S. Aonuma

A new molecular superconductor β′-Et2Me2P[Pd(dmit)2]2 (dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate)

Abstract The anion radical salt β′-Et 2 Me 2 P[Pd(dmit) 2 ] 2 is non-metallic at ambient pressure. An application of pressure induces a metallic behavior and the system exhibits a superconducting transition in the pressure region of 6.9–10.4 kbar with T c = 4.0−1.8 K (onset). Under higher pressure, however, non-metallic behavior appears in the low-temperature region. The isostructural salt β′-Et 2 Me 2 Sb[Pd(dmit) 2 ] 2 also shows pressure-induced metallic behavior. However, not a superconducting transition above 1.6 K, nor a high-pressure non-metallic behavior is observed at pressures up to 16.4 kbar. These Pd(dmit) 2 salts can be characterized by an existence of two different types of bands near the Fermi level. The counter cation dependence of the electronic state under pressure suggests that dimensionality of the electronic structure plays an important role.

Preparation and physical properties of an alloyed (DMe-DCNQI)2Cu with fully deuterated DMe-DCNQI (DMe-DCNQI = 2,5-dimethyl- N,N′-dicyanoquinonediimine)

Abstract Preparation, electrical resistivity, static magnetic susceptibility, and low-temperature X-ray study of the alloyed molecular conductor [(DMe-DCNQI-h 8 ) 1− x (DMe-DCNQI-d 8 ) x ] 2 Cu are described. The very low-pressure (P ca. 500 bar) region in the pressure-temperature phase diagram for (DMe-DCNQI) 2 Cu obtained by the helium gas pressure technique could be reproduced under ambient pressure by the alloying with DMe-DCNQI-d 8 . The results are discussed in terms of the strong correlation picture.

Se-Substitution and Cation Effects on the High-Pressure Molecular Superconducior, β-Me4N[Pd(dmit)2]2 -A Unique Two-Band System

Abstract Crystal and electronic structures and physical properties of the molecular conductors Me4Z [Pd(L)2]2 (Z = N, P, As, and Sb; L = dmit and dmise) are described. Every salt has very similar crystal structure with “solid-crossing” columns. The behavior of resistivity under pressure depends on the Se-substitution and the choice of the cation. Tight-binding band calculations indicate that these salts form a unique two-band system. The narrow and two-dimensional HOMO (anti-bonding) band is thought to form the half-filled conduction band. Magnetic properties suggest that the insulating state under ambient pressure comes from the strong electron-electron correlation. The one-dimensional LUMO (bonding) band is located below the HOMO band. The application of pressure induces an overlap of these two bands and changes the band filling, which brings forth the metallic state. The system under higher pressure, however, shows non-metallic behavior probably due to the one-dimensional instability associated with th...

Synthesis of DMe-DCNQI-d7 and deuterium-induced metal-insulator transition of (DMe-DCNQI-d7)2Cu

Abstract DMe-DCNQI-d 7 (DCNQI = dicyanoquiodiimine) was synthesized by applying 1 H/ 2 D exchange reaction, and its Cu and Li salts were prepared. An extremely sharp metal-insulator transition was brought about by deuteration, whereas (DMe-DCNQI-h 8 ) 2 Cu maintained its metallic state. A negligible isotope effect was observed on the resistivity behavior of the Li salt. The difference in reduction potential of the sole molecule in solution was found to be very small. Small but definite contraction of the stacking axis was caused by deuteration in both the Cu and Li salts. This ‘chemical pressure’ would be a plausible explanation of the drastic isotope effect observed in the Cu salt.

Metallic and superconducting salts based on an unsymmetrical π-donor dimethyl(ethylenedithio)tetraselenafulvalene (DMET-TSeF)

Abstract A series of radical cation salts (DMET-TSeF)2X (X = I3, AuI2, AuBr2, AuCl2, Au(CN)2) crystallize in the triclinic P 1 group. The DMET-TSeF molecules form zigzag columnar stacks. Short intermolecular chalcogen ⋯ chalcogen contacts are observed within and between stacks. The AuI2 salt exhibits a sample-dependent resistivity minimum around 25 K. The AuI2 salt without the resistivity minimum shows superconductivity with Tc = 0.58 K at ambient pressure. The AuBr2 salt exhibits a resistivity minimum around 35 K. Other salts remain metallic down to 4.2 K. These salts should be classified into a quasi one-dimensional system.

Anomalous pressure-temperature phase diagram of the molecular conductor, (DI-DCNQI)2Cu (DI-DCNQI=2,5-DIIODO-N,N′-dicyanoquinonediimine)

Abstract We report the temperature-dependent resistivity under pressure up to 22 kbar and the crystal and electronic band structures for the molecular conductor (DI-DCNQI) 2 Cu. In the (DCNQI) 2 Cu system, the application of pressure induces the metal-insulator (M-I) transition. (DI-DCNQI) 2 Cu shows the highest critical pressure (15.3 kbar). In contrast to previously reported (DCNQI) 2 Cu systems, however, this system turns into metallic state again above 20 kbar. This anomalous phenomenon is discussed in terms of an enhancement of the intercolumn LUMO…LUMO interaction.

Enhanced magnetic susceptibility of (DI-DCNQI)2Cu

Abstract The Cu salt of 2,5-diiodo-N,N′-dicyanoquinonediimine, (DI-DCNQI)2Cu, which is metallic down to 4 K, is found to exhibit enhanced and temperature dependent magnetic susceptibility, in contrast to other metallic DCNQI salts of Cu. The susceptibility shows a broad maximum around 110 K. The temperature dependence can be analyzed on the basis of a phenomenology using a characteristic temperature, T ∗ ≈ 200 K . This suggests an important role of spin fluctuations in a narrow 3d band near the Fermi level. To our knowledge, this is the first evidence for the effect of spin fluctuation in a molecular conductor based on π-d interactions.

Cation size effect in molecular conductors related to the high-pressure superconductor β-Me4N[Pd(dmit)2]2 (dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate)

Abstract Cation size effect in the anion radical salts of Pd(dmit) 2 with the tetrahedral cation Me 4 Z + (Z=N, P, As, and Sb) has been examined by resistivity measurements under pressure, X-ray structure analyses and tight-binding band calculations. Although all compounds possess very similar dimerized column structure, resistivity behaviors under pressure are quite different. The cation size affects an overlapping mode between the Pd(dmit) 2 dimers. Tight-binding band calculations show that subtle differences in the overlapping mode have a profound effect on their electronic structures. Our results suggest that the stronger two-dimensional nature tends to induce the high-pressure metallic phase. This system is a unique dimer system, where the dimerization is closely linked to the dimensionality of the electronic structure.

Charge-Transfer-Controlled Phase Transition in a Molecular Conductor, (DMe-DCNQI)2Cu–Doping Effect–

Li + -doping in the Cu salt of deuterated DMe-DCNQI- d 7 has shown drastic effects on electrical and magnetic properties. Undoped (DMe-DCNQI- d 7 ) 2 Cu exhibits a first-order metal-insulator (M-I) transition at 80 K. A small amount of Li + -doping suppresses the M-I transition. The M-I-M (reentrant) transition, which is similar to that in selectively deuterated (DMe-DCNQI) 2 Cu, appears in a narrow region of Li content x . The x - T phase diagram under ambient pressure well reproduces the P - T phase diagram of the (DMe-DCNQI) 2 Cu system. This doping effect is explained by the change in charge transfer amount from metal ions to DCNQI molecules. The effect of pressure or deuteration on the M-I transition in the DCNQI 2 Cu system is generally understood from this viewpoint.

Preparation, structure and conductivity of Et2Me2Z (ZP,As,Sb) salts of M(dmit)2 and M(dmise)2 (MNi,Pd)

Abstract Et 2 Me 2 P + was synthesized by Grignard reaction of MePCl 2 with EtMgBr, followed by quaternarization using MeI. Et 2 Me 2 Sb + was prepared by similar method, but obtained as mixture of Et n Me (4− n ) Sb + ( n = 0,1,2,3). Fortunately, (Et 2 Me 2 Sb)[Pd(dmit) 2 ] 2 could be obtained as single crystals in a certain condition. Both (Et 2 Me 2 P)[Pd(dmit) 2 ] 2 and (Et 2 Me 2 Sb)[Pd(dmit) 2 ] 2 are isostructural with β′-Me 4 Z salts (ZP, As, Sb), and indicate stronger dimerization and enhanced two-dimensionality.