Abstract
The BETS conductor with mixed anions, λ-BETS 2Fe 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 2Fe 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 2Fe 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 2Fe 0.7Ga 0.3Cl 4. Based on these resistivity measurements, the preliminary P-T- x phase diagram of λ-BETS 2Fe x Ga 1− x Cl 4 was obtained. The anisotropy and field dependence of the susceptibilities of λ-BETS 2Fe 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 2Fe x Ga 1− x BrCl 3 was semiconducting at ambient pressure. But λ-BETS 2Fe 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 2Fe 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 2Fe 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.
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