Nuclear spin-lattice relaxation in κ-(BETS)2FeBr4

Abstract We report results of 77 Se NMR study for a newly found field-induced superconductor, κ-(BETS) 2 FeBr 4 . The nuclear spin lattice relaxation rate (1/T 1 ) above 25 K has a linear relation with temperature, 1/ T 1 = a T +b, which can be analyzed in a framework of normal metal with dipolar fields from uncorrelated 3 d local moments to the Se nuclei. However, 1/T 1 has an evident kink at 25 K and shows a remarkable decrease within a narrow temperature range of 2.5 K. Below 22.5 K, we found that 1/ T 1 follows a power law 1/ T 1 ∼ T 1.43 , although the material is still conductive. The temperature where 1/ T 1 shows a kink agrees with the temperature where the temperature devivative of the resistivity dρ/d T diverses. We discuss a possible reconstruction of the electronic state, which may attain magnetic correlation below this temperature.