Hisashi Tanaka

New BETS Superconductors and Metals with Tetrahedral Non-Magnetic and Magnetic Anions (BETS = Bis(ethylenedithio)tetraselenafulvalene)

Abstract Two types of BETS conductors are reported. Needle crystals of λ-BETS2GaXxY4-x gave a series of new superconductors: λ-BETS2GaCl4 (Tc=7 K), λ-BETS2GaBrCl3 (8 K), λ-BETS2GaCl3F (3.5 K), λ-BETS2GaBr0.5Cl3.5 (6.5 K). λ-BETS2GaBr1.5Cl2.5, which is a semiconductor at 1 bar, exhibited a superconducting transition at 8 K and ca. 1 kbar. Pressure and temperature dependencies of the resistivities of λ-BETS2GaXxY4-x closely resemble those of κ-type BEDT-TTF superconductors. Many BETS metals with magnetic ions (Fe3+, Mn2+, Co2+, Cu2+) were prepared. Among them, κ-type MX4 (M=Fe, Co; X=Cl, Br) salts and θ-type Cu2Cl6 salts are metallic down to at least 4 K. The static susceptibility of λ-BETS2FeCl4 shows a characteristic magnetic field dependence below its metal-insulator transition temperature.

A novel superconductor with insulating magnetic ground state

Abstract The BETS conductor with mixed anions, λ-BETS 2 Fe x Ga 1− x Cl 4 (BETS=bis(ethylenedithio)tetraselenafulvalene) exhibited an unprecedented superconductor-to-insulator (SC–I) transition around x =0.43 at ambient pressure. The large drop in magnetic susceptibility at T c (≈4.2 K) and its recovery at T SC–I (≈3.2 K) clearly showed that this SC–I transition reflects an intrinsic nature of the crystal. The magnitude of ac susceptibility (≈−40 emu mol −1 ) at 4 K, where the system is in the superconducting state, gave the Meissner volume fraction of about 75%. The anisotropy and field dependence of magnetization below T SC–I indicate the insulating ground state to be antiferromagnetic with easy axis parallel to c . The systematic examination of the resistivities of λ-BETS 2 Fe x Ga 1− x Cl 4 showed that the system undergoes a MI transition at x > x c1 (≈0.5), SC transition at x x c2 (≈0.35) and SC–I transition at x c2 x x c1 . The SC–I transition was also observed in λ-BETS 2 Fe x Ga 1− x Cl 4 ( x ≈0.55) at 1 kbar, which exhibited a metal–insulator (M–I) transition at ambient pressure. The high-pressure resistivity measurements revealed a superconducting transition in λ-BETS 2 Fe 0.7 Ga 0.3 Cl 4 . Based on these resistivity measurements, the preliminary P-T- x phase diagram of λ-BETS 2 Fe x Ga 1− x Cl 4 was obtained. The anisotropy and field dependence of the susceptibilities of λ-BETS 2 Fe x Ga 1− x Cl 4 ( x =1.0, 0.7, 0.55 and 0.43) suggested the characteristic π-d coupled antiferromagnetic structure of the insulating ground state. The Br-containing system, λ-BETS 2 Fe x Ga 1− x BrCl 3 was semiconducting at ambient pressure. But λ-BETS 2 Fe x Ga 1− x BrCl ( x =0.5) exhibited a SC–I transition ( T c =7 K and T SC–I =3 K) at 2 kbar. While λ-BETS 2 Fe x Ga 1− x BrCl ( x ≈0.1–0.2) showed a SC–M (metal) transition around 1.5 kbar. It was quite surprising that the susceptibility of λ-BETS 2 Fe x Ga 1− x BrCl ( x ≈0.1) showed the antiferromagnetic ordering of the diluted Fe 3+ spins around 4 K at ambient pressure. The recovery of the metallic state below T SC–M in this system seems to be connected with the magnetic ordering of Fe spins.

Structural and electrical properties of BETS compounds with transition metal complex anions

Abstract BETS (=bis(ethylenedithio)tetraselenafulvalene) conductors with M(CN) 4 anions (M=Pd, Pt) are metallic down to 4K. Crystals of MCl 4 salts (M=Co, Ni, In) are metallic down to 4 K. BETS 2 InCl 5 [(C 2 H 5 ) 4 N]0.7 undergoes a gradual metal-insulator transition around 50 K. The magnetic susceptibility of BETS 4 CU 2 Cl 6 exhibits a round maximum showing an antiferromagnetic interaction between Cu 2+ in Cu 2 Cl 6 2− anions (J=163 K). The electrical properties and crystal structures of BETS complexes with M(dts) 2 (M=Au, Pd, Pt; dts= dithiosquarate) are briefly mentioned.

Structural and electrical properties of a series of organic superconductors λ-BETS2GaXzY4-z (X=Br, Cl, F; 0<z<4)

A series of organic superconductors with tetrahalide GaX 4 - anions λ-BETS 2 GaX z Y 4-z (X=Br, Cl, F; 0<z<4, BETS= bis(ethylenedithio)tetraselenafulvalene) are reported together with related BETS conductors. Electric, magnetic and structural properties are briefly described. The resistivity behavior and the superconducting transition temperature have a strong correlation with the size of the anion (or unit cell volume).