Search

The family of layered organic conductors, θ-(BEDT-TTF)2MM′(SCN)4, are anisotropic triangular-lattice systems with quarter-filled bands and exhibit charge order instability or glassy charge freezing...

Glassy dynamics in the charge degrees of freedom in the organic conductor θ-(BEDT-TTF)2CsZn(SCN)4 is investigated through fluctuation spectroscopy. We observed the 1/f-type noise spectrum over the temperature range studied (20 < T < 300 K). The analysis of the data demonstrates that the DDH model, which assumes the 1/f noise to originate from the independent superposition of thermal activations over a range of energy gaps, explains the dynamics of the charge glass. The distribution of the energy gaps deduced from the analysis reveals that a gap of 0.3 eV dominates the glassy fluctuations in qualitative agreement with the strong-glass nature in θ-(BEDT-TTF)2CsZn(SCN)4.

The θ-(BEDT-TTF)2I3 system is suggested to be the second organic system of massless Dirac fermions, which emerge from the metallic phase at a pressure exceeding 5 kbar. We have performed 13C NMR measurements on a single crystal of θ-(BEDT-TTF)2I3 under variable pressure across the critical pressure, 5 kbar. The Knight shift and spin–lattice relaxation rate show that a metallic state with an NMR enhancement factor of 2.2–2.4 at ambient pressure persists up to the critical pressure of 5 kbar. Above that, a marked spectral change indicative of a first-order phase transition is found. The angular dependence of NMR spectra indicates a structural phase transition from the θ-type molecular arrangement to the α-type one, which hosts the tilted Dirac cone.

La-based high- T C oxides La 2- x Ba x CuO 4 (LBCO) and La 2- x Sr x CuO 4 (LSCO) show a local minimum in the curve of T C versus x around 0.125 for LBCO and around 0.115 for LSCO. In order to investigate the origin of this local minimum, temperature dependence of 63/65 Cu/ 139 La-NMR/NQR spectra has been studied on spray-dry-made samples with various Ba/Sr-concentrations. Existence of the antiferromagnetic ordering was confirmed by a significant broadening in the resonance lines at low temperatures below 35 K for LBCO (\(x\cong 0.125\)) and 32 K for LSCO (\(x\cong 0.115\)). By the analysis on the anisotropy in the broadening, the direction of the ordered field was found to be within the CuO plane. The hyperfine field and the Neel temperature showed a maximum at the Ba/Sr-concentration around which T C is most suppressed, suggesting the crucial interplay between the antiferromagnetic ordering and the suppression in T C .

We performed electrical resistivity measurements for the quasi-two-dimensional ionic Mott insulator M2P-TCNQF4 (M2P: 5,10-dihydro-5,10-dimethylphenazine, TCNQF4: 2,3,5-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) under high pressures up to 80 kbar, to search for a metallic state emerging from an ionic insulator. At room temperature, the resistivity is markedly decreased by five orders of magnitude with increasing pressure up to 80 kbar. Although the temperature dependence of resistivity shows insulating behavior at 80 kbar, the activation energy of 0.36 eV at ambient pressure is reduced to 0.077 eV at 80 kbar, indicating that the system is brought close to the metallic state. At 60 and 80 kbar, low-temperature resistivity is described by variable range hopping rather than by thermal activation over a gap, which suggests that some disorder is induced in the system at high pressures.

We present the results of our 13 C NMR study of the quasi-two-dimensional organic conductor θ-(BEDT-TTF) 2 I 3 under pressure, which is suggested to be a zero-gap conductor by transport measurements. We found that NMR spin shift is proportional to T and that spin–lattice relaxation rate follows the power law T α (α=3–4), where T is the temperature. This behavior is consistent with the cone-like band dispersion and provides microscopic evidence for the realization of the zero-gap state in the present material under pressure.

79 Br NQR measurement is performed to determine the low-energy charge-transfer/dimerization fluctuations in the organic charge-transfer complex DMTTF–2,6-QBr 2 Cl 2 . With decreasing temperature, NQR line width gradually increases and the spin–lattice relaxation rate 1/ T 1 shows a large enhancement from the two-phonon relaxation law ( T 7 ). This behavior is consistent with the picture that the present material is situated in the critical region of the charge-lattice-coupled quantum transition.

We present the results of a ^{1}H NMR study of the single-component molecular conductor, [Au(tmdt)_{2}]. A steep increase in the NMR line width and a peak formation of the nuclear spin-lattice relaxation rate, 1/T_{1}, were observed at around 110 K. This behavior provides clear and microscopic evidences for a magnetic phase transition at considerably high temperature among organic conductors. The observed variation in 1/T_{1} with respect to temperature indicates the highly correlated nature of the metallic phase., Comment: 5pages, 6figures to be published in J. Phys. Soc. Jpn