Keiichi Yokogawa

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.

Solidification of high-pressure medium daphne 7373

The solidification pressure of Daphne 7373, which is widely used as a pressure medium in high pressure studies, was examined at room temperature. Using a new generation clamp-type pressure cell, we found that Daphne 7373 solidifies at 2.2 GPa at room temperature. This is exactly on the natural extrapolation of the melting curve obtained at lower pressures and temperatures in our previous report. The solidification pressure of Daphne 7373 is twice as high as that of another well-known medium Fluorinert 77/70 (0.9 GPa). This allows us to hold hydrostatic pressure even in the newly developed BeCu–NiCrAl clamp-type pressure cell, which exceeds the limit of 1.5 GPa generated by a conventional BeCu cell.

Uniaxial strain orientation dependence of superconducting transition temperature (Tc) and critical superconducting pressure (Pc) in β-(BDA-TTP)2I3.

Dependence of the superconducting transition temperature (T(c)) and critial superconducting pressure (P(c)) of the pressure-induced superconductor β-(BDA-TTP)(2)I(3) [BDA-TTP = 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene] on the orientation of uniaxial strain has been investigated. On the basis of the overlap between the upper and lower bands in the energy dispersion curve, the pressure orientation is thought to change the half-filled band to the quarter-filled one. The observed variations in T(c) and P(c) are explained by considering the degree of application of the pressure and the degree of contribution of the effective electronic correlation at uniaxial strains with different orientations parallel to the conducting donor layer.

Transport properties of HMTSF-TCNQ up to 8 GPa and a novel hysteresis and quantum oscillatory behavior in magnetoresistance in magnetic field up to 31 Tesla

With the interest of ground state near CDW at low temperature, the electronic properties under high pressure, at low temperature and in high magnetic field of HMTSF-TCNQ are examined. Up to 8 GPa, the overall resistivity-temperature behaviour are unchanged, i.e. broad resistance minimum around 100–150 K and subtle resistance decrease below around 30 K. The pressure insensitive nature is not consistent with previous data. At 1.5–1.6 GPa, but not at P = 0, a novel hysteretic behaviour and probable quantum oscillations in magnetoresistance are found in a clear form.

Field-Induced Successive Phase Transitions in the Charge Density Wave Organic Conductor HMTSF-TCNQ

The magnetoresistance of quasi-1D two-chain organic conductor HMTSF-TCNQ (hexamethylenetetraselenafulvalene-tetracyanoquinodimethane), which shows charge density wave transition at ambient pressure, was studied under pressure up to 27 T and 31 T, in two high field facilities. We found a kink structure in the magnetoresistance reminiscent of field-induced spin density wave at an intermediate pressure of 1.5 GPa between 0 and 2 GPa. We speculate that these are successive quantum phases induced by magnetic field.

Metal–semiconductor structural phase transitions and antiferromagnetic orderings in (Benzo-TTFVO)2·MX4 (M = Fe, Ga; X = Cl, Br) salts

Crystals of the 2 : 1 salts of a new donor molecule, benzotetrathiafulvalenoquinone-1,3-dithiolemethide (4, Benzo-TTFVO) with magnetic FeX4− and non-magnetic GaX4− (X = Cl, Br) ions, 42·FeX4 and 42·GaX4, are isostructural to each other and showed a β-type packing of the donor molecules where they form a uniform-stacked structure with an interplanar distance of 3.50 A. These salts exhibited metallic behavior down to 140–170 K, but at these temperatures (∼TM−I) an abrupt increase in the resistivities (ρ) occurred and thereafter semiconducting behavior with an activation energy of 40–100 meV was observed. A structural change in the donor column from uniform to tetramer-unit stacks was observed in the 42·FeBr4 crystal before and after TM−I. By application of pressures up to 1.0 GPa, the metallic behavior in the higher temperature region was gradually strengthened and TM−I gradually became lower with increasing pressure, but the transitions could not be suppressed at all. In response to the metal–semiconductor transition at TM−I, there was a sharp decrease in the paramagnetic susceptibility of the π electron system, where the transition from Pauli paramagnetism due to the metal-conducting behavior to the spin singlet state caused by tetramer formation of the donor molecules was observed. In addition, the FeX4− (X = Cl, Br) salts showed comparatively strong antiferromagnetic interactions between the Fe(III) d spins of the FeCl4− and FeBr4− ions (Weiss temperature: −11 K for 42·FeCl4 and −37 K for 42·FeBr4), giving rise to antiferromagnetic orderings at 1.6 K for 42·FeCl4 and 9.3 K for 42·FeBr4. The magnitudes of the d–d and π–d interactions in 42·FeBr4 are calculated to be Jdd = 2.06 K and Jπd = 2.32 K, respectively. The comparison of these J values with the other magnetic conductors based on our system suggests that the d–d interaction of 42·FeBr4 is stronger than the π–d interaction. Since the three-dimensional antiferromagnetic ordering appears at the comparatively high temperature of 9.3 K, there is an important contribution of the π electrons to the antiferromagnetic ordering of the Fe(III) d spins in order to mediate the magnetic interaction between two-dimensional magnetic anion layers.

Evidence for theπ-d interaction comparing magneto-resistance in (EDT-DSDTFVO)2X, X=FeCl4, GaCl4

We have measured the electrical resistivities and magnetoresistances (MR) of (EDT-DSDTFVO)2X (X=FeCl4, GaCl4), where EDT-DSDTFVO stands for ethylenedithiodiselenadithiafulvalenothioquinone-1,3-dithiolemethide. These materials undergo gradual metal-insulator transitions at Tmin=52 K for FeCl4-salt and Tmin=30 K for GaCl4-salt, respectively. In spite of the similarity of the temperature dependence of the resistivity and its pressure effect, MR of both salts exhibit a clear contrast, i.e. FeCl4-salt shows negative and GaCl4, positive. Origin of the difference in the sign of MR between these salts are discussed in terms of the existence of π-d interaction.

Magnetic-field-induced phase transitions in the quasi-one-dimensional organic conductor HMTSF–TCNQ

Motivated by an interest to see if the field-induced (FI) phase in the charge-density wave (CDW) system is similar to the field-induced-SDW (FISDW) in (TMTSF)2X, (TMTSF: tetramethyltetraselenafulvalene), we examined the magnetic-field-induced phases in a quasi-one-dimensional (Q1D) organic conductor HMTSF–TCNQ (hexamethylene- tetraselenafulvalene- tetracyanoquinodimethane) under a pressure of 1.1 GPa, where the CDW occurring at 30 K is suppressed. The work was carried out by measurements of angular-dependent magnetoresistance oscillations and exploratory work on the Hall effect. It turned out that the FI-phase, most likely a FICDW for B > 0.1 T, accompany a quantum Hall effect, and the FI-phase transitions are controlled by the field component along the least conducting axis. Above 10 T, the lowest Landau level of the small 2D Fermi pocket (due to incomplete nesting of Fermi surface) exceeds the Fermi level, reaching the quantum limit. Although there are many differences between the CDW (HMTSF–TCNQ) and S...