Keizo Murata

Superconductivity with the Onset at 8 K in the Organic Conductor β-(BEDT-TTF)2I3 under Pressure

We report the superconductivity with the onset temperature as high as 8 K in the organic conductor of β-(BEDT-TTF) 2 I 3 , bis-ethylenedithio otetrathiofulvalene-triiodide. The increased superconducting transition temperature was achieved by an application of a soft pressure, i.e. 1.3 kb. High value of the upper critical field, H c2 was observed. ( H c2 >13 kOe at 1.5 K along the c * -axis.)

Study on the electrical conduction mechanism of polypyrrole films

Abstract The electrical conduction mechanism of polypyrrole films containing various counter ions was investigated. Above 100 K the temperature dependence of electrical conductivity obeyed the Mott equation based on variable range hopping. The exponential factor ( T o ) increased as the size of counter ion increased, while the pre-exponential factor ( K o ) was independent of the size. The enhancement of conductivity ( σ ∣ ) in the stretch direction upon stretching was explained by the increase of the hopping attempt frequency ( ν o ). The temperature dependence of the conductivity of the films containing small counter ions fitted well the Sheng equation from room temperature down to 10 K suggesting that the conduction is dominated more strongly by tunneling than by hopping in a lower temperature region. The conductivity of a film containing PF 6 − showed a metallic-like temperature dependence below 20 K where the apparent magnetoresistance was positive.

Precise Pressure Dependence of the Superconducting Transition Temperature of FeSe: Resistivity and 77Se-NMR Study

We report the precise pressure dependence of FeSe from a resistivity measurement up to 4.15 GPa. Superconducting transition temperature ( T c ) increases sensitively under pressure, but shows a plateau between 0.5–1.5 GPa. The maximum T c , which is determined by zero resistance, is 21 K at approximately 3.5 GPa. The onset value reaches ∼37 K at 4.15 GPa. We also measure the nuclear spin–lattice relaxation rate 1/ T 1 under pressure using 77 Se-NMR measurement. 1/ T 1 shows that bulk superconductivity is realized in the zero-resistance state. The pressure dependence of 1/ T 1 T just above T c shows a plateau as well as the pressure dependence of T c , which gives clear evidence of the close relationship between 1/ T 1 T and T c . Spin fluctuations are suggested to contribute to the mechanism of superconductivity.

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.

Pressure Phase Diagram of the Organic Superconductor β-(BEDT-TTF)2I3

Low temperature phase diagram of β-(BEDT-TTF) 2 I 3 as a function of pressure ( P ) is determined by resistivity measurement. The superconducting transition temperature ( T c ) rises from 1.1–1.5 K ( P =0 kbar) to around 7.4 K ( P =1.3 kbar) and decreases to 1.2 K ( P =7 kbar). We propose that two kinds of superconducting states exist in this salt and that ambient pressure gives two stable states which have T c s of around 8 K as well as around 1.3 K. The upper critical field, H c2 , along the c * -axis is also presented.

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.

Ambient-pressure superconductivity at 8 K in the organic conductor β-(BEDT-TTF)2I3

Abstract We report that the superconductivity with the onset temperature as high as 8 K in the organic conductor of β-(BEDT-TTF)2I3, which was achieved by an application of a soft pressure, i.e. 1.3 kb, remains stable with even higher transition temperature after a subsequent release of pressure to the ambient pressure. Correspondingly, higher values of the upper critical field, Hc2, are observed. (Hc2 > 20 k0e at 2 K along the c∗-axis.)

Superconductivity and the possibility of semiconductor-metal transition in (DMET)2AuBr2

An organic conductor, (DMET) 2 AuBrd 2 , was found to undergo superconducting transition around 1 K under a pressure of 1.5 kbar. Furthermore, this material was found to be semiconducting above 190 K. Below this temperature, it became metallic, contrary to usual metal-semiconductor transition. This semiconductor-metal transition also occurred at 180 K at ambient pressure.

Crystal and electronic structures of new molecular conductors tetramethylammonium and tetramethylarsonium complexes of Pd(dmit)2

The synthesis, structural characterization and electrical conductivity measurements of α-,β-[(CH3)4N][Pd(dmit)2]2 and [(CH3)4As][Pd(dmit)2]2 are reported. They have similar crystal structures made of stacked Pd(dmit)2 dimers. The mode of the intra-dimer molecular overlapping is that of the eclipsed type. Pd atoms deviate from the planes formed by ligand S atoms by 0.07–0.08 A to approach to each other. Simple tight-binding band calculations were made on these three Pd(dmit)2 salts. Each energy band is made up of two energy branches separated by a large mid-gap. The lower energy branch is a narrow half-filled band which seems to be consistent with the weakly metallic or semiconducting behaviour of these compounds around room temperature.

Ambient-pressure superconductivity in organic metals, BEDT-TTF trihalides

Abstract The ambient-pressure superconductivity in β-(BEDT-TTF) 2 I 3 and its mixed trihalide homologues has been studied, where BEDT-TTF is bis-(ethylenedithio-tetrathiafulvalene. In β-(BEDT-TTF) 2 I 3 , two superconductivity states are observed, i.e. , a low- T c state with T c = 1.1 – 1.5 K and a high- T c state with T c = 7 – 8 K. The high- T c state, which was originally discovered under a pressure of 1.3 kbar, is found to remain stable after removal of pressure. Moreover it has also been observed even without application of pressure. In order to realize an intrinsic high- T c ambient-pressure organic superconductor, we propose an introduction of ‘lattice pressure’ by substitution of triiodide anion (I 3 − ) with smaller mixed trihalide anions, such as I 2 Br − and IBr 2 − . The result is discussed using a model based on the T c vs. pressure phase diagram for β-(BEDT-TTF) 2 I 3 .