure is of a novel type accompanied with a charge re-distribution in the organic layer. In the insulating state, the electric charges order as a stripe along the a-axes and the charge rich and poor sites alternate along the c-axes. The spin-singlet phase below 30 K is recognized as a spin-Peielrs transition of the 1D spin system.

77Se NMR Evidence for the Jaccarino-Peter Mechanism in the Field Induced Superconductor, λ-(BETS)2FeCl4

We performed 77 Se NMR on a single crystal sample of the field-induced superconductor λ-(BETS) 2 FeCl 4 . Our result obtained in the paramagnetic state provide a microscopic insight on the exchange interaction J between the spins s of the BETS π conduction electrons and the Fe localized d spins S . The absolute value of the Knight shift K decreases when the polarization of the Fe spins increases. This reflects the “negative” spin polarization of the π electrons through the exchange interaction J . The value of J has been estimated from the temperature and the magnetic field dependence of K and found to be in good agreement with that deduced from transport measurements [Balicas et al. : Phys. Rev. Lett. 87 (2001) 067002]. This provides direct microscopic evidence that the field-induced superconductivity is due to the compensation effect predicted by Jaccarino and Peter [Phys. Rev. Lett. 9 (1962) 290]. Furthermore, an anomalous broadening of the NMR line has been observed at low temperatures, which suggests...

Magnetism of DCNQI-Cu salts

Abstract Magnetic properties of (DMe-DCNQI)2Cu, (DMeO-DCNQI)2Cu and (DBr-DCNQI)2Cu were investigated by 1H-NMR, ESR and magnetic susceptibility measurements. 1H-T1−1 was found to exhibit a step-like enhancement at the metal-insulator (M-I) transitions of DBr-salt at ambient pressure and of DMe- and DMeO-salts under pressure. This reflects the appearance of localized Cu2+ spins at the M-I transition. An antiferromagnetic (AF) ordering was observed in DBr- (at ambient pressure) and DMe-salts (under pressure). In both salts, 1H-NMR relaxation rates, T 1 −1 , T 2 ∗−1 and T 2 −1 , indicate the onset of inhomogeneous local field at the transition temperature, TN: The lineshape below TN is well explained by the assumption that the magnetic Cu2+-sites with μB form a regular three-fold lattice along the c-axis. In the DMe-salt, the pressure dependence of TN was determined. It was shown that the enhancement of static susceptibility in the metallic state of DMe-salt was due to the insulating domain induced by some stress within the sample.

Charge ordering in the one dimensional half-filled band system, (TTM-TTP)I3

13 C-NMR measurements have been carried out to investigate the nature of the electronic properties of the one-dimensional half-filled band system, (TTM-TTP)I 3 . It is known that this compound is metallic down to 160K. The metal-insulator transition into a non-magnetic ground state was confirmed from the temperature dependence of NMR relaxation rate. In addition, it was found that 13 C-NMR spectra split into two in the insulating state. This is a evidence of charge disproportionation among the organic molecules. It may be considered as a typical CDW formation in a 1D half-filled system with large intersite Coulomb repulsion.

X-ray structural study of 4kF superlattice in (DIDCNQI)2Ag

Abstract X-ray diffuse scattering study using monochromatic camera revealed c * /2 satellite reflections indicating three dimensional 4 k F CDW formation in quasi one-dimensional compound (DIDCNQI) 2 Ag below 200 K.

Variation of electronic phases in (R1R2DCNQI)2Ag: systematic change of the one-dimensional band by chemical substitution of R1 and R2

Abstract The width and dimensionality of the quasi-one dimensional electronic band of (R 1 ,R 2 -DCNQI) 2 Ag are controlled by substitution of R 1 /R 2 (R 1 /R 2 =Me/Me, Br/Br, Br/I, I/I). Electrical resistivity, magnetic susceptibility, and NMR measurements have been performed to characterize the spin/charge states in this family. We observed a systematic change from the well-known spin-Peierls state in the Me/Me salt to the spin gapless Wigner crystal state in the I/I salt corresponding to the band parameters. Substitution of R1 and R2 in (R 1 ,R 2 -DCNQI) 2 Ag successfully controls the interchain and intrachain transfer integrals of the quarter-filled π band.

High-Pressure Synthesis, Structure, Dielectric and Magnetic Properties for SrCu3Ti4O12

An A-site ordered perovskite oxide SrCu3Ti4O12 was synthesized under high pressure and temperature conditions. The sample was characterized by the synchrotron X-ray diffraction, dielectric and magnetic susceptibility measurements. The synchrotron X-ray Rietveld refinement revealed that the strontium ion fully occupied on the A-site, while the copper ion at the A′-site was slightly deficient. The dielectric constant was about 160 at 1 kHz at room temperature. The anti-ferromagnetic (AFM) order with a Néel-temperature TN of at 24.5 K was observed. The ionic radii of the A-site ion had little influence on AFM indirect exchange interaction between Cu2+ ions in ACu3Ti4O12.

Giant Shubnikov-de Haas oscillation and the new metallic state in the organic τ-type conductors

Two-dimensional organic T-type conductors show metallic temperature dependence of resistivity down to T min = 30-50 K, below which a resistivity upturn is observed. A large negative magnetoresistance appears below T min suddenly in temperature. Below about 4 K, magnetoresistance in any direction is extremely hysteretic in field-, temperature- or field-angle-sweep, and p⊥(H//plane) shows switching between two- and four-fold depending on magnetic and thermal history, indicating magnetic domain generation. Static susceptibility shows slight ferromagnetic component. NMR is in favor of anti-ferromagnetism below about 15 K. These two results imply that the system is a weak ferromagnetic one. Preliminary optical reflectance and specific heat studies implied metallic state at low temperature, but no clear-cut proof has been shown whether or not the ground state is metallic. This report presents giant Shubnikov de Haas (SdH) oscillation in both τ-(EDO-S,S-DMEDT-TTF) 2 (AuBr 2 ) 1+y , and τ-(P-S,S-DMEDT-TTF) 2 (AuBr 2 ) 1+ν , (y ∼ 0.75), which is a clear evidence for the metallic ground state. However, the observed Fermi surfaces (FS) are 0.65 and 6.1% of the first Brillouin zone for T-(EDO-S,S-DMEDT-TTF) 2 (AuBr 2 ) 1+ν , and 2.3 and 6.9% for τ-(P-S,S-DMEDT-TTF) 2 (AuBr 2 ) 1+y , (y ∼ 0.75), which suggest extremely small FS pockets, and were not expected by the previous band calculation. Further, it is surprising that the Dingle temperature was around 1 K, indicating unexpectedly clean, while residual resistivity ratio is about 1. We note that the fundamental oscillation of 0.65% is already n = 2 state, which is already next to the Landau quantum limit.