A novel superconductor with insulating magnetic ground state

The BETS conductor with mixed anions, lambda-BETS2FexGa1-xCl4 (BETS = bis(e thylenedithio)tetraselenafulvalene) exhibited an unprecedented superconduct or-to-insulator (SC-I) transition around x = 0.43 at ambient pressure. The large drop in magnetic susceptibility at T-c (approximate to 4.2 K) and its recovery at TSC-I (approximate to 3.2 K) clearly showed that this SC-I tra nsition reflects an intrinsic nature of the crystal. The magnitude of ac su sceptibility (approximate to -40 emu mol(-1)) at 4 K, where the system is i n the superconducting state, gave the Meissner volume fraction of about 75% . The anisotropy and held dependence of magnetization below TSC-I indicate the insulating ground state to be antiferromagnetic with easy axis parallel to c. The systematic examination of the resistivities of lambda-BETS2FexGa 1-xCl4 showed that the system undergoes a MI transition at x > x(c1) (appro ximate to 0.5), SC transition at x < x(c2) (approximate to 0.35) and SC-I t ransition at x(c2) < x < x(c1). The SC-I transition was also observed in la mbda-BETS2FexGa1-xCl4 (x approximate to 0.55) at 1 kbar, which exhibited a metal-insulator (M-I) transition at ambient pressure. The high-pressure res istivity measurements revealed a superconducting transition in lambda-BETS2 Fe0.7Ga0.3Cl4. Based on these resistivity measurements, the preliminary P-T -x phase diagram of lambda-BETS2FexGa1-xCl4 was obtained. The anisotropy an d field dependence of the susceptibilities of lambda-BETS2FexGa1-xCl4 (x = 1.0, 0.7, 0.55 and 0.43) suggested the characteristic pi-d coupled antiferr omagnetic structure of the insulating ground state. The Br-containing syste m, lambda-BETS2FexGa1-xBrCl3 was semiconducting at ambient pressure. But la mbda-BETS2FexGa1-xBrCl (x = 0.5) exhibited a SC-I transition (T-c = 7 K and TSC-I = 3 K) at 2 kbar. While lambda-BETS2FexGa1-xBrCl (x approximate to 0 .1-0.2) showed a SC-M (metal) transition around 1.5 kbar. It was quite surp rising that the susceptibility of lambda-BETS2FexGa1-xBrCl (x approximate t o 0.1) showed the antiferromagnetic ordering of the diluted Fe3+ spins arou nd 4 K at ambient pressure. The recovery of the metallic state below TSC-M in this system seems to be connected with the magnetic ordering of Fe spins . (C) 1999 Elsevier Science S.A. All rights reserved.