Collapse of charge order in a quasi-one-dimensional organic conductor with a quarter-filled band

Correlating conduction properties with the molecular symmetry: segregation of Z and E isomers in the charge-assisted, halogen-bonded cocrystal [(Z,E)-Me2I2TTF]2Br

Co-crystallization of theZandEisomers of Me₂I₂TTF in a mixed-valence bromide salt leads to segregated stacks with two different charge order patterns and associated charge-assisted halogen bonding.

The effects of the next-nearest-neighbour density-density interaction in the atomic limit of the extended Hubbard model

We have studied the extended Hubbard model in the atomic limit. The Hamiltonian analysed consists of the effective on-site interaction U and the intersite density-density interactions W(ij) (both nearest-neighbour and next-nearest-neighbour). The model can be considered as a simple effective model of charge ordered insulators. The phase diagrams and thermodynamic properties of this system have been determined within the variational approach, which treats the on-site interaction term exactly and the intersite interactions within the mean-field approximation. Our investigation of the general case taking into account for the first time the effects of longer-ranged density-density interaction (repulsive and attractive) as well as possible phase separations shows that, depending on the values of the interaction parameters and the electron concentration, the system can exhibit not only several homogeneous charge ordered (CO) phases, but also various phase separated states (CO-CO and CO-nonordered). One finds that the model considered exhibits very interesting multicritical behaviours and features, including bicritical, tricritical, and critical end points and isolated critical points.

Short piston-cylinder pressure cells based on Ni-Cr-Al cylinders and their application to fragile materials

In this decade, the performance of piston-cylinder pressure cells has been drastically improved by using robust materials such as nickel-chromium-aluminum and cobalt-nickel-chromium-molybdenum alloys to construct the inner cylinders. In this article, we present several experimental techniques for carrying out resistivity measurements under high pressure by applying the piston-cylinder devices based on the nickel-chromium-aluminum cylinders to fragile materials such as organics. These techniques are, in principle, applicable to measurements on any solid-state conductor. First, we introduce the construction of our piston-cylinder cells including two kinds of wired platforms for transport measurements. Second, we describe the construction of the platforms and the method of introducing the samples. After reporting test results for conventional materials such as ammonium fluoride, bismuth, and tellurium, lastly, we present examples of the successful application of our method to organic materials.

Phase diagram of quarter-filled band organic salts [EDT-TTF-CONMe2]2X, X=AsF6 and Br

An investigation of the P-T phase diagram of the quarter-filled organic conductors, [EDT-TTF-CONMe2]2X is reported on the basis of transport and NMR studies of two members, X=AsF6 and Br of the family. The strongly insulating character of these materials in the low-pressure regime has been attributed to a remarkably stable charge-ordered state confirmed by 13C NMR and the only existence of 1/4 Umklapp e-e scattering favoring a charge ordering instead of the 1D Mott localization seen in (TMTTF)2X and (TMTSF)2X which are 1/4-filled compounds with dimerization. A nonmagnetic insulating phase instead of the spin density wave state is stabilized in the deconfined regime of the phase diagram. This sequence of phases observed under pressure may be considered as a generic behavior for 1/4-filled conductors with correlations.

Spiral charge frustration in the molecular conductor (DI-DCNQI)2Ag

We theoretically study the effect of spiral-type charge frustration in a quasi-one-dimensional molecular conductor (DI-DCNQI)2Ag. We clarify how the spiral frustration in the interchain Coulomb repulsion is relieved and leads to a self-organization of complex charge-lattice ordered chains, in agreement with the recent synchrotron x-ray study [T. Kakiuchi, Phys. Rev. Lett. 98, 066402 (2007)10.1103/PhysRevLett.98.066402]. In addition, we find that a keen competition between charge and lattice degrees of freedom under the frustration gives rise to a characteristic temperature within the ordered phase, below which a drastic growth of molecular displacements occurs. Our results enlighten the relevance of the spiral frustration and provide a possible reconciliation among puzzling experimental data.

Recent progress in high-pressure studies on organic conductors

Recent high-pressure studies of organic conductors and superconductors are reviewed. The discovery of the highest T_c superconductivity among organics under high pressure has triggered the further progress of the high-pressure research. Owing to this finding, various organic conductors with the strong electron correlation were investigated under high pressures. This review includes the pressure techniques using the cubic anvil apparatus, as well as high-pressure studies of the organic conductors up to 10 GPa showing extraordinary temperature and pressure dependent transport phenomena.

Tetrathiapentalene-based organic conductors

The synthesis, structure and properties of tetrathiapentalene-based (TTP) organic conductors are reviewed. Among various TTP-type donors, bis-fused tetrathiafulvalene, 2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene (BDT-TTP) and its derivatives afford many metallic radical cation salts stable down to low temperatures, regardless of the size and shape of the counter anions. Most BDT-TTP conductors have a β-type donor arrangement with almost uniform stacks. Introduction of appropriate substituents results in molecular packing that differs from the β-type. A vinylogous TTP, 2-(1,3-dithiol-2-ylidene)-5-(2-ethanediylidene-1,3-dithiole)-1,3,4,6-tetrathiapentalene (DTEDT) has yielded an organic superconductor (DTEDT)3Au(CN)2 as well as metallic radical cation salts, regardless of the counter anions. (Thio)pyran analogs of TTP, namely (T)PDT-TTP and its derivatives produce molecular conductors with novel molecular arrangements. A TTP analog with reduced π-electron system 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene (BDA-TTP) has afforded several organic superconductors. Highly conducting molecular metals with unusual oxidation states (+1, +5/3 and neutral) have been developed on the basis of 2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene (BDT-TTP) derivatives and analogous metal derivatives M(dt)2 (M = Ni, Au).

Pressure transmitting medium Daphne 7474 solidifying at 3.7 GPa at room temperature

A pressure transmitting medium named Daphne 7474, which solidifies at P(s)=3.7 GPa at room temperature, is presented. The value of P(s) increases almost linearly with temperature up to 6.7 GPa at 100 degrees C. The high pressure realized by a medium at the liquid state allows a higher limit of pressurization, which assures an ideal hydrostatic pressure. We show a volume change against pressure, pressure reduction from room to liquid helium temperature in a clamped piston cylinder cell, pressure distribution and its standard deviation in a diamond anvil cell, and infrared properties, which might be useful for experimental applications.

Adaptation of the Bridgman anvil cell to liquid pressure mediums

The advantage of Bridgman anvil pressure cells is their wide pressure range and the large number of wires which can be introduced into the pressure chamber. In these pressure cells, soft solid pressure mediums such as steatite are used. We have succeeded in adapting the Bridgman cell to liquid pressure mediums. With this breakthrough, it is now possible to measure in very good hydrostatic pressure conditions up to 7 GPa, which is about twice the pressure attainable in piston-cylinder cells. The pressure gradient in the cell, estimated from the superconducting transition width of lead, is reduced by a factor of 5 in the liquid medium with respect to steatite. By using nonmagnetic materials for the anvils and the clamp and due to the small dimensions of the latter, our device is specially suitable for magnetotransport measurements in dilution fridges. This pressure cell has been developed to measure very fragile and brittle samples such as organic conductors. Resistivity measurements of (TMTTF)(2)BF(4) performed in a solid and a liquid pressure medium demonstrate the necessity of hydrostatic pressure conditions for the study of organic conductors at high pressures.