Syuma Yasuzuka

High-Pressure Research in Organic Conductors

High-pressure studies on organic conductors and superconductors are reviewed. It is stressed that almost all the organic conductors are studied by means of high pressures, but the role or meaning o...

Temperature-pressure phase diagram in TTF-TCNQ : Strong suppression of charge-density-wave state under extremely high pressure

We have measured the resistivity of the organic conductor TTF-TCNQ under high pressure up to 8.0 GPa using a cubic anvil apparatus. Below 3 GPa, the pressure dependence of the charge-density-wave (CDW) transition temperature, T CDW , is consistent with previous results, except for the observation of the “49 K-transition” at around 1 GPa. Above 3 GPa, T CDW decreases with increasing pressure but the CDW state survives even at 8.0 GPa. At 8.0 GPa, however, the resistivity shows metallic behavior below 25 K after the CDW transition with T CDW = 31 K. The origin of suppression is discussed in terms of the imperfect nesting originating from the increase in the interchain transfer integral, the significant deviation from the third-order commensurability, as well as the bandwidth broadening for both TTF and TCNQ stacks. The complete suppression of the CDW phase is expected near 9 GPa.

Global Phase Diagram of the Magnetic Field-Induced Organic Superconductors λ-(BETS)2FexGa1-xCl4

Organic alloys λ-(BETS) 2 Fe x Ga 1- x Cl 4 show superconductivity only under very high magnetic fields parallel to the conducting layer for x ≥0.47. As x decreases, the field induced superconducti...

Experimental assessment of quasi-binary picture of thermotropics: Induced smectic A phase in 7CB–n-heptane system

Quasi-binary (QB) picture of thermotropics, which regards a neat thermotropic liquid crystal as a binary system consisting of (semi)rigid core and molten alkyl chain, was assessed experimentally for the most famous (and representative) thermotropic mesogenic series nCB. By adding n-heptane as solvent, the smectic A phase was induced in 7CB-n-heptane system. Small angle x-ray diffraction showed that the QB picture holds not only in the phase behavior but also in the structure. It is suggested that the melting of 8CB and 9CB to isotropic liquid via smectic and nematic liquid crystals can be understood as a thermotropic-lyotropic crossover.

Calorimetric and dielectric study of organic ferroelectrics, phenazine-chloranilic acid, and its bromo analog

The heat capacities of single crystals of organic ferroelectric complexes phenazine-chloranilic acid (Phz-H(2)ca) and phenazine-bromanilic acid (Phz-H(2)ba) were measured. At temperatures below those of the reported ferroelectric phase transitions, heat capacity anomalies due to successive phase transitions were found in both complexes. Excess entropies involved in the low-temperature successive phase transitions are much larger than those due to the ferroelectric phase transitions. The temperature dependence of the complex dielectric constants showed the existence of multiple dielectric relaxation modes in both complexes and their deuterated analogs (Phz-D(2)ca and Phz-D(2)ba). We discuss the possibility of concerted hopping of neighboring protons within a hydrogen-bonded chain while taking into account the one-dimensional nature of the chain.

Quantum Criticality in Reduced π-Donor System (MeDH-TTP)2AsF6

The temperature–pressure ( T – P ) phase diagram of the reduced π-donor system (MeDH-TTP) 2 AsF 6 is studied using resistivity measurement, where MeDH-TTP stands for 2-methyl-5-(1,3-dithiolan-2-yli...

Fermi surface and resistance anomalies in ET-TCNQ

Recently, a new organic conductor ET-TCNQ, which exhibits two resistance anomalies at T 1 = 80 K and T 2 = 20 K, was synthesized. We present a systematic study of transport properties of ET-TCNQ under magnetic fields. Shubnikov-de Haas (SdH) oscillations are clearly observed and we find five distinct small Fermi surfaces (FSs). The cross-sectional areas of FS are estimated to be from I %(α) to 3.5 %(e) of the first Brillouin zone. We detect Lebed resonance and Danner-Chaikin oscillation in measurements of angular-dependent magnetoresistance oscillation (AMRO), suggesting the existence of the quasi-one dimensional FS related to ET molecules. The origins of the two resistance anomalies are discussed in terms of nesting-driven density-wave transition.

Coexistence of the Upper Charge-Density-Wave and the Superconductivity in NbSe3

We have extensively measured the resistance of NbSe 3 near critical pressure ( P 1 = 3.2 GPa) for the upper charge-density-wave ( T 1 -CDW) phase. We reveal that the T 1 -CDW coexists with the superconducting (SC) phase between 2.0 GPa and 2.9 GPa. Near P 1 on the T 1 -CDW side, the T 1 -CDW transition temperature appears to be singularly, T 1 ∼( P 1 - P ) 0.49±0.03 and the SC transition is very broad and becomes much sharpened with increasing pressure. We discuss mechanism of the coexistence of the T 1 -CDW and the SC phase.

Neat liquid consisting of hydrogen-bonded tetramers: dicyclohexylmethanol.

Nonmonotonous temperature dependence was detected in the heat capacity and dielectric constant of the title compound in the liquid state. A coherent analysis of FT-IR spectra, heat capacity, and dielectric constants shows that the neat liquid at low temperatures consists of closed (square) tetramers via H-bonds.

Metallization of (TTM-TTP)I3 with a highly one-dimensional half-filled band under extremely high pressure

The title compound (TTM-TTP)I 3 with a highly one-dimensional (1D) half-filled energy band is almost metallized by the application of pressure beyond 7 GPa. It is theoretically predicted that the g...