K. Hiraki

Charge ordering in α-(BEDT-TTF)2I3

13C-NMR measurements were performed to the quasi two-dimensional organic conductor α-(BEDT-TTF) 2 I 3 , which exhibits metal-insulator transition at 135 K (=T MI ) at ambient pressure. The relaxation rate T 1 -1 has two components below T MI indicating two nonequivalent sites. T-dependence of the lineshape clearly indicates that there exist two differently charged BEDT-TTF molecules below T MI .

Integer spin-chain antiferromagnetism of the 4doxide CaRuO3with post-perovskite structure

A quasi-one dimensional magnetism was discovered in the post-perovskite CaRuO3 (Ru4+: 4d4, Cmcm), which is iso-compositional with the perovskite CaRuO3 (Pbnm). An antiferromagnetic spin-chain function with -J/kB = 350 K well reproduces the experimental curve of the magnetic susceptibility vs. temperature, suggesting long-range antiferromagnetic correlations. The anisotropic magnetism is probably owing to the dyz - 2p- dzx and dzx - 2p- dyz superexchange bonds along a-axis. The Sommerfeld coefficient of the specific heat is fairly small, 0.16(2) mJ mol-1 K-2, indicating that the magnetism reflects localized nature of the 4d electrons. As far as we know, this is the first observation of an integer (S = 1) spin-chain antiferromagnetism in the 4d electron system.

Low temperature electric nature of τ-phase conductors

Magnetoresistance, magnetization, and reflectance spectra were measured for τ-(EDO-S,S-DMEDT-TTF) 2 (AuBr 2 ) 1+y , (y∼0.75) (EDO-S,S-DMEDT-TTF = ethylenedioxy-S,S-dimethylethylenedithio-tetrathiafulvalene) and τ-(P-S,S-DMEDT-TTF) 2 (AuBr 2 ) 1+y , (y∼0.75) (P-S,S-DMEDT-TTF = pyrazino-S,S-dimethylethylenedithio-tetrathiafulvalene). The saturation of magnetization curve and Drude dispersion imply the weak ferromagnetic and metallic nature of former compound at low temperature. For the comparison with this compound, magnetoresistance of latter salt was measured up to 24 T.

Magnetic Structure in the Antiferromagnetic State of the Organic Conductor, (DMe-DCNQI[3,3:1]d7)2Cu: 1H-NMR Analysis

The antiferromagnetic state of the title compound has been investigated by the measurements of 1 H-NMR lineshape and nuclear spin-lattice relaxation rate, T 1 -1 , on a single crystal sample. Below the Neel temperature, T N ∼ 6.7 K, the resonance line was observed to split into three pairs of peaks. The angular dependence of the lineshape has revealed the magnetic structure of this compound; i) the antiferromagnetic moments are localized at Cu 2+ sites and are regularly ordered with a three-fold periodicity along the stacking c-axis; ii) the easy axis is in the a b -plane and the c -axis is the hard axis; iii) a model of the three dimensional arrangement of the magnetic moments well explaining the experimental results is proposed.

Unexpected isotope effect in 13C-substituted (−C ≡ N) molecular conductor (DMe-DCNQI)2Cu (DMe-DCNQI = 2,5-dimethyl-N,N′-dicyanoquinonediimine)

Abstract The introduction of 13 C atoms into two cyano groups in the mixed-valence molecular conductor (DMe-DCNQI) 2 Cu, where DMe-DCNQI is 2,5-dimethyl- N,N′ -dicyanoquinonediimine, non-deuterated and selectively deuterated at the methyl groups, was carried out. Isotope purity for 13 C was higher than 98 atom%. The impurity content (Li, Na, K, Mg, Ti, Mn, Fe, Co, Ni, Zn, Si) was less than 0.002 wt.% (inductively coupled plasma mass spectrometry method). Electrical resistivity measurements have shown apparent and reproducible shifts of the temperatures for phase transitions in each 13 C-containing salt. This unexpected isotope effect indicates that the electronic state of (DMe-DCNQI) 2 Cu is quite sensitive to perturbations in the coordination around Cu.

Magnetic properties of high-pressure phase of CaRuO3 with post-perovskite structure

CaRuO3 post-perovskite which has quasi-two-dimensional lattice shows one dimensional antiferromagnetism such as Bonner-Fisher type above 400 K. The Neel temperature, TN, is around 270 K.

Electronic state of (DI-DCNQI)2Ag with Cu doping or under pressure

(DI-DCNQI) 2 Ag, which has a quasi-one-dimensional quarter-filled π band, is a charge-ordered insulator. In the present study, the charge-ordered phase has been melted in two ways and characterized by the resistivity measurements. One is Cu doping into the Ag sites and the other is application of pressure. As the Cu content is increased, the charge-gapped insulator changes to the nongapped insulator with the variable range hopping behaviour, eventually followed by the metallic phase. When (DI-DCNQI) 2 Ag is pressurized, clear metallic behaviours appear in resistivity above the metal-insulator transition, the temperature of which is decreased at elevated pressures.

Electronic state of (DI-DCNQI)2Ag under ambient and applied pressures

Abstract The resistivity and susceptibility of (DI-DCNQI)2Ag were found to show anomalies corresponding to the charge ordering and the antiferromagnetic transition, respectively, at ambient pressure. The 13 C NMR measurements evidenced that the charge ordering is depressed toward the lower temperatures by application of pressure.

Electronic properties of novel donor-acceptor type charge transfer complexes, (BETS)2(X2TCNQ) (X=Cl, Br) : 1H, 77Se and 13C-NMR

13 C and 77 Se NMR measurements were performed to investigate the electronic properties of (BETS) 2 (X 2 TCNQ) (X=Cl, Br). The susceptibility was decomposed into the contributions of BETS and X 2 TCNQ site. The line broadening of the 1 H/ 77 Se NMR spectra were observed in the Br 2 TCNQ complex at the metal insulator transition temperature, 12 K. Non magnetic spin singlet formation on the Br 2 TCNQ site was observed at 80 K by 13 C NMR. Line broadening of the 1 H NMR spectra were also observed in the Cl 2 TCNQ complex at the metal insulator transition temperature. The insulating state of the Br and Cl 2 TCNQ complexes under pressure are both magnetic. The holes on the BETS layers are relevant to the magnetic ordering.

13C-NMR studies of the metallic state of (DCNQI)2Cu

Abstract 13 C-NMR relaxation rate, T 1 −1 and Knight shift, Δ H/H , were measured in the metallic state of the organic conductors, (DMe-DCNQI) 2 Cu, (DMeO-DCNQI) 2 Cu, and (DI-DCNQI) 2 Cu, the CN groups of which were substituted by 13 CN. The Korringa relation, (T 1 T) −1 /(ΔH/H) 2 = (4πκ B / h )(γ n 2 /γ e 2 )K(α) , was found to to hold for the three compounds over the whole temperature region. The enhancement factor, K ( α ), is quite large (about 4.6 for the DI-salt), suggesting that these salts are highly correlated conductors.