Electronic properties of BETS superconductors with magnetic anions (BETS=bis(ethylenedithio)tetraselenafulvalene)

Abstract

The physical properties of BETS (=bis(ethylenedithio)tetraselenafulvalene) conductors with tetrahalide magnetic anions and related systems were investigated. We have recently succeeded to find the superconducting transition of the crystal of κ-type TlCl 4 − salt coated with epoxy resin ( T c(onset)=2.5 K). The specific heat measurements on the antiferromagnetic organic superconductor, κ-(BETS) 2FeCl 4 was performed down to 60 mK, which gave sharp peak at 0.45 K ( T N). The peak trails a tail at T> T N, suggesting low-dimensionality of the spin system. Resistivity measurements under magnetic field of λ-(BETS) 2Fe x Ga 1− x Cl 4 ( x≈0.4) exhibiting superconductor-to-insulating (SC–I) transition, I–SC phase transition was observed around 4 T at T<3 K. The “low-field superconducting region” between metallic and insulating phases around zero magnetic field is connected with the “high-field superconducting region”.